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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



 NAME
      libpng - Portable Network Graphics (PNG) Reference Library 1.6.29

 SYNOPSIS
      #include <png.h>

      png_uint_32 png_access_version_number (void);

      void png_benign_error (png_structp png_ptr, png_const_charp error);

      void png_build_grayscale_palette (int bit_depth, png_colorp palette);

      png_voidp png_calloc (png_structp png_ptr, png_alloc_size_t size);

      void png_chunk_benign_error (png_structp png_ptr, png_const_charp
      error);

      void png_chunk_error (png_structp png_ptr, png_const_charp error);

      void png_chunk_warning (png_structp png_ptr, png_const_charp message);

      void png_convert_from_struct_tm (png_timep ptime, struct tm FAR *
      ttime);

      void png_convert_from_time_t (png_timep ptime, time_t ttime);

      png_charp png_convert_to_rfc1123 (png_structp png_ptr, png_timep
      ptime);

      png_infop png_create_info_struct (png_structp png_ptr);

      png_structp png_create_read_struct (png_const_charp user_png_ver,
      png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

      png_structp png_create_read_struct_2 (png_const_charp user_png_ver,
      png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn,
      png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

      png_structp png_create_write_struct (png_const_charp user_png_ver,
      png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn);

      png_structp png_create_write_struct_2 (png_const_charp user_png_ver,
      png_voidp error_ptr, png_error_ptr error_fn, png_error_ptr warn_fn,
      png_voidp mem_ptr, png_malloc_ptr malloc_fn, png_free_ptr free_fn);

      void png_data_freer (png_structp png_ptr, png_infop info_ptr, int
      freer, png_uint_32 mask));

      void png_destroy_info_struct (png_structp png_ptr, png_infopp
      info_ptr_ptr);




                                    - 1 -            Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      void png_destroy_read_struct (png_structpp png_ptr_ptr, png_infopp
      info_ptr_ptr, png_infopp end_info_ptr_ptr);

      void png_destroy_write_struct (png_structpp png_ptr_ptr, png_infopp
      info_ptr_ptr);

      void png_err (png_structp png_ptr);

      void png_error (png_structp png_ptr, png_const_charp error);

      void png_free (png_structp png_ptr, png_voidp ptr);

      void png_free_chunk_list (png_structp png_ptr);

      void png_free_default (png_structp png_ptr, png_voidp ptr);

      void png_free_data (png_structp png_ptr, png_infop info_ptr, int num);

      png_byte png_get_bit_depth (png_const_structp png_ptr, png_const_infop
      info_ptr);

      png_uint_32 png_get_bKGD (png_const_structp png_ptr, png_infop
      info_ptr, png_color_16p *background);

      png_byte png_get_channels (png_const_structp png_ptr, png_const_infop
      info_ptr);

      png_uint_32 png_get_cHRM (png_const_structp png_ptr, png_const_infop
      info_ptr, double *white_x, double *white_y, double *red_x, double
      *red_y, double *green_x, double *green_y, double *blue_x, double
      *blue_y);

      png_uint_32 png_get_cHRM_fixed (png_const_structp png_ptr,
      png_const_infop info_ptr, png_uint_32 *white_x, png_uint_32 *white_y,
      png_uint_32 *red_x, png_uint_32 *red_y, png_uint_32 *green_x,
      png_uint_32 *green_y, png_uint_32 *blue_x, png_uint_32 *blue_y);

      png_uint_32 png_get_cHRM_XYZ (png_structp png_ptr, png_const_infop
      info_ptr, double *red_X, double *red_Y, double *red_Z, double *green_X,
      double *green_Y, double *green_Z, double *blue_X, double *blue_Y,
      double *blue_Z);

      png_uint_32 png_get_cHRM_XYZ_fixed (png_structp png_ptr,
      png_const_infop info_ptr, png_fixed_point *int_red_X, png_fixed_point
      *int_red_Y, png_fixed_point *int_red_Z, png_fixed_point *int_green_X,
      png_fixed_point *int_green_Y, png_fixed_point *int_green_Z,
      png_fixed_point *int_blue_X, png_fixed_point *int_blue_Y,
      png_fixed_point *int_blue_Z);

      png_uint_32 png_get_chunk_cache_max (png_const_structp png_ptr);




                                    - 2 -            Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      png_alloc_size_t png_get_chunk_malloc_max (png_const_structp png_ptr);

      png_byte png_get_color_type (png_const_structp png_ptr, png_const_infop
      info_ptr);

      png_uint_32 png_get_compression_buffer_size (png_const_structp
      png_ptr);

      png_byte png_get_compression_type (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_byte png_get_copyright (png_const_structp png_ptr);

      png_uint_32 png_get_current_row_number (png_const_structp);

      png_byte png_get_current_pass_number (png_const_structp);

      png_voidp png_get_error_ptr (png_const_structp png_ptr);

      png_byte png_get_filter_type (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_uint_32 png_get_gAMA (png_const_structp png_ptr, png_const_infop
      info_ptr, double *file_gamma);

      png_uint_32 png_get_gAMA_fixed (png_const_structp png_ptr,
      png_const_infop info_ptr, png_uint_32 *int_file_gamma);

      png_byte png_get_header_ver (png_const_structp png_ptr);

      png_byte png_get_header_version (png_const_structp png_ptr);

      png_uint_32 png_get_hIST (png_const_structp png_ptr, png_const_infop
      info_ptr, png_uint_16p *hist);

      png_uint_32 png_get_iCCP (png_const_structp png_ptr, png_const_infop
      info_ptr, png_charpp name, int *compression_type, png_bytepp profile,
      png_uint_32 *proflen);

      png_uint_32 png_get_IHDR (png_structp png_ptr, png_infop info_ptr,
      png_uint_32 *width, png_uint_32 *height, int *bit_depth, int
      *color_type, int *interlace_type, int *compression_type, int
      *filter_type);

      png_uint_32 png_get_image_height (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_uint_32 png_get_image_width (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_int_32 png_get_int_32 (png_bytep buf);



                                    - 3 -            Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      png_byte png_get_interlace_type (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_uint_32 png_get_io_chunk_type (png_const_structp png_ptr);

      png_voidp png_get_io_ptr (png_structp png_ptr);

      png_uint_32 png_get_io_state (png_structp png_ptr);

      png_byte png_get_libpng_ver (png_const_structp png_ptr);

      int png_get_palette_max(png_const_structp png_ptr, png_const_infop
      info_ptr);

      png_voidp png_get_mem_ptr (png_const_structp png_ptr);

      png_uint_32 png_get_oFFs (png_const_structp png_ptr, png_const_infop
      info_ptr, png_uint_32 *offset_x, png_uint_32 *offset_y, int
      *unit_type);

      png_uint_32 png_get_pCAL (png_const_structp png_ptr, png_const_infop
      info_ptr, png_charp *purpose, png_int_32 *X0, png_int_32 *X1, int
      *type, int *nparams, png_charp *units, png_charpp *params);

      png_uint_32 png_get_pHYs (png_const_structp png_ptr, png_const_infop
      info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int *unit_type);

      float png_get_pixel_aspect_ratio (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_uint_32 png_get_pHYs_dpi (png_const_structp png_ptr,
      png_const_infop info_ptr, png_uint_32 *res_x, png_uint_32 *res_y, int
      *unit_type);

      png_fixed_point png_get_pixel_aspect_ratio_fixed (png_const_structp
      png_ptr, png_const_infop info_ptr);

      png_uint_32 png_get_pixels_per_inch (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_uint_32 png_get_pixels_per_meter (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_voidp png_get_progressive_ptr (png_const_structp png_ptr);

      png_uint_32 png_get_PLTE (png_const_structp png_ptr, png_const_infop
      info_ptr, png_colorp *palette, int *num_palette);

      png_byte png_get_rgb_to_gray_status (png_const_structp png_ptr);

      png_uint_32 png_get_rowbytes (png_const_structp png_ptr,



                                    - 4 -            Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      png_const_infop info_ptr);

      png_bytepp png_get_rows (png_const_structp png_ptr, png_const_infop
      info_ptr);

      png_uint_32 png_get_sBIT (png_const_structp png_ptr, png_infop
      info_ptr, png_color_8p *sig_bit);

      void png_get_sCAL (png_const_structp png_ptr, png_const_infop info_ptr,
      int* unit, double* width, double* height);

      void png_get_sCAL_fixed (png_const_structp png_ptr, png_const_infop
      info_ptr, int* unit, png_fixed_pointp width, png_fixed_pointp height);

      void png_get_sCAL_s (png_const_structp png_ptr, png_const_infop
      info_ptr, int* unit, png_charpp width, png_charpp height);

      png_bytep png_get_signature (png_const_structp png_ptr, png_infop
      info_ptr);

      png_uint_32 png_get_sPLT (png_const_structp png_ptr, png_const_infop
      info_ptr, png_spalette_p *splt_ptr);

      png_uint_32 png_get_sRGB (png_const_structp png_ptr, png_const_infop
      info_ptr, int *file_srgb_intent);

      png_uint_32 png_get_text (png_const_structp png_ptr, png_const_infop
      info_ptr, png_textp *text_ptr, int *num_text);

      png_uint_32 png_get_tIME (png_const_structp png_ptr, png_infop
      info_ptr, png_timep *mod_time);

      png_uint_32 png_get_tRNS (png_const_structp png_ptr, png_infop
      info_ptr, png_bytep *trans_alpha, int *num_trans, png_color_16p
      *trans_color);

      /* This function is really an inline macro. */

      png_uint_16 png_get_uint_16 (png_bytep buf);

      png_uint_32 png_get_uint_31 (png_structp png_ptr, png_bytep buf);

      /* This function is really an inline macro. */

      png_uint_32 png_get_uint_32 (png_bytep buf);

      png_uint_32 png_get_unknown_chunks (png_const_structp png_ptr,
      png_const_infop info_ptr, png_unknown_chunkpp unknowns);

      png_voidp png_get_user_chunk_ptr (png_const_structp png_ptr);




                                    - 5 -            Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      png_uint_32 png_get_user_height_max (png_const_structp png_ptr);

      png_voidp png_get_user_transform_ptr (png_const_structp png_ptr);

      png_uint_32 png_get_user_width_max (png_const_structp png_ptr);

      png_uint_32 png_get_valid (png_const_structp png_ptr, png_const_infop
      info_ptr, png_uint_32 flag);

      float png_get_x_offset_inches (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_fixed_point png_get_x_offset_inches_fixed (png_structp png_ptr,
      png_const_infop info_ptr);

      png_int_32 png_get_x_offset_microns (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_int_32 png_get_x_offset_pixels (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_uint_32 png_get_x_pixels_per_inch (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_uint_32 png_get_x_pixels_per_meter (png_const_structp png_ptr,
      png_const_infop info_ptr);

      float png_get_y_offset_inches (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_fixed_point png_get_y_offset_inches_fixed (png_structp png_ptr,
      png_const_infop info_ptr);

      png_int_32 png_get_y_offset_microns (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_int_32 png_get_y_offset_pixels (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_uint_32 png_get_y_pixels_per_inch (png_const_structp png_ptr,
      png_const_infop info_ptr);

      png_uint_32 png_get_y_pixels_per_meter (png_const_structp png_ptr,
      png_const_infop info_ptr);

      int png_handle_as_unknown (png_structp png_ptr, png_bytep chunk_name);

      int png_image_begin_read_from_file (png_imagep image, const char
      *file_name);

      int png_image_begin_read_from_stdio (png_imagep image, FILE* file);



                                    - 6 -            Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      int, png_image_begin_read_from_memory (png_imagep image,
      png_const_voidp memory, png_size_t size);

      int png_image_finish_read (png_imagep image, png_colorp background,
      void *buffer, png_int_32 row_stride, void *colormap);

      void png_image_free (png_imagep image);

      int png_image_write_to_file (png_imagep image, const char *file, int
      convert_to_8bit, const void *buffer, png_int_32 row_stride, void
      *colormap);

      int png_image_write_to_memory (png_imagep image, void *memory,
      png_alloc_size_t * PNG_RESTRICT memory_bytes, int convert_to_8_bit,
      const void *buffer, png_int_32 row_stride, const void *colormap));

      int png_image_write_to_stdio (png_imagep image, FILE *file, int
      convert_to_8_bit, const void *buffer, png_int_32 row_stride, void
      *colormap));

      void png_info_init_3 (png_infopp info_ptr, png_size_t
      png_info_struct_size);

      void png_init_io (png_structp png_ptr, FILE *fp);

      void png_longjmp (png_structp png_ptr, int val);

      png_voidp png_malloc (png_structp png_ptr, png_alloc_size_t size);

      png_voidp png_malloc_default (png_structp png_ptr, png_alloc_size_t
      size);

      png_voidp png_malloc_warn (png_structp png_ptr, png_alloc_size_t size);

      png_uint_32 png_permit_mng_features (png_structp png_ptr, png_uint_32
      mng_features_permitted);

      void png_process_data (png_structp png_ptr, png_infop info_ptr,
      png_bytep buffer, png_size_t buffer_size);

      png_size_t png_process_data_pause (png_structp, int save);

      png_uint_32 png_process_data_skip (png_structp);

      void png_progressive_combine_row (png_structp png_ptr, png_bytep
      old_row, png_bytep new_row);

      void png_read_end (png_structp png_ptr, png_infop info_ptr);

      void png_read_image (png_structp png_ptr, png_bytepp image);




                                    - 7 -            Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      void png_read_info (png_structp png_ptr, png_infop info_ptr);

      void png_read_png (png_structp png_ptr, png_infop info_ptr, int
      transforms, png_voidp params);

      void png_read_row (png_structp png_ptr, png_bytep row, png_bytep
      display_row);

      void png_read_rows (png_structp png_ptr, png_bytepp row, png_bytepp
      display_row, png_uint_32 num_rows);

      void png_read_update_info (png_structp png_ptr, png_infop info_ptr);

      int png_reset_zstream (png_structp png_ptr);

      void png_save_int_32 (png_bytep buf, png_int_32 i);

      void png_save_uint_16 (png_bytep buf, unsigned int i);

      void png_save_uint_32 (png_bytep buf, png_uint_32 i);

      void png_set_add_alpha (png_structp png_ptr, png_uint_32 filler, int
      flags);

      void png_set_alpha_mode (png_structp png_ptr, int mode, double
      output_gamma);

      void png_set_alpha_mode_fixed (png_structp png_ptr, int mode,
      png_fixed_point output_gamma);

      void png_set_background (png_structp png_ptr, png_color_16p
      background_color, int background_gamma_code, int need_expand, double
      background_gamma);

      void png_set_background_fixed (png_structp png_ptr, png_color_16p
      background_color, int background_gamma_code, int need_expand,
      png_uint_32 background_gamma);

      void png_set_benign_errors (png_structp png_ptr, int allowed);

      void png_set_bgr (png_structp png_ptr);

      void png_set_bKGD (png_structp png_ptr, png_infop info_ptr,
      png_color_16p background);

      void png_set_check_for_invalid_index(png_structrp png_ptr, int
      allowed);

      void png_set_cHRM (png_structp png_ptr, png_infop info_ptr, double
      white_x, double white_y, double red_x, double red_y, double green_x,
      double green_y, double blue_x, double blue_y);



                                    - 8 -            Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      void png_set_cHRM_fixed (png_structp png_ptr, png_infop info_ptr,
      png_uint_32 white_x, png_uint_32 white_y, png_uint_32 red_x,
      png_uint_32 red_y, png_uint_32 green_x, png_uint_32 green_y,
      png_uint_32 blue_x, png_uint_32 blue_y);

      void png_set_cHRM_XYZ (png_structp png_ptr, png_infop info_ptr, double
      red_X, double red_Y, double red_Z, double green_X, double green_Y,
      double green_Z, double blue_X, double blue_Y, double blue_Z);

      void png_set_cHRM_XYZ_fixed (png_structp png_ptr, png_infop info_ptr,
      png_fixed_point int_red_X, png_fixed_point int_red_Y, png_fixed_point
      int_red_Z, png_fixed_point int_green_X, png_fixed_point int_green_Y,
      png_fixed_point int_green_Z, png_fixed_point int_blue_X,
      png_fixed_point int_blue_Y, png_fixed_point int_blue_Z);

      void png_set_chunk_cache_max (png_structp png_ptr, png_uint_32
      user_chunk_cache_max);

      void png_set_compression_level (png_structp png_ptr, int level);

      void png_set_compression_mem_level (png_structp png_ptr, int
      mem_level);

      void png_set_compression_method (png_structp png_ptr, int method);

      void png_set_compression_strategy (png_structp png_ptr, int strategy);

      void png_set_compression_window_bits (png_structp png_ptr, int
      window_bits);

      void png_set_crc_action (png_structp png_ptr, int crit_action, int
      ancil_action);

      void png_set_error_fn (png_structp png_ptr, png_voidp error_ptr,
      png_error_ptr error_fn, png_error_ptr warning_fn);

      void png_set_expand (png_structp png_ptr);

      void png_set_expand_16 (png_structp png_ptr);

      void png_set_expand_gray_1_2_4_to_8 (png_structp png_ptr);

      void png_set_filler (png_structp png_ptr, png_uint_32 filler, int
      flags);

      void png_set_filter (png_structp png_ptr, int method, int filters);

      void png_set_filter_heuristics (png_structp png_ptr, int
      heuristic_method, int num_weights, png_doublep filter_weights,
      png_doublep filter_costs);




                                    - 9 -            Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      void png_set_filter_heuristics_fixed (png_structp png_ptr, int
      heuristic_method, int num_weights, png_fixed_point_p filter_weights,
      png_fixed_point_p filter_costs);

      void png_set_flush (png_structp png_ptr, int nrows);

      void png_set_gamma (png_structp png_ptr, double screen_gamma, double
      default_file_gamma);

      void png_set_gamma_fixed (png_structp png_ptr, png_uint_32
      screen_gamma, png_uint_32 default_file_gamma);

      void png_set_gAMA (png_structp png_ptr, png_infop info_ptr, double
      file_gamma);

      void png_set_gAMA_fixed (png_structp png_ptr, png_infop info_ptr,
      png_uint_32 file_gamma);

      void png_set_gray_1_2_4_to_8 (png_structp png_ptr);

      void png_set_gray_to_rgb (png_structp png_ptr);

      void png_set_hIST (png_structp png_ptr, png_infop info_ptr,
      png_uint_16p hist);

      void png_set_iCCP (png_structp png_ptr, png_infop info_ptr,
      png_const_charp name, int compression_type, png_const_bytep profile,
      png_uint_32 proflen);

      int png_set_interlace_handling (png_structp png_ptr);

      void png_set_invalid (png_structp png_ptr, png_infop info_ptr, int
      mask);

      void png_set_invert_alpha (png_structp png_ptr);

      void png_set_invert_mono (png_structp png_ptr);

      void png_set_IHDR (png_structp png_ptr, png_infop info_ptr, png_uint_32
      width, png_uint_32 height, int bit_depth, int color_type, int
      interlace_type, int compression_type, int filter_type);

      void png_set_keep_unknown_chunks (png_structp png_ptr, int keep,
      png_bytep chunk_list, int num_chunks);

      jmp_buf* png_set_longjmp_fn (png_structp png_ptr, png_longjmp_ptr
      longjmp_fn, size_t jmp_buf_size);

      void png_set_chunk_malloc_max (png_structp png_ptr, png_alloc_size_t
      user_chunk_cache_max);




                                    - 10 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      void png_set_compression_buffer_size (png_structp png_ptr, png_uint_32
      size);

      void png_set_mem_fn (png_structp png_ptr, png_voidp mem_ptr,
      png_malloc_ptr malloc_fn, png_free_ptr free_fn);

      void png_set_oFFs (png_structp png_ptr, png_infop info_ptr, png_uint_32
      offset_x, png_uint_32 offset_y, int unit_type);

      int png_set_option(png_structrp png_ptr, int option, int onoff);

      void png_set_packing (png_structp png_ptr);

      void png_set_packswap (png_structp png_ptr);

      void png_set_palette_to_rgb (png_structp png_ptr);

      void png_set_pCAL (png_structp png_ptr, png_infop info_ptr, png_charp
      purpose, png_int_32 X0, png_int_32 X1, int type, int nparams, png_charp
      units, png_charpp params);

      void png_set_pHYs (png_structp png_ptr, png_infop info_ptr, png_uint_32
      res_x, png_uint_32 res_y, int unit_type);

      void png_set_progressive_read_fn (png_structp png_ptr, png_voidp
      progressive_ptr, png_progressive_info_ptr info_fn,
      png_progressive_row_ptr row_fn, png_progressive_end_ptr end_fn);

      void png_set_PLTE (png_structp png_ptr, png_infop info_ptr, png_colorp
      palette, int num_palette);

      void png_set_quantize (png_structp png_ptr, png_colorp palette, int
      num_palette, int maximum_colors, png_uint_16p histogram, int
      full_quantize);

      void png_set_read_fn (png_structp png_ptr, png_voidp io_ptr, png_rw_ptr
      read_data_fn);

      void png_set_read_status_fn (png_structp png_ptr, png_read_status_ptr
      read_row_fn);

      void png_set_read_user_chunk_fn (png_structp png_ptr, png_voidp
      user_chunk_ptr, png_user_chunk_ptr read_user_chunk_fn);

      void png_set_read_user_transform_fn (png_structp png_ptr,
      png_user_transform_ptr read_user_transform_fn);

      void png_set_rgb_to_gray (png_structp png_ptr, int error_action, double
      red, double green);

      void png_set_rgb_to_gray_fixed (png_structp png_ptr, int error_action



                                    - 11 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      png_uint_32 red, png_uint_32 green);

      void png_set_rows (png_structp png_ptr, png_infop info_ptr, png_bytepp
      row_pointers);

      void png_set_sBIT (png_structp png_ptr, png_infop info_ptr,
      png_color_8p sig_bit);

      void png_set_sCAL (png_structp png_ptr, png_infop info_ptr, int unit,
      double width, double height);

      void png_set_sCAL_fixed (png_structp png_ptr, png_infop info_ptr, int
      unit, png_fixed_point width, png_fixed_point height);

      void png_set_sCAL_s (png_structp png_ptr, png_infop info_ptr, int unit,
      png_charp width, png_charp height);

      void png_set_scale_16 (png_structp png_ptr);

      void png_set_shift (png_structp png_ptr, png_color_8p true_bits);

      void png_set_sig_bytes (png_structp png_ptr, int num_bytes);

      void png_set_sPLT (png_structp png_ptr, png_infop info_ptr,
      png_spalette_p splt_ptr, int num_spalettes);

      void png_set_sRGB (png_structp png_ptr, png_infop info_ptr, int
      srgb_intent);

      void png_set_sRGB_gAMA_and_cHRM (png_structp png_ptr, png_infop
      info_ptr, int srgb_intent);

      void png_set_strip_16 (png_structp png_ptr);

      void png_set_strip_alpha (png_structp png_ptr);

      void png_set_strip_error_numbers (png_structp png_ptr, png_uint_32
      strip_mode);

      void png_set_swap (png_structp png_ptr);

      void png_set_swap_alpha (png_structp png_ptr);

      void png_set_text (png_structp png_ptr, png_infop info_ptr, png_textp
      text_ptr, int num_text);

      void png_set_text_compression_level (png_structp png_ptr, int level);

      void png_set_text_compression_mem_level (png_structp png_ptr, int
      mem_level);




                                    - 12 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      void png_set_text_compression_strategy (png_structp png_ptr, int
      strategy);

      void png_set_text_compression_window_bits (png_structp png_ptr, int
      window_bits);

      void png_set_text_compression_method, (png_structp png_ptr, int
      method));

      void png_set_tIME (png_structp png_ptr, png_infop info_ptr, png_timep
      mod_time);

      void png_set_tRNS (png_structp png_ptr, png_infop info_ptr, png_bytep
      trans_alpha, int num_trans, png_color_16p trans_color);

      void png_set_tRNS_to_alpha (png_structp png_ptr);

      png_uint_32 png_set_unknown_chunks (png_structp png_ptr, png_infop
      info_ptr, png_unknown_chunkp unknowns, int num, int location);

      void png_set_unknown_chunk_location (png_structp png_ptr, png_infop
      info_ptr, int chunk, int location);

      void png_set_user_limits (png_structp png_ptr, png_uint_32
      user_width_max, png_uint_32 user_height_max);

      void png_set_user_transform_info (png_structp png_ptr, png_voidp
      user_transform_ptr, int user_transform_depth, int
      user_transform_channels);

      void png_set_write_fn (png_structp png_ptr, png_voidp io_ptr,
      png_rw_ptr write_data_fn, png_flush_ptr output_flush_fn);

      void png_set_write_status_fn (png_structp png_ptr, png_write_status_ptr
      write_row_fn);

      void png_set_write_user_transform_fn (png_structp png_ptr,
      png_user_transform_ptr write_user_transform_fn);

      int png_sig_cmp (png_bytep sig, png_size_t start, png_size_t
      num_to_check);

      void png_start_read_image (png_structp png_ptr);

      void png_warning (png_structp png_ptr, png_const_charp message);

      void png_write_chunk (png_structp png_ptr, png_bytep chunk_name,
      png_bytep data, png_size_t length);

      void png_write_chunk_data (png_structp png_ptr, png_bytep data,
      png_size_t length);



                                    - 13 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                          LIBPNG(3)
                                March 16, 2017



      void png_write_chunk_end (png_structp png_ptr);

      void png_write_chunk_start (png_structp png_ptr, png_bytep chunk_name,
      png_uint_32 length);

      void png_write_end (png_structp png_ptr, png_infop info_ptr);

      void png_write_flush (png_structp png_ptr);

      void png_write_image (png_structp png_ptr, png_bytepp image);

      void png_write_info (png_structp png_ptr, png_infop info_ptr);

      void png_write_info_before_PLTE (png_structp png_ptr, png_infop
      info_ptr);

      void png_write_png (png_structp png_ptr, png_infop info_ptr, int
      transforms, png_voidp params);

      void png_write_row (png_structp png_ptr, png_bytep row);

      void png_write_rows (png_structp png_ptr, png_bytepp row, png_uint_32
      num_rows);

      void png_write_sig (png_structp png_ptr);


 DESCRIPTION
      The libpng library supports encoding, decoding, and various
      manipulations of the Portable Network Graphics (PNG) format image
      files.  It uses the zlib(3) compression library.  Following is a copy
      of the libpng-manual.txt file that accompanies libpng.

 LIBPNG.TXT
      libpng-manual.txt - A description on how to use and modify libpng

       libpng version 1.6.29 - March 16, 2017
       Updated and distributed by Glenn Randers-Pehrson
       <glennrp at users.sourceforge.net>
       Copyright (c) 1998-2016 Glenn Randers-Pehrson

       This document is released under the libpng license.
       For conditions of distribution and use, see the disclaimer
       and license in png.h

       Based on:

       libpng versions 0.97, January 1998, through 1.6.29 - March 16, 2017
       Updated and distributed by Glenn Randers-Pehrson
       Copyright (c) 1998-2016 Glenn Randers-Pehrson




                                   - 14 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



       libpng 1.0 beta 6 - version 0.96 - May 28, 1997
       Updated and distributed by Andreas Dilger
       Copyright (c) 1996, 1997 Andreas Dilger

       libpng 1.0 beta 2 - version 0.88 - January 26, 1996
       For conditions of distribution and use, see copyright
       notice in png.h. Copyright (c) 1995, 1996 Guy Eric
       Schalnat, Group 42, Inc.

       Updated/rewritten per request in the libpng FAQ
       Copyright (c) 1995, 1996 Frank J. T. Wojcik
       December 18, 1995 & January 20, 1996

       TABLE OF CONTENTS

          I. Introduction
         II. Structures
        III. Reading
         IV. Writing
          V. Simplified API
         VI. Modifying/Customizing libpng
        VII. MNG support
       VIII. Changes to Libpng from version 0.88
         IX. Changes to Libpng from version 1.0.x to 1.2.x
          X. Changes to Libpng from version 1.0.x/1.2.x to 1.4.x
         XI. Changes to Libpng from version 1.4.x to 1.5.x
        XII. Changes to Libpng from version 1.5.x to 1.6.x
       XIII. Detecting libpng
        XIV. Source code repository
         XV. Coding style
        XVI. Y2K Compliance in libpng


 I. Introduction
      This file describes how to use and modify the PNG reference library
      (known as libpng) for your own use.  In addition to this file,
      example.c is a good starting point for using the library, as it is
      heavily commented and should include everything most people will need.
      We assume that libpng is already installed; see the INSTALL file for
      instructions on how to configure and install libpng.

      For examples of libpng usage, see the files "example.c", "pngtest.c",
      and the files in the "contrib" directory, all of which are included in
      the libpng distribution.

      Libpng was written as a companion to the PNG specification, as a way
      of reducing the amount of time and effort it takes to support the PNG
      file format in application programs.

      The PNG specification (second edition), November 2003, is available as
      a W3C Recommendation and as an ISO Standard (ISO/IEC 15948:2004 (E))



                                   - 15 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      at <http://www.w3.org/TR/2003/REC-PNG-20031110/ The W3C and ISO
      documents have identical technical content.

      The PNG-1.2 specification is available at <http://png-
      mng.sourceforge.net/pub/png/spec/1.2/>.  It is technically equivalent
      to the PNG specification (second edition) but has some additional
      material.

      The PNG-1.0 specification is available as RFC 2083 <http://png-
      mng.sourceforge.net/pub/png/spec/1.0/> and as a W3C Recommendation
      <http://www.w3.org/TR/REC-png-961001>.

      Some additional chunks are described in the special-purpose public
      chunks documents at <http://www.libpng.org/pub/png/spec/register/>

      Other information about PNG, and the latest version of libpng, can be
      found at the PNG home page, <http://www.libpng.org/pub/png/>.

      Most users will not have to modify the library significantly; advanced
      users may want to modify it more.  All attempts were made to make it
      as complete as possible, while keeping the code easy to understand.
      Currently, this library only supports C.  Support for other languages
      is being considered.

      Libpng has been designed to handle multiple sessions at one time, to
      be easily modifiable, to be portable to the vast majority of machines
      (ANSI, K&R, 16-, 32-, and 64-bit) available, and to be easy to use.
      The ultimate goal of libpng is to promote the acceptance of the PNG
      file format in whatever way possible.  While there is still work to be
      done (see the TODO file), libpng should cover the majority of the
      needs of its users.

      Libpng uses zlib for its compression and decompression of PNG files.
      Further information about zlib, and the latest version of zlib, can be
      found at the zlib home page, <http://zlib.net/>.  The zlib compression
      utility is a general purpose utility that is useful for more than PNG
      files, and can be used without libpng.  See the documentation
      delivered with zlib for more details.  You can usually find the source
      files for the zlib utility wherever you find the libpng source files.

      Libpng is thread safe, provided the threads are using different
      instances of the structures.  Each thread should have its own
      png_struct and png_info instances, and thus its own image.  Libpng
      does not protect itself against two threads using the same instance of
      a structure.


 II. Structures
      There are two main structures that are important to libpng, png_struct
      and png_info.  Both are internal structures that are no longer exposed
      in the libpng interface (as of libpng 1.5.0).



                                   - 16 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      The png_info structure is designed to provide information about the
      PNG file.  At one time, the fields of png_info were intended to be
      directly accessible to the user.  However, this tended to cause
      problems with applications using dynamically loaded libraries, and as
      a result a set of interface functions for png_info (the png_get_*()
      and png_set_*() functions) was developed, and direct access to the
      png_info fields was deprecated..

      The png_struct structure is the object used by the library to decode a
      single image.  As of 1.5.0 this structure is also not exposed.

      Almost all libpng APIs require a pointer to a png_struct as the first
      argument.  Many (in particular the png_set and png_get APIs) also
      require a pointer to png_info as the second argument.  Some
      application visible macros defined in png.h designed for basic data
      access (reading and writing integers in the PNG format) don't take a
      png_info pointer, but it's almost always safe to assume that a
      (png_struct*) has to be passed to call an API function.

      You can have more than one png_info structure associated with an
      image, as illustrated in pngtest.c, one for information valid prior to
      the IDAT chunks and another (called "end_info" below) for things after
      them.

      The png.h header file is an invaluable reference for programming with
      libpng.  And while I'm on the topic, make sure you include the libpng
      header file:

      #include <png.h>

      and also (as of libpng-1.5.0) the zlib header file, if you need it:

      #include <zlib.h>


    Types
      The png.h header file defines a number of integral types used by the
      APIs.  Most of these are fairly obvious; for example types
      corresponding to integers of particular sizes and types for passing
      color values.

      One exception is how non-integral numbers are handled.  For
      application convenience most APIs that take such numbers have C
      (double) arguments; however, internally PNG, and libpng, use 32 bit
      signed integers and encode the value by multiplying by 100,000.  As of
      libpng 1.5.0 a convenience macro PNG_FP_1 is defined in png.h along
      with a type (png_fixed_point) which is simply (png_int_32).

      All APIs that take (double) arguments also have a matching API that
      takes the corresponding fixed point integer arguments.  The fixed
      point API has the same name as the floating point one with "_fixed"



                                   - 17 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      appended.  The actual range of values permitted in the APIs is
      frequently less than the full range of (png_fixed_point) (-21474 to
      +21474).  When APIs require a non-negative argument the type is
      recorded as png_uint_32 above.  Consult the header file and the text
      below for more information.

      Special care must be take with sCAL chunk handling because the chunk
      itself uses non-integral values encoded as strings containing decimal
      floating point numbers.  See the comments in the header file.


    Configuration
      The main header file function declarations are frequently protected by
      C preprocessing directives of the form:

          #ifdef PNG_feature_SUPPORTED
          declare-function
          #endif
          ...
          #ifdef PNG_feature_SUPPORTED
          use-function
          #endif

      The library can be built without support for these APIs, although a
      standard build will have all implemented APIs.  Application programs
      should check the feature macros before using an API for maximum
      portability.  From libpng 1.5.0 the feature macros set during the
      build of libpng are recorded in the header file "pnglibconf.h" and
      this file is always included by png.h.

      If you don't need to change the library configuration from the
      default, skip to the next section ("Reading").

      Notice that some of the makefiles in the 'scripts' directory and (in
      1.5.0) all of the build project files in the 'projects' directory
      simply copy scripts/pnglibconf.h.prebuilt to pnglibconf.h.  This means
      that these build systems do not permit easy auto-configuration of the
      library - they only support the default configuration.

      The easiest way to make minor changes to the libpng configuration when
      auto-configuration is supported is to add definitions to the command
      line using (typically) CPPFLAGS.  For example:

      CPPFLAGS=-DPNG_NO_FLOATING_ARITHMETIC

      will change the internal libpng math implementation for gamma
      correction and other arithmetic calculations to fixed point, avoiding
      the need for fast floating point support.  The result can be seen in
      the generated pnglibconf.h - make sure it contains the changed feature
      macro setting.




                                   - 18 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      If you need to make more extensive configuration changes - more than
      one or two feature macro settings - you can either add
      -DPNG_USER_CONFIG to the build command line and put a list of feature
      macro settings in pngusr.h or you can set DFA_XTRA (a makefile
      variable) to a file containing the same information in the form of
      'option' settings.

      A. Changing pnglibconf.h

      A variety of methods exist to build libpng.  Not all of these support
      reconfiguration of pnglibconf.h.  To reconfigure pnglibconf.h it must
      either be rebuilt from scripts/pnglibconf.dfa using awk or it must be
      edited by hand.

      Hand editing is achieved by copying scripts/pnglibconf.h.prebuilt to
      pnglibconf.h and changing the lines defining the supported features,
      paying very close attention to the 'option' information in
      scripts/pnglibconf.dfa that describes those features and their
      requirements.  This is easy to get wrong.

      B. Configuration using DFA_XTRA

      Rebuilding from pnglibconf.dfa is easy if a functioning 'awk', or a
      later variant such as 'nawk' or 'gawk', is available.  The configure
      build will automatically find an appropriate awk and build
      pnglibconf.h.  The scripts/pnglibconf.mak file contains a set of make
      rules for doing the same thing if configure is not used, and many of
      the makefiles in the scripts directory use this approach.

      When rebuilding simply write a new file containing changed options and
      set DFA_XTRA to the name of this file.  This causes the build to
      append the new file to the end of scripts/pnglibconf.dfa.  The
      pngusr.dfa file should contain lines of the following forms:

      everything = off

      This turns all optional features off.  Include it at the start of
      pngusr.dfa to make it easier to build a minimal configuration.  You
      will need to turn at least some features on afterward to enable either
      reading or writing code, or both.

      option feature on option feature off

      Enable or disable a single feature.  This will automatically enable
      other features required by a feature that is turned on or disable
      other features that require a feature which is turned off.
      Conflicting settings will cause an error message to be emitted by awk.

      setting feature default value

      Changes the default value of setting 'feature' to 'value'.  There are



                                   - 19 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      a small number of settings listed at the top of pnglibconf.h, they are
      documented in the source code.  Most of these values have performance
      implications for the library but most of them have no visible effect
      on the API.  Some can also be overridden from the API.

      This method of building a customized pnglibconf.h is illustrated in
      contrib/pngminim/*.  See the "$(PNGCONF):" target in the makefile and
      pngusr.dfa in these directories.

      C. Configuration using PNG_USER_CONFIG

      If -DPNG_USER_CONFIG is added to the CPPFLAGS when pnglibconf.h is
      built, the file pngusr.h will automatically be included before the
      options in scripts/pnglibconf.dfa are processed.  Your pngusr.h file
      should contain only macro definitions turning features on or off or
      setting settings.

      Apart from the global setting "everything = off" all the options
      listed above can be set using macros in pngusr.h:

      #define PNG_feature_SUPPORTED

      is equivalent to:

      option feature on

      #define PNG_NO_feature

      is equivalent to:

      option feature off

      #define PNG_feature value

      is equivalent to:

      setting feature default value

      Notice that in both cases, pngusr.dfa and pngusr.h, the contents of
      the pngusr file you supply override the contents of
      scripts/pnglibconf.dfa

      If confusing or incomprehensible behavior results it is possible to
      examine the intermediate file pnglibconf.dfn to find the full set of
      dependency information for each setting and option.  Simply locate the
      feature in the file and read the C comments that precede it.

      This method is also illustrated in the contrib/pngminim/* makefiles
      and pngusr.h.





                                   - 20 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



 III. Reading
      We'll now walk you through the possible functions to call when reading
      in a PNG file sequentially, briefly explaining the syntax and purpose
      of each one.  See example.c and png.h for more detail.  While
      progressive reading is covered in the next section, you will still
      need some of the functions discussed in this section to read a PNG
      file.


    Setup
      You will want to do the I/O initialization(*) before you get into
      libpng, so if it doesn't work, you don't have much to undo.  Of
      course, you will also want to insure that you are, in fact, dealing
      with a PNG file.  Libpng provides a simple check to see if a file is a
      PNG file.  To use it, pass in the first 1 to 8 bytes of the file to
      the function png_sig_cmp(), and it will return 0 (false) if the bytes
      match the corresponding bytes of the PNG signature, or nonzero (true)
      otherwise.  Of course, the more bytes you pass in, the greater the
      accuracy of the prediction.

      If you are intending to keep the file pointer open for use in libpng,
      you must ensure you don't read more than 8 bytes from the beginning of
      the file, and you also have to make a call to png_set_sig_bytes() with
      the number of bytes you read from the beginning.  Libpng will then
      only check the bytes (if any) that your program didn't read.

      (*): If you are not using the standard I/O functions, you will need to
      replace them with custom functions.  See the discussion under
      Customizing libpng.

          FILE *fp = fopen(file_name, "rb");
          if (!fp)
          {
             return (ERROR);
          }

          if (fread(header, 1, number, fp) != number)
          {
             return (ERROR);
          }

          is_png = !png_sig_cmp(header, 0, number);
          if (!is_png)
          {
             return (NOT_PNG);
          }

      Next, png_struct and png_info need to be allocated and initialized.
      In order to ensure that the size of these structures is correct even
      with a dynamically linked libpng, there are functions to initialize
      and allocate the structures.  We also pass the library version,



                                   - 21 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      optional pointers to error handling functions, and a pointer to a data
      struct for use by the error functions, if necessary (the pointer and
      functions can be NULL if the default error handlers are to be used).
      See the section on Changes to Libpng below regarding the old
      initialization functions.  The structure allocation functions quietly
      return NULL if they fail to create the structure, so your application
      should check for that.

          png_structp png_ptr = png_create_read_struct
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
              user_error_fn, user_warning_fn);

          if (!png_ptr)
             return (ERROR);

          png_infop info_ptr = png_create_info_struct(png_ptr);

          if (!info_ptr)
          {
             png_destroy_read_struct(&png_ptr,
                 (png_infopp)NULL, (png_infopp)NULL);
             return (ERROR);
          }

      If you want to use your own memory allocation routines, use a libpng
      that was built with PNG_USER_MEM_SUPPORTED defined, and use
      png_create_read_struct_2() instead of png_create_read_struct():

          png_structp png_ptr = png_create_read_struct_2
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
              user_error_fn, user_warning_fn, (png_voidp)
              user_mem_ptr, user_malloc_fn, user_free_fn);

      The error handling routines passed to png_create_read_struct() and the
      memory alloc/free routines passed to png_create_struct_2() are only
      necessary if you are not using the libpng supplied error handling and
      memory alloc/free functions.

      When libpng encounters an error, it expects to longjmp back to your
      routine.  Therefore, you will need to call setjmp and pass your
      png_jmpbuf(png_ptr).  If you read the file from different routines,
      you will need to update the longjmp buffer every time you enter a new
      routine that will call a png_*() function.

      See your documentation of setjmp/longjmp for your compiler for more
      information on setjmp/longjmp.  See the discussion on libpng error
      handling in the Customizing Libpng section below for more information
      on the libpng error handling.  If an error occurs, and libpng
      longjmp's back to your setjmp, you will want to call
      png_destroy_read_struct() to free any memory.




                                   - 22 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          if (setjmp(png_jmpbuf(png_ptr)))
          {
             png_destroy_read_struct(&png_ptr, &info_ptr,
                 &end_info);
             fclose(fp);
             return (ERROR);
          }

      Pass (png_infopp)NULL instead of &end_info if you didn't create an
      end_info structure.

      If you would rather avoid the complexity of setjmp/longjmp issues, you
      can compile libpng with PNG_NO_SETJMP, in which case errors will
      result in a call to PNG_ABORT() which defaults to abort().

      You can #define PNG_ABORT() to a function that does something more
      useful than abort(), as long as your function does not return.

      Now you need to set up the input code.  The default for libpng is to
      use the C function fread().  If you use this, you will need to pass a
      valid FILE * in the function png_init_io().  Be sure that the file is
      opened in binary mode.  If you wish to handle reading data in another
      way, you need not call the png_init_io() function, but you must then
      implement the libpng I/O methods discussed in the Customizing Libpng
      section below.

          png_init_io(png_ptr, fp);

      If you had previously opened the file and read any of the signature
      from the beginning in order to see if this was a PNG file, you need to
      let libpng know that there are some bytes missing from the start of
      the file.

          png_set_sig_bytes(png_ptr, number);

      You can change the zlib compression buffer size to be used while
      reading compressed data with

          png_set_compression_buffer_size(png_ptr, buffer_size);

      where the default size is 8192 bytes.  Note that the buffer size is
      changed immediately and the buffer is reallocated immediately, instead
      of setting a flag to be acted upon later.

      If you want CRC errors to be handled in a different manner than the
      default, use

          png_set_crc_action(png_ptr, crit_action, ancil_action);

      The values for png_set_crc_action() say how libpng is to handle CRC
      errors in ancillary and critical chunks, and whether to use the data



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      contained therein. Starting with libpng-1.6.26, this also governs how
      an ADLER32 error is handled while reading the IDAT chunk. Note that it
      is impossible to "discard" data in a critical chunk.

      Choices for (int) crit_action are
         PNG_CRC_DEFAULT      0  error/quit
         PNG_CRC_ERROR_QUIT   1  error/quit
         PNG_CRC_WARN_USE     3  warn/use data
         PNG_CRC_QUIET_USE    4  quiet/use data
         PNG_CRC_NO_CHANGE    5  use the current value

      Choices for (int) ancil_action are
         PNG_CRC_DEFAULT      0  error/quit
         PNG_CRC_ERROR_QUIT   1  error/quit
         PNG_CRC_WARN_DISCARD 2  warn/discard data
         PNG_CRC_WARN_USE     3  warn/use data
         PNG_CRC_QUIET_USE    4  quiet/use data
         PNG_CRC_NO_CHANGE    5  use the current value

      When the setting for crit_action is PNG_CRC_QUIET_USE, the CRC and
      ADLER32 checksums are not only ignored, but they are not evaluated.


    Setting up callback code
      You can set up a callback function to handle any unknown chunks in the
      input stream. You must supply the function

          read_chunk_callback(png_structp png_ptr,
               png_unknown_chunkp chunk);
          {
             /* The unknown chunk structure contains your
                chunk data, along with similar data for any other
                unknown chunks: */

                 png_byte name[5];
                 png_byte *data;
                 png_size_t size;

             /* Note that libpng has already taken care of
                the CRC handling */

             /* put your code here.  Search for your chunk in the
                unknown chunk structure, process it, and return one
                of the following: */

             return (-n); /* chunk had an error */
             return (0); /* did not recognize */
             return (n); /* success */
          }

      (You can give your function another name that you like instead of



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      "read_chunk_callback")

      To inform libpng about your function, use

          png_set_read_user_chunk_fn(png_ptr, user_chunk_ptr,
              read_chunk_callback);

      This names not only the callback function, but also a user pointer
      that you can retrieve with

          png_get_user_chunk_ptr(png_ptr);

      If you call the png_set_read_user_chunk_fn() function, then all
      unknown chunks which the callback does not handle will be saved when
      read.  You can cause them to be discarded by returning '1' ("handled")
      instead of '0'.  This behavior will change in libpng 1.7 and the
      default handling set by the png_set_keep_unknown_chunks() function,
      described below, will be used when the callback returns 0.  If you
      want the existing behavior you should set the global default to
      PNG_HANDLE_CHUNK_IF_SAFE now; this is compatible with all current
      versions of libpng and with 1.7.  Libpng 1.6 issues a warning if you
      keep the default, or PNG_HANDLE_CHUNK_NEVER, and the callback returns
      0.

      At this point, you can set up a callback function that will be called
      after each row has been read, which you can use to control a progress
      meter or the like.  It's demonstrated in pngtest.c.  You must supply a
      function

          void read_row_callback(png_structp png_ptr,
             png_uint_32 row, int pass);
          {
            /* put your code here */
          }

      (You can give it another name that you like instead of
      "read_row_callback")

      To inform libpng about your function, use

          png_set_read_status_fn(png_ptr, read_row_callback);

      When this function is called the row has already been completely
      processed and the 'row' and 'pass' refer to the next row to be
      handled.  For the non-interlaced case the row that was just handled is
      simply one less than the passed in row number, and pass will always be
      0.  For the interlaced case the same applies unless the row value is
      0, in which case the row just handled was the last one from one of the
      preceding passes.  Because interlacing may skip a pass you cannot be
      sure that the preceding pass is just 'pass-1'; if you really need to
      know what the last pass is record (row,pass) from the callback and use



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      the last recorded value each time.

      As with the user transform you can find the output row using the
      PNG_ROW_FROM_PASS_ROW macro.


    Unknown-chunk handling
      Now you get to set the way the library processes unknown chunks in the
      input PNG stream. Both known and unknown chunks will be read.  Normal
      behavior is that known chunks will be parsed into information in
      various info_ptr members while unknown chunks will be discarded. This
      behavior can be wasteful if your application will never use some known
      chunk types. To change this, you can call:

          png_set_keep_unknown_chunks(png_ptr, keep,
              chunk_list, num_chunks);

          keep       - 0: default unknown chunk handling
                       1: ignore; do not keep
                       2: keep only if safe-to-copy
                       3: keep even if unsafe-to-copy

                     You can use these definitions:
                       PNG_HANDLE_CHUNK_AS_DEFAULT   0
                       PNG_HANDLE_CHUNK_NEVER        1
                       PNG_HANDLE_CHUNK_IF_SAFE      2
                       PNG_HANDLE_CHUNK_ALWAYS       3

          chunk_list - list of chunks affected (a byte string,
                       five bytes per chunk, NULL or ' ' if
                       num_chunks is positive; ignored if
                       numchunks <= 0).

          num_chunks - number of chunks affected; if 0, all
                       unknown chunks are affected.  If positive,
                       only the chunks in the list are affected,
                       and if negative all unknown chunks and
                       all known chunks except for the IHDR,
                       PLTE, tRNS, IDAT, and IEND chunks are
                       affected.

      Unknown chunks declared in this way will be saved as raw data onto a
      list of png_unknown_chunk structures.  If a chunk that is normally
      known to libpng is named in the list, it will be handled as unknown,
      according to the "keep" directive.  If a chunk is named in successive
      instances of png_set_keep_unknown_chunks(), the final instance will
      take precedence.  The IHDR and IEND chunks should not be named in
      chunk_list; if they are, libpng will process them normally anyway.  If
      you know that your application will never make use of some particular
      chunks, use PNG_HANDLE_CHUNK_NEVER (or 1) as demonstrated below.




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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      Here is an example of the usage of png_set_keep_unknown_chunks(),
      where the private "vpAg" chunk will later be processed by a user chunk
      callback function:

          png_byte vpAg[5]={118, 112,  65, 103, (png_byte) ' '};

          #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
            png_byte unused_chunks[]=
            {
              104,  73,  83,  84, (png_byte) ' ',   /* hIST */
              105,  84,  88, 116, (png_byte) ' ',   /* iTXt */
              112,  67,  65,  76, (png_byte) ' ',   /* pCAL */
              115,  67,  65,  76, (png_byte) ' ',   /* sCAL */
              115,  80,  76,  84, (png_byte) ' ',   /* sPLT */
              116,  73,  77,  69, (png_byte) ' ',   /* tIME */
            };
          #endif

          ...

          #if defined(PNG_UNKNOWN_CHUNKS_SUPPORTED)
            /* ignore all unknown chunks
             * (use global setting "2" for libpng16 and earlier):
             */
            png_set_keep_unknown_chunks(read_ptr, 2, NULL, 0);

            /* except for vpAg: */
            png_set_keep_unknown_chunks(read_ptr, 2, vpAg, 1);

            /* also ignore unused known chunks: */
            png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
               (int)(sizeof unused_chunks)/5);
          #endif


    User limits
      The PNG specification allows the width and height of an image to be as
      large as 2^(31-1 (0x7fffffff), or about 2.147 billion rows and
      columns.  For safety, libpng imposes a default limit of 1 million rows
      and columns.  Larger images will be rejected immediately with a
      png_error() call. If you wish to change these limits, you can use

         png_set_user_limits(png_ptr, width_max, height_max);

      to set your own limits (libpng may reject some very wide images anyway
      because of potential buffer overflow conditions).

      You should put this statement after you create the PNG structure and
      before calling png_read_info(), png_read_png(), or png_process_data().

      When writing a PNG datastream, put this statement before calling



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      png_write_info() or png_write_png().

      If you need to retrieve the limits that are being applied, use

         width_max = png_get_user_width_max(png_ptr);
         height_max = png_get_user_height_max(png_ptr);

      The PNG specification sets no limit on the number of ancillary chunks
      allowed in a PNG datastream.  By default, libpng imposes a limit of a
      total of 1000 sPLT, tEXt, iTXt, zTXt, and unknown chunks to be stored.
      If you have set up both info_ptr and end_info_ptr, the limit applies
      separately to each.  You can change the limit on the total number of
      such chunks that will be stored, with

         png_set_chunk_cache_max(png_ptr, user_chunk_cache_max);

      where 0x7fffffffL means unlimited.  You can retrieve this limit with

         chunk_cache_max = png_get_chunk_cache_max(png_ptr);

      Libpng imposes a limit of 8 Megabytes (8,000,000 bytes) on the amount
      of memory that a compressed chunk other than IDAT can occupy, when
      decompressed.  You can change this limit with

         png_set_chunk_malloc_max(png_ptr, user_chunk_malloc_max);

      and you can retrieve the limit with

         chunk_malloc_max = png_get_chunk_malloc_max(png_ptr);

      Any chunks that would cause either of these limits to be exceeded will
      be ignored.


    Information about your system
      If you intend to display the PNG or to incorporate it in other image
      data you need to tell libpng information about your display or drawing
      surface so that libpng can convert the values in the image to match
      the display.

      From libpng-1.5.4 this information can be set before reading the PNG
      file header.  In earlier versions png_set_gamma() existed but behaved
      incorrectly if called before the PNG file header had been read and
      png_set_alpha_mode() did not exist.

      If you need to support versions prior to libpng-1.5.4 test the version
      number as illustrated below using "PNG_LIBPNG_VER >= 10504" and follow
      the procedures described in the appropriate manual page.

      You give libpng the encoding expected by your system expressed as a
      'gamma' value.  You can also specify a default encoding for the PNG



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      file in case the required information is missing from the file.  By
      default libpng assumes that the PNG data matches your system, to keep
      this default call:

         png_set_gamma(png_ptr, screen_gamma, output_gamma);

      or you can use the fixed point equivalent:

         png_set_gamma_fixed(png_ptr, PNG_FP_1*screen_gamma,
            PNG_FP_1*output_gamma);

      If you don't know the gamma for your system it is probably 2.2 - a
      good approximation to the IEC standard for display systems (sRGB).  If
      images are too contrasty or washed out you got the value wrong - check
      your system documentation!

      Many systems permit the system gamma to be changed via a lookup table
      in the display driver, a few systems, including older Macs, change the
      response by default.  As of 1.5.4 three special values are available
      to handle common situations:

         PNG_DEFAULT_sRGB: Indicates that the system conforms to the
                           IEC 61966-2-1 standard.  This matches almost
                           all systems.
         PNG_GAMMA_MAC_18: Indicates that the system is an older
                           (pre Mac OS 10.6) Apple Macintosh system with
                           the default settings.
         PNG_GAMMA_LINEAR: Just the fixed point value for 1.0 - indicates
                           that the system expects data with no gamma
                           encoding.

      You would use the linear (unencoded) value if you need to process the
      pixel values further because this avoids the need to decode and re-
      encode each component value whenever arithmetic is performed.  A lot
      of graphics software uses linear values for this reason, often with
      higher precision component values to preserve overall accuracy.


      The output_gamma value expresses how to decode the output values, not
      how they are encoded.  The values used correspond to the normal
      numbers used to describe the overall gamma of a computer display
      system; for example 2.2 for an sRGB conformant system.  The values are
      scaled by 100000 in the _fixed version of the API (so 220000 for
      sRGB.)

      The inverse of the value is always used to provide a default for the
      PNG file encoding if it has no gAMA chunk and if png_set_gamma() has
      not been called to override the PNG gamma information.

      When the ALPHA_OPTIMIZED mode is selected the output gamma is used to
      encode opaque pixels however pixels with lower alpha values are not



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      encoded, regardless of the output gamma setting.

      When the standard Porter Duff handling is requested with mode 1 the
      output encoding is set to be linear and the output_gamma value is only
      relevant as a default for input data that has no gamma information.
      The linear output encoding will be overridden if png_set_gamma() is
      called - the results may be highly unexpected!

      The following numbers are derived from the sRGB standard and the
      research behind it.  sRGB is defined to be approximated by a PNG gAMA
      chunk value of 0.45455 (1/2.2) for PNG.  The value implicitly includes
      any viewing correction required to take account of any differences in
      the color environment of the original scene and the intended display
      environment; the value expresses how to *decode* the image for
      display, not how the original data was *encoded*.

      sRGB provides a peg for the PNG standard by defining a viewing
      environment.  sRGB itself, and earlier TV standards, actually use a
      more complex transform (a linear portion then a gamma 2.4 power law)
      than PNG can express.  (PNG is limited to simple power laws.)  By
      saying that an image for direct display on an sRGB conformant system
      should be stored with a gAMA chunk value of 45455 (11.3.3.2 and
      11.3.3.5 of the ISO PNG specification) the PNG specification makes it
      possible to derive values for other display systems and environments.

      The Mac value is deduced from the sRGB based on an assumption that the
      actual extra viewing correction used in early Mac display systems was
      implemented as a power 1.45 lookup table.

      Any system where a programmable lookup table is used or where the
      behavior of the final display device characteristics can be changed
      requires system specific code to obtain the current characteristic.
      However this can be difficult and most PNG gamma correction only
      requires an approximate value.

      By default, if png_set_alpha_mode() is not called, libpng assumes that
      all values are unencoded, linear, values and that the output device
      also has a linear characteristic.  This is only very rarely correct -
      it is invariably better to call png_set_alpha_mode() with
      PNG_DEFAULT_sRGB than rely on the default if you don't know what the
      right answer is!

      The special value PNG_GAMMA_MAC_18 indicates an older Mac system (pre
      Mac OS 10.6) which used a correction table to implement a somewhat
      lower gamma on an otherwise sRGB system.

      Both these values are reserved (not simple gamma values) in order to
      allow more precise correction internally in the future.

      NOTE: the values can be passed to either the fixed or floating point
      APIs, but the floating point API will also accept floating point



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      values.

      The second thing you may need to tell libpng about is how your system
      handles alpha channel information.  Some, but not all, PNG files
      contain an alpha channel.  To display these files correctly you need
      to compose the data onto a suitable background, as described in the
      PNG specification.

      Libpng only supports composing onto a single color (using
      png_set_background; see below).  Otherwise you must do the composition
      yourself and, in this case, you may need to call png_set_alpha_mode:

         #if PNG_LIBPNG_VER >= 10504
            png_set_alpha_mode(png_ptr, mode, screen_gamma);
         #else
            png_set_gamma(png_ptr, screen_gamma, 1.0/screen_gamma);
         #endif

      The screen_gamma value is the same as the argument to png_set_gamma;
      however, how it affects the output depends on the mode.
      png_set_alpha_mode() sets the file gamma default to 1/screen_gamma, so
      normally you don't need to call png_set_gamma.  If you need different
      defaults call png_set_gamma() before png_set_alpha_mode() - if you
      call it after it will override the settings made by
      png_set_alpha_mode().

      The mode is as follows:

          PNG_ALPHA_PNG: The data is encoded according to the PNG
      specification.  Red, green and blue, or gray, components are gamma
      encoded color values and are not premultiplied by the alpha value.
      The alpha value is a linear measure of the contribution of the pixel
      to the corresponding final output pixel.

      You should normally use this format if you intend to perform color
      correction on the color values; most, maybe all, color correction
      software has no handling for the alpha channel and, anyway, the math
      to handle pre-multiplied component values is unnecessarily complex.

      Before you do any arithmetic on the component values you need to
      remove the gamma encoding and multiply out the alpha channel.  See the
      PNG specification for more detail.  It is important to note that when
      an image with an alpha channel is scaled, linear encoded, pre-
      multiplied component values must be used!

      The remaining modes assume you don't need to do any further color
      correction or that if you do, your color correction software knows all
      about alpha (it probably doesn't!).  They 'associate' the alpha with
      the color information by storing color channel values that have been
      scaled by the alpha.  The advantage is that the color channels can be
      resampled (the image can be scaled) in this form.  The disadvantage is



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      that normal practice is to store linear, not (gamma) encoded, values
      and this requires 16-bit channels for still images rather than the 8-
      bit channels that are just about sufficient if gamma encoding is used.
      In addition all non-transparent pixel values, including completely
      opaque ones, must be gamma encoded to produce the final image.  These
      are the 'STANDARD', 'ASSOCIATED' or 'PREMULTIPLIED' modes described
      below (the latter being the two common names for associated alpha
      color channels). Note that PNG files always contain non-associated
      color channels; png_set_alpha_mode() with one of the modes causes the
      decoder to convert the pixels to an associated form before returning
      them to your application.

      Since it is not necessary to perform arithmetic on opaque color values
      so long as they are not to be resampled and are in the final color
      space it is possible to optimize the handling of alpha by storing the
      opaque pixels in the PNG format (adjusted for the output color space)
      while storing partially opaque pixels in the standard, linear, format.
      The accuracy required for standard alpha composition is relatively
      low, because the pixels are isolated, therefore typically the accuracy
      loss in storing 8-bit linear values is acceptable.  (This is not true
      if the alpha channel is used to simulate transparency over large areas
      - use 16 bits or the PNG mode in this case!)  This is the 'OPTIMIZED'
      mode.  For this mode a pixel is treated as opaque only if the alpha
      value is equal to the maximum value.

          PNG_ALPHA_STANDARD:  The data libpng produces is encoded in the
      standard way assumed by most correctly written graphics software.  The
      gamma encoding will be removed by libpng and the linear component
      values will be pre-multiplied by the alpha channel.

      With this format the final image must be re-encoded to match the
      display gamma before the image is displayed.  If your system doesn't
      do that, yet still seems to perform arithmetic on the pixels without
      decoding them, it is broken - check out the modes below.

      With PNG_ALPHA_STANDARD libpng always produces linear component
      values, whatever screen_gamma you supply.  The screen_gamma value is,
      however, used as a default for the file gamma if the PNG file has no
      gamma information.

      If you call png_set_gamma() after png_set_alpha_mode() you will
      override the linear encoding.  Instead the pre-multiplied pixel values
      will be gamma encoded but the alpha channel will still be linear.
      This may actually match the requirements of some broken software, but
      it is unlikely.

      While linear 8-bit data is often used it has insufficient precision
      for any image with a reasonable dynamic range.  To avoid problems, and
      if your software supports it, use png_set_expand_16() to force all
      components to 16 bits.




                                   - 32 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          PNG_ALPHA_OPTIMIZED: This mode is the same as PNG_ALPHA_STANDARD
      except that completely opaque pixels are gamma encoded according to
      the screen_gamma value.  Pixels with alpha less than 1.0 will still
      have linear components.

      Use this format if you have control over your compositing software and
      so don't do other arithmetic (such as scaling) on the data you get
      from libpng.  Your compositing software can simply copy opaque pixels
      to the output but still has linear values for the non-opaque pixels.

      In normal compositing, where the alpha channel encodes partial pixel
      coverage (as opposed to broad area translucency), the inaccuracies of
      the 8-bit representation of non-opaque pixels are irrelevant.

      You can also try this format if your software is broken; it might look
      better.

          PNG_ALPHA_BROKEN: This is PNG_ALPHA_STANDARD; however, all
      component values, including the alpha channel are gamma encoded.  This
      is broken because, in practice, no implementation that uses this
      choice correctly undoes the encoding before handling alpha
      composition.  Use this choice only if other serious errors in the
      software or hardware you use mandate it.  In most cases of broken
      software or hardware the bug in the final display manifests as a
      subtle halo around composited parts of the image.  You may not even
      perceive this as a halo; the composited part of the image may simply
      appear separate from the background, as though it had been cut out of
      paper and pasted on afterward.

      If you don't have to deal with bugs in software or hardware, or if you
      can fix them, there are three recommended ways of using
      png_set_alpha_mode():

         png_set_alpha_mode(png_ptr, PNG_ALPHA_PNG,
             screen_gamma);

      You can do color correction on the result (libpng does not currently
      support color correction internally).  When you handle the alpha
      channel you need to undo the gamma encoding and multiply out the
      alpha.

         png_set_alpha_mode(png_ptr, PNG_ALPHA_STANDARD,
             screen_gamma);
         png_set_expand_16(png_ptr);

      If you are using the high level interface, don't call
      png_set_expand_16(); instead pass PNG_TRANSFORM_EXPAND_16 to the
      interface.

      With this mode you can't do color correction, but you can do
      arithmetic, including composition and scaling, on the data without



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      further processing.

         png_set_alpha_mode(png_ptr, PNG_ALPHA_OPTIMIZED,
             screen_gamma);

      You can avoid the expansion to 16-bit components with this mode, but
      you lose the ability to scale the image or perform other linear
      arithmetic.  All you can do is compose the result onto a matching
      output.  Since this mode is libpng-specific you also need to write
      your own composition software.

      The following are examples of calls to png_set_alpha_mode to achieve
      the required overall gamma correction and, where necessary, alpha
      premultiplication.

          png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);

      This is the default libpng handling of the alpha channel - it is not
      pre-multiplied into the color components.  In addition the call states
      that the output is for a sRGB system and causes all PNG files without
      gAMA chunks to be assumed to be encoded using sRGB.

          png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);

      In this case the output is assumed to be something like an sRGB
      conformant display preceeded by a power-law lookup table of power
      1.45.  This is how early Mac systems behaved.

          png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_GAMMA_LINEAR);

      This is the classic Jim Blinn approach and will work in academic
      environments where everything is done by the book.  It has the
      shortcoming of assuming that input PNG data with no gamma information
      is linear - this is unlikely to be correct unless the PNG files where
      generated locally.  Most of the time the output precision will be so
      low as to show significant banding in dark areas of the image.

          png_set_expand_16(pp);
          png_set_alpha_mode(pp, PNG_ALPHA_STANDARD, PNG_DEFAULT_sRGB);

      This is a somewhat more realistic Jim Blinn inspired approach.  PNG
      files are assumed to have the sRGB encoding if not marked with a gamma
      value and the output is always 16 bits per component.  This permits
      accurate scaling and processing of the data.  If you know that your
      input PNG files were generated locally you might need to replace
      PNG_DEFAULT_sRGB with the correct value for your system.

          png_set_alpha_mode(pp, PNG_ALPHA_OPTIMIZED, PNG_DEFAULT_sRGB);

      If you just need to composite the PNG image onto an existing
      background and if you control the code that does this you can use the



                                   - 34 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      optimization setting.  In this case you just copy completely opaque
      pixels to the output.  For pixels that are not completely transparent
      (you just skip those) you do the composition math using png_composite
      or png_composite_16 below then encode the resultant 8-bit or 16-bit
      values to match the output encoding.

          Other cases

      If neither the PNG nor the standard linear encoding work for you
      because of the software or hardware you use then you have a big
      problem.  The PNG case will probably result in halos around the image.
      The linear encoding will probably result in a washed out, too bright,
      image (it's actually too contrasty.)  Try the ALPHA_OPTIMIZED mode
      above - this will probably substantially reduce the halos.
      Alternatively try:

          png_set_alpha_mode(pp, PNG_ALPHA_BROKEN, PNG_DEFAULT_sRGB);

      This option will also reduce the halos, but there will be slight dark
      halos round the opaque parts of the image where the background is
      light.  In the OPTIMIZED mode the halos will be light halos where the
      background is dark.  Take your pick - the halos are unavoidable unless
      you can get your hardware/software fixed!  (The OPTIMIZED approach is
      slightly faster.)

      When the default gamma of PNG files doesn't match the output gamma.
      If you have PNG files with no gamma information png_set_alpha_mode
      allows you to provide a default gamma, but it also sets the ouput
      gamma to the matching value.  If you know your PNG files have a gamma
      that doesn't match the output you can take advantage of the fact that
      png_set_alpha_mode always sets the output gamma but only sets the PNG
      default if it is not already set:

          png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_DEFAULT_sRGB);
          png_set_alpha_mode(pp, PNG_ALPHA_PNG, PNG_GAMMA_MAC);

      The first call sets both the default and the output gamma values, the
      second call overrides the output gamma without changing the default.
      This is easier than achieving the same effect with png_set_gamma.  You
      must use PNG_ALPHA_PNG for the first call - internal checking in
      png_set_alpha will fire if more than one call to png_set_alpha_mode
      and png_set_background is made in the same read operation, however
      multiple calls with PNG_ALPHA_PNG are ignored.

      If you don't need, or can't handle, the alpha channel you can call
      png_set_background() to remove it by compositing against a fixed
      color.  Don't call png_set_strip_alpha() to do this - it will leave
      spurious pixel values in transparent parts of this image.

         png_set_background(png_ptr, &background_color,
             PNG_BACKGROUND_GAMMA_SCREEN, 0, 1);



                                   - 35 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      The background_color is an RGB or grayscale value according to the
      data format libpng will produce for you.  Because you don't yet know
      the format of the PNG file, if you call png_set_background at this
      point you must arrange for the format produced by libpng to always
      have 8-bit or 16-bit components and then store the color as an 8-bit
      or 16-bit color as appropriate.  The color contains separate gray and
      RGB component values, so you can let libpng produce gray or RGB output
      according to the input format, but low bit depth grayscale images must
      always be converted to at least 8-bit format.  (Even though low bit
      depth grayscale images can't have an alpha channel they can have a
      transparent color!)

      You set the transforms you need later, either as flags to the high
      level interface or libpng API calls for the low level interface.  For
      reference the settings and API calls required are:

      8-bit values:
         PNG_TRANSFORM_SCALE_16 | PNG_EXPAND
         png_set_expand(png_ptr); png_set_scale_16(png_ptr);

         If you must get exactly the same inaccurate results
         produced by default in versions prior to libpng-1.5.4,
         use PNG_TRANSFORM_STRIP_16 and png_set_strip_16(png_ptr)
         instead.

      16-bit values:
         PNG_TRANSFORM_EXPAND_16
         png_set_expand_16(png_ptr);

      In either case palette image data will be expanded to RGB.  If you
      just want color data you can add PNG_TRANSFORM_GRAY_TO_RGB or
      png_set_gray_to_rgb(png_ptr) to the list.

      Calling png_set_background before the PNG file header is read will not
      work prior to libpng-1.5.4.  Because the failure may result in
      unexpected warnings or errors it is therefore much safer to call
      png_set_background after the head has been read.  Unfortunately this
      means that prior to libpng-1.5.4 it cannot be used with the high level
      interface.


    The high-level read interface
      At this point there are two ways to proceed; through the high-level
      read interface, or through a sequence of low-level read operations.
      You can use the high-level interface if (a) you are willing to read
      the entire image into memory, and (b) the input transformations you
      want to do are limited to the following set:

          PNG_TRANSFORM_IDENTITY      No transformation
          PNG_TRANSFORM_SCALE_16      Strip 16-bit samples to
                                      8-bit accurately



                                   - 36 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          PNG_TRANSFORM_STRIP_16      Chop 16-bit samples to
                                      8-bit less accurately
          PNG_TRANSFORM_STRIP_ALPHA   Discard the alpha channel
          PNG_TRANSFORM_PACKING       Expand 1, 2 and 4-bit
                                      samples to bytes
          PNG_TRANSFORM_PACKSWAP      Change order of packed
                                      pixels to LSB first
          PNG_TRANSFORM_EXPAND        Perform set_expand()
          PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
          PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                      sBIT depth
          PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                      to BGRA
          PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                      to AG
          PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                      to transparency
          PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
          PNG_TRANSFORM_GRAY_TO_RGB   Expand grayscale samples
                                      to RGB (or GA to RGBA)
          PNG_TRANSFORM_EXPAND_16     Expand samples to 16 bits

      (This excludes setting a background color, doing gamma transformation,
      quantizing, and setting filler.)  If this is the case, simply do this:

          png_read_png(png_ptr, info_ptr, png_transforms, NULL)

      where png_transforms is an integer containing the bitwise OR of some
      set of transformation flags.  This call is equivalent to
      png_read_info(), followed the set of transformations indicated by the
      transform mask, then png_read_image(), and finally png_read_end().

      (The final parameter of this call is not yet used.  Someday it might
      point to transformation parameters required by some future input
      transform.)

      You must use png_transforms and not call any png_set_transform()
      functions when you use png_read_png().

      After you have called png_read_png(), you can retrieve the image data
      with

         row_pointers = png_get_rows(png_ptr, info_ptr);

      where row_pointers is an array of pointers to the pixel data for each
      row:

         png_bytep row_pointers[height];

      If you know your image size and pixel size ahead of time, you can
      allocate row_pointers prior to calling png_read_png() with



                                   - 37 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



         if (height > PNG_UINT_32_MAX/(sizeof (png_byte)))
            png_error (png_ptr,
                "Image is too tall to process in memory");

         if (width > PNG_UINT_32_MAX/pixel_size)
            png_error (png_ptr,
                "Image is too wide to process in memory");

         row_pointers = png_malloc(png_ptr,
             height*(sizeof (png_bytep)));

         for (int i=0; i<height, i++)
            row_pointers[i]=NULL;  /* security precaution */

         for (int i=0; i<height, i++)
            row_pointers[i]=png_malloc(png_ptr,
                width*pixel_size);

         png_set_rows(png_ptr, info_ptr, &row_pointers);

      Alternatively you could allocate your image in one big block and
      define row_pointers[i] to point into the proper places in your block.

      If you use png_set_rows(), the application is responsible for freeing
      row_pointers (and row_pointers[i], if they were separately allocated).

      If you don't allocate row_pointers ahead of time, png_read_png() will
      do it, and it'll be free'ed by libpng when you call png_destroy_*().


    The low-level read interface
      If you are going the low-level route, you are now ready to read all
      the file information up to the actual image data.  You do this with a
      call to png_read_info().

          png_read_info(png_ptr, info_ptr);

      This will process all chunks up to but not including the image data.

      This also copies some of the data from the PNG file into the decode
      structure for use in later transformations.  Important information
      copied in is:

      1) The PNG file gamma from the gAMA chunk.  This overwrites the
      default value provided by an earlier call to png_set_gamma or
      png_set_alpha_mode.

      2) Prior to libpng-1.5.4 the background color from a bKGd chunk.  This
      damages the information provided by an earlier call to
      png_set_background resulting in unexpected behavior.  Libpng-1.5.4 no
      longer does this.



                                   - 38 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      3) The number of significant bits in each component value.  Libpng
      uses this to optimize gamma handling by reducing the internal lookup
      table sizes.

      4) The transparent color information from a tRNS chunk.  This can be
      modified by a later call to png_set_tRNS.


    Querying the info structure
      Functions are used to get the information from the info_ptr once it
      has been read.  Note that these fields may not be completely filled in
      until png_read_end() has read the chunk data following the image.

          png_get_IHDR(png_ptr, info_ptr, &width, &height,
             &bit_depth, &color_type, &interlace_type,
             &compression_type, &filter_method);

          width          - holds the width of the image
                           in pixels (up to 2^31).

          height         - holds the height of the image
                           in pixels (up to 2^31).

          bit_depth      - holds the bit depth of one of the
                           image channels.  (valid values are
                           1, 2, 4, 8, 16 and depend also on
                           the color_type.  See also
                           significant bits (sBIT) below).

          color_type     - describes which color/alpha channels
                               are present.
                           PNG_COLOR_TYPE_GRAY
                              (bit depths 1, 2, 4, 8, 16)
                           PNG_COLOR_TYPE_GRAY_ALPHA
                              (bit depths 8, 16)
                           PNG_COLOR_TYPE_PALETTE
                              (bit depths 1, 2, 4, 8)
                           PNG_COLOR_TYPE_RGB
                              (bit_depths 8, 16)
                           PNG_COLOR_TYPE_RGB_ALPHA
                              (bit_depths 8, 16)

                           PNG_COLOR_MASK_PALETTE
                           PNG_COLOR_MASK_COLOR
                           PNG_COLOR_MASK_ALPHA

          interlace_type - (PNG_INTERLACE_NONE or
                           PNG_INTERLACE_ADAM7)

          compression_type - (must be PNG_COMPRESSION_TYPE_BASE
                           for PNG 1.0)



                                   - 39 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          filter_method  - (must be PNG_FILTER_TYPE_BASE
                           for PNG 1.0, and can also be
                           PNG_INTRAPIXEL_DIFFERENCING if
                           the PNG datastream is embedded in
                           a MNG-1.0 datastream)

          Any of width, height, color_type, bit_depth,
          interlace_type, compression_type, or filter_method can
          be NULL if you are not interested in their values.

          Note that png_get_IHDR() returns 32-bit data into
          the application's width and height variables.
          This is an unsafe situation if these are not png_uint_32
          variables.  In such situations, the
          png_get_image_width() and png_get_image_height()
          functions described below are safer.

          width            = png_get_image_width(png_ptr,
                               info_ptr);

          height           = png_get_image_height(png_ptr,
                               info_ptr);

          bit_depth        = png_get_bit_depth(png_ptr,
                               info_ptr);

          color_type       = png_get_color_type(png_ptr,
                               info_ptr);

          interlace_type   = png_get_interlace_type(png_ptr,
                               info_ptr);

          compression_type = png_get_compression_type(png_ptr,
                               info_ptr);

          filter_method    = png_get_filter_type(png_ptr,
                               info_ptr);

          channels = png_get_channels(png_ptr, info_ptr);

          channels       - number of channels of info for the
                           color type (valid values are 1 (GRAY,
                           PALETTE), 2 (GRAY_ALPHA), 3 (RGB),
                           4 (RGB_ALPHA or RGB + filler byte))

          rowbytes = png_get_rowbytes(png_ptr, info_ptr);

          rowbytes       - number of bytes needed to hold a row

          signature = png_get_signature(png_ptr, info_ptr);




                                   - 40 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          signature      - holds the signature read from the
                           file (if any).  The data is kept in
                           the same offset it would be if the
                           whole signature were read (i.e. if an
                           application had already read in 4
                           bytes of signature before starting
                           libpng, the remaining 4 bytes would
                           be in signature[4] through signature[7]
                           (see png_set_sig_bytes())).

      These are also important, but their validity depends on whether the
      chunk has been read.  The png_get_valid(png_ptr, info_ptr,
      PNG_INFO_<chunk>) and png_get_<chunk>(png_ptr, info_ptr, ...)
      functions return non-zero if the data has been read, or zero if it is
      missing.  The parameters to the png_get_<chunk> are set directly if
      they are simple data types, or a pointer into the info_ptr is returned
      for any complex types.

      The colorspace data from gAMA, cHRM, sRGB, iCCP, and sBIT chunks is
      simply returned to give the application information about how the
      image was encoded.  Libpng itself only does transformations using the
      file gamma when combining semitransparent pixels with the background
      color, and, since libpng-1.6.0, when converting between 8-bit sRGB and
      16-bit linear pixels within the simplified API.  Libpng also uses the
      file gamma when converting RGB to gray, beginning with libpng-1.0.5,
      if the application calls png_set_rgb_to_gray()).

          png_get_PLTE(png_ptr, info_ptr, &palette,
                           &num_palette);

          palette        - the palette for the file
                           (array of png_color)

          num_palette    - number of entries in the palette

          png_get_gAMA(png_ptr, info_ptr, &file_gamma);
          png_get_gAMA_fixed(png_ptr, info_ptr, &int_file_gamma);

          file_gamma     - the gamma at which the file is
                           written (PNG_INFO_gAMA)

          int_file_gamma - 100,000 times the gamma at which the
                           file is written

          png_get_cHRM(png_ptr, info_ptr,  &white_x, &white_y, &red_x,
                           &red_y, &green_x, &green_y, &blue_x, &blue_y)
          png_get_cHRM_XYZ(png_ptr, info_ptr, &red_X, &red_Y, &red_Z,
                           &green_X, &green_Y, &green_Z, &blue_X, &blue_Y,
                           &blue_Z)
          png_get_cHRM_fixed(png_ptr, info_ptr, &int_white_x,
                           &int_white_y, &int_red_x, &int_red_y,



                                   - 41 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



                           &int_green_x, &int_green_y, &int_blue_x,
                           &int_blue_y)
          png_get_cHRM_XYZ_fixed(png_ptr, info_ptr, &int_red_X, &int_red_Y,
                           &int_red_Z, &int_green_X, &int_green_Y,
                           &int_green_Z, &int_blue_X, &int_blue_Y,
                           &int_blue_Z)

          {white,red,green,blue}_{x,y}
                           A color space encoding specified using the
                           chromaticities of the end points and the
                           white point. (PNG_INFO_cHRM)

          {red,green,blue}_{X,Y,Z}
                           A color space encoding specified using the
                           encoding end points - the CIE tristimulus
                           specification of the intended color of the red,
                           green and blue channels in the PNG RGB data.
                           The white point is simply the sum of the three
                           end points. (PNG_INFO_cHRM)

          png_get_sRGB(png_ptr, info_ptr, &srgb_intent);

          srgb_intent -    the rendering intent (PNG_INFO_sRGB)
                           The presence of the sRGB chunk
                           means that the pixel data is in the
                           sRGB color space.  This chunk also
                           implies specific values of gAMA and
                           cHRM.

          png_get_iCCP(png_ptr, info_ptr, &name,
             &compression_type, &profile, &proflen);

          name             - The profile name.

          compression_type - The compression type; always
                             PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                             You may give NULL to this argument to
                             ignore it.

          profile          - International Color Consortium color
                             profile data. May contain NULs.

          proflen          - length of profile data in bytes.

          png_get_sBIT(png_ptr, info_ptr, &sig_bit);

          sig_bit        - the number of significant bits for
                           (PNG_INFO_sBIT) each of the gray,
                           red, green, and blue channels,
                           whichever are appropriate for the
                           given color type (png_color_16)



                                   - 42 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          png_get_tRNS(png_ptr, info_ptr, &trans_alpha,
                           &num_trans, &trans_color);

          trans_alpha    - array of alpha (transparency)
                           entries for palette (PNG_INFO_tRNS)

          num_trans      - number of transparent entries
                           (PNG_INFO_tRNS)

          trans_color    - graylevel or color sample values of
                           the single transparent color for
                           non-paletted images (PNG_INFO_tRNS)

          png_get_hIST(png_ptr, info_ptr, &hist);
                           (PNG_INFO_hIST)

          hist           - histogram of palette (array of
                           png_uint_16)

          png_get_tIME(png_ptr, info_ptr, &mod_time);

          mod_time       - time image was last modified
                          (PNG_VALID_tIME)

          png_get_bKGD(png_ptr, info_ptr, &background);

          background     - background color (of type
                           png_color_16p) (PNG_VALID_bKGD)
                           valid 16-bit red, green and blue
                           values, regardless of color_type

          num_comments   = png_get_text(png_ptr, info_ptr,
                           &text_ptr, &num_text);

          num_comments   - number of comments

          text_ptr       - array of png_text holding image
                           comments

          text_ptr[i].compression - type of compression used
                       on "text" PNG_TEXT_COMPRESSION_NONE
                                 PNG_TEXT_COMPRESSION_zTXt
                                 PNG_ITXT_COMPRESSION_NONE
                                 PNG_ITXT_COMPRESSION_zTXt

          text_ptr[i].key   - keyword for comment.  Must contain
                               1-79 characters.

          text_ptr[i].text  - text comments for current
                               keyword.  Can be empty.




                                   - 43 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          text_ptr[i].text_length - length of text string,
                       after decompression, 0 for iTXt

          text_ptr[i].itxt_length - length of itxt string,
                       after decompression, 0 for tEXt/zTXt

          text_ptr[i].lang  - language of comment (empty
                               string for unknown).

          text_ptr[i].lang_key  - keyword in UTF-8
                               (empty string for unknown).

          Note that the itxt_length, lang, and lang_key
          members of the text_ptr structure only exist when the
          library is built with iTXt chunk support.  Prior to
          libpng-1.4.0 the library was built by default without
          iTXt support. Also note that when iTXt is supported,
          they contain NULL pointers when the "compression"
          field contains PNG_TEXT_COMPRESSION_NONE or
          PNG_TEXT_COMPRESSION_zTXt.

          num_text       - number of comments (same as
                           num_comments; you can put NULL here
                           to avoid the duplication)

          Note while png_set_text() will accept text, language,
          and translated keywords that can be NULL pointers, the
          structure returned by png_get_text will always contain
          regular zero-terminated C strings.  They might be
          empty strings but they will never be NULL pointers.

          num_spalettes = png_get_sPLT(png_ptr, info_ptr,
             &palette_ptr);

          num_spalettes  - number of sPLT chunks read.

          palette_ptr    - array of palette structures holding
                           contents of one or more sPLT chunks
                           read.

          png_get_oFFs(png_ptr, info_ptr, &offset_x, &offset_y,
             &unit_type);

          offset_x       - positive offset from the left edge
                           of the screen (can be negative)

          offset_y       - positive offset from the top edge
                           of the screen (can be negative)

          unit_type      - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER




                                   - 44 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          png_get_pHYs(png_ptr, info_ptr, &res_x, &res_y,
             &unit_type);

          res_x          - pixels/unit physical resolution in
                           x direction

          res_y          - pixels/unit physical resolution in
                           x direction

          unit_type      - PNG_RESOLUTION_UNKNOWN,
                           PNG_RESOLUTION_METER

          png_get_sCAL(png_ptr, info_ptr, &unit, &width,
             &height)

          unit        - physical scale units (an integer)

          width       - width of a pixel in physical scale units

          height      - height of a pixel in physical scale units
                       (width and height are doubles)

          png_get_sCAL_s(png_ptr, info_ptr, &unit, &width,
             &height)

          unit        - physical scale units (an integer)

          width       - width of a pixel in physical scale units
                        (expressed as a string)

          height      - height of a pixel in physical scale units
                       (width and height are strings like "2.54")

          num_unknown_chunks = png_get_unknown_chunks(png_ptr,
             info_ptr, &unknowns)

          unknowns          - array of png_unknown_chunk
                              structures holding unknown chunks

          unknowns[i].name  - name of unknown chunk

          unknowns[i].data  - data of unknown chunk

          unknowns[i].size  - size of unknown chunk's data

          unknowns[i].location - position of chunk in file

          The value of "i" corresponds to the order in which the
          chunks were read from the PNG file or inserted with the
          png_set_unknown_chunks() function.




                                   - 45 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          The value of "location" is a bitwise "or" of

               PNG_HAVE_IHDR  (0x01)
               PNG_HAVE_PLTE  (0x02)
               PNG_AFTER_IDAT (0x08)

      The data from the pHYs chunk can be retrieved in several convenient
      forms:

          res_x = png_get_x_pixels_per_meter(png_ptr,
             info_ptr)

          res_y = png_get_y_pixels_per_meter(png_ptr,
             info_ptr)

          res_x_and_y = png_get_pixels_per_meter(png_ptr,
             info_ptr)

          res_x = png_get_x_pixels_per_inch(png_ptr,
             info_ptr)

          res_y = png_get_y_pixels_per_inch(png_ptr,
             info_ptr)

          res_x_and_y = png_get_pixels_per_inch(png_ptr,
             info_ptr)

          aspect_ratio = png_get_pixel_aspect_ratio(png_ptr,
             info_ptr)

          Each of these returns 0 [signifying "unknown"] if
             the data is not present or if res_x is 0;
             res_x_and_y is 0 if res_x != res_y

          Note that because of the way the resolutions are
             stored internally, the inch conversions won't
             come out to exactly even number.  For example,
             72 dpi is stored as 0.28346 pixels/meter, and
             when this is retrieved it is 71.9988 dpi, so
             be sure to round the returned value appropriately
             if you want to display a reasonable-looking result.

      The data from the oFFs chunk can be retrieved in several convenient
      forms:

          x_offset = png_get_x_offset_microns(png_ptr, info_ptr);

          y_offset = png_get_y_offset_microns(png_ptr, info_ptr);

          x_offset = png_get_x_offset_inches(png_ptr, info_ptr);




                                   - 46 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          y_offset = png_get_y_offset_inches(png_ptr, info_ptr);

          Each of these returns 0 [signifying "unknown" if both
             x and y are 0] if the data is not present or if the
             chunk is present but the unit is the pixel.  The
             remark about inexact inch conversions applies here
             as well, because a value in inches can't always be
             converted to microns and back without some loss
             of precision.

      For more information, see the PNG specification for chunk contents.
      Be careful with trusting rowbytes, as some of the transformations
      could increase the space needed to hold a row (expand, filler,
      gray_to_rgb, etc.).  See png_read_update_info(), below.

      A quick word about text_ptr and num_text.  PNG stores comments in
      keyword/text pairs, one pair per chunk, with no limit on the number of
      text chunks, and a 2^31 byte limit on their size.  While there are
      suggested keywords, there is no requirement to restrict the use to
      these strings.  It is strongly suggested that keywords and text be
      sensible to humans (that's the point), so don't use abbreviations.
      Non-printing symbols are not allowed.  See the PNG specification for
      more details.  There is also no requirement to have text after the
      keyword.

      Keywords should be limited to 79 Latin-1 characters without leading or
      trailing spaces, but non-consecutive spaces are allowed within the
      keyword.  It is possible to have the same keyword any number of times.
      The text_ptr is an array of png_text structures, each holding a
      pointer to a language string, a pointer to a keyword and a pointer to
      a text string.  The text string, language code, and translated keyword
      may be empty or NULL pointers.  The keyword/text pairs are put into
      the array in the order that they are received.  However, some or all
      of the text chunks may be after the image, so, to make sure you have
      read all the text chunks, don't mess with these until after you read
      the stuff after the image.  This will be mentioned again below in the
      discussion that goes with png_read_end().


    Input transformations
      After you've read the header information, you can set up the library
      to handle any special transformations of the image data.  The various
      ways to transform the data will be described in the order that they
      should occur.  This is important, as some of these change the color
      type and/or bit depth of the data, and some others only work on
      certain color types and bit depths.

      Transformations you request are ignored if they don't have any meaning
      for a particular input data format.  However some transformations can
      have an effect as a result of a previous transformation.  If you
      specify a contradictory set of transformations, for example both



                                   - 47 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      adding and removing the alpha channel, you cannot predict the final
      result.

      The color used for the transparency values should be supplied in the
      same format/depth as the current image data.  It is stored in the same
      format/depth as the image data in a tRNS chunk, so this is what libpng
      expects for this data.

      The color used for the background value depends on the need_expand
      argument as described below.

      Data will be decoded into the supplied row buffers packed into bytes
      unless the library has been told to transform it into another format.
      For example, 4 bit/pixel paletted or grayscale data will be returned 2
      pixels/byte with the leftmost pixel in the high-order bits of the
      byte, unless png_set_packing() is called.  8-bit RGB data will be
      stored in RGB RGB RGB format unless png_set_filler() or
      png_set_add_alpha() is called to insert filler bytes, either before or
      after each RGB triplet.

      16-bit RGB data will be returned RRGGBB RRGGBB, with the most
      significant byte of the color value first, unless png_set_scale_16()
      is called to transform it to regular RGB RGB triplets, or
      png_set_filler() or png_set_add alpha() is called to insert two filler
      bytes, either before or after each RRGGBB triplet.  Similarly, 8-bit
      or 16-bit grayscale data can be modified with png_set_filler(),
      png_set_add_alpha(), png_set_strip_16(), or png_set_scale_16().

      The following code transforms grayscale images of less than 8 to 8
      bits, changes paletted images to RGB, and adds a full alpha channel if
      there is transparency information in a tRNS chunk.  This is most
      useful on grayscale images with bit depths of 2 or 4 or if there is a
      multiple-image viewing application that wishes to treat all images in
      the same way.

          if (color_type == PNG_COLOR_TYPE_PALETTE)
              png_set_palette_to_rgb(png_ptr);

          if (png_get_valid(png_ptr, info_ptr,
              PNG_INFO_tRNS)) png_set_tRNS_to_alpha(png_ptr);

          if (color_type == PNG_COLOR_TYPE_GRAY &&
              bit_depth < 8) png_set_expand_gray_1_2_4_to_8(png_ptr);

      The first two functions are actually aliases for png_set_expand(),
      added in libpng version 1.0.4, with the function names expanded to
      improve code readability.  In some future version they may actually do
      different things.

      As of libpng version 1.2.9, png_set_expand_gray_1_2_4_to_8() was
      added.  It expands the sample depth without changing tRNS to alpha.



                                   - 48 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      As of libpng version 1.5.2, png_set_expand_16() was added.  It behaves
      as png_set_expand(); however, the resultant channels have 16 bits
      rather than 8.  Use this when the output color or gray channels are
      made linear to avoid fairly severe accuracy loss.

         if (bit_depth < 16)
            png_set_expand_16(png_ptr);

      PNG can have files with 16 bits per channel.  If you only can handle 8
      bits per channel, this will strip the pixels down to 8-bit.

          if (bit_depth == 16) #if PNG_LIBPNG_VER >= 10504
             png_set_scale_16(png_ptr); #else
             png_set_strip_16(png_ptr); #endif

      (The more accurate "png_set_scale_16()" API became available in libpng
      version 1.5.4).

      If you need to process the alpha channel on the image separately from
      the image data (for example if you convert it to a bitmap mask) it is
      possible to have libpng strip the channel leaving just RGB or gray
      data:

          if (color_type & PNG_COLOR_MASK_ALPHA)
             png_set_strip_alpha(png_ptr);

      If you strip the alpha channel you need to find some other way of
      dealing with the information.  If, instead, you want to convert the
      image to an opaque version with no alpha channel use
      png_set_background; see below.

      As of libpng version 1.5.2, almost all useful expansions are
      supported, the major ommissions are conversion of grayscale to indexed
      images (which can be done trivially in the application) and conversion
      of indexed to grayscale (which can be done by a trivial manipulation
      of the palette.)

      In the following table, the 01 means grayscale with depth<8, 31 means
      indexed with depth<8, other numerals represent the color type, "T"
      means the tRNS chunk is present, A means an alpha channel is present,
      and O means tRNS or alpha is present but all pixels in the image are
      opaque.

        FROM  01  31   0  0T  0O   2  2T  2O   3  3T  3O  4A  4O  6A  6O
         TO
         01    -  [G]  -   -   -   -   -   -   -   -   -   -   -   -   -
         31   [Q]  Q  [Q] [Q] [Q]  Q   Q   Q   Q   Q   Q  [Q] [Q]  Q   Q
          0    1   G   +   .   .   G   G   G   G   G   G   B   B  GB  GB
         0T    lt  Gt  t   +   .   Gt  G   G   Gt  G   G   Bt  Bt GBt GBt
         0O    lt  Gt  t   .   +   Gt  Gt  G   Gt  Gt  G   Bt  Bt GBt GBt
          2    C   P   C   C   C   +   .   .   C   -   -  CB  CB   B   B



                                   - 49 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



         2T    Ct  -   Ct  C   C   t   +   t   -   -   -  CBt CBt  Bt  Bt
         2O    Ct  -   Ct  C   C   t   t   +   -   -   -  CBt CBt  Bt  Bt
          3   [Q]  p  [Q] [Q] [Q]  Q   Q   Q   +   .   .  [Q] [Q]  Q   Q
         3T   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   +   t  [Qt][Qt] Qt  Qt
         3O   [Qt] p  [Qt][Q] [Q]  Qt  Qt  Qt  t   t   +  [Qt][Qt] Qt  Qt
         4A    lA  G   A   T   T   GA  GT  GT  GA  GT  GT  +   BA  G  GBA
         4O    lA GBA  A   T   T   GA  GT  GT  GA  GT  GT  BA  +  GBA  G
         6A    CA  PA  CA  C   C   A   T  tT   PA  P   P   C  CBA  +   BA
         6O    CA PBA  CA  C   C   A  tT   T   PA  P   P  CBA  C   BA  +

      Within the matrix,
           "+" identifies entries where 'from' and 'to' are the same.
           "-" means the transformation is not supported.
           "." means nothing is necessary (a tRNS chunk can just be
      ignored).
           "t" means the transformation is obtained by png_set_tRNS.
           "A" means the transformation is obtained by png_set_add_alpha().
           "X" means the transformation is obtained by png_set_expand().
           "1" means the transformation is obtained by
               png_set_expand_gray_1_2_4_to_8() (and by png_set_expand()
               if there is no transparency in the original or the final
               format).
           "C" means the transformation is obtained by
      png_set_gray_to_rgb().
           "G" means the transformation is obtained by
      png_set_rgb_to_gray().
           "P" means the transformation is obtained by
               png_set_expand_palette_to_rgb().
           "p" means the transformation is obtained by png_set_packing().
           "Q" means the transformation is obtained by png_set_quantize().
           "T" means the transformation is obtained by
               png_set_tRNS_to_alpha().
           "B" means the transformation is obtained by
               png_set_background(), or png_strip_alpha().

      When an entry has multiple transforms listed all are required to cause
      the right overall transformation.  When two transforms are separated
      by a comma either will do the job.  When transforms are enclosed in []
      the transform should do the job but this is currently unimplemented -
      a different format will result if the suggested transformations are
      used.

      In PNG files, the alpha channel in an image is the level of opacity.
      If you need the alpha channel in an image to be the level of
      transparency instead of opacity, you can invert the alpha channel (or
      the tRNS chunk data) after it's read, so that 0 is fully opaque and
      255 (in 8-bit or paletted images) or 65535 (in 16-bit images) is fully
      transparent, with

          png_set_invert_alpha(png_ptr);




                                   - 50 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
      they can, resulting in, for example, 8 pixels per byte for 1 bit
      files.  This code expands to 1 pixel per byte without changing the
      values of the pixels:

          if (bit_depth < 8)
             png_set_packing(png_ptr);

      PNG files have possible bit depths of 1, 2, 4, 8, and 16.  All pixels
      stored in a PNG image have been "scaled" or "shifted" up to the next
      higher possible bit depth (e.g. from 5 bits/sample in the range [0,31]
      to 8 bits/sample in the range [0, 255]).  However, it is also possible
      to convert the PNG pixel data back to the original bit depth of the
      image.  This call reduces the pixels back down to the original bit
      depth:

          png_color_8p sig_bit;

          if (png_get_sBIT(png_ptr, info_ptr, &sig_bit))
             png_set_shift(png_ptr, sig_bit);

      PNG files store 3-color pixels in red, green, blue order.  This code
      changes the storage of the pixels to blue, green, red:

          if (color_type == PNG_COLOR_TYPE_RGB ||
              color_type == PNG_COLOR_TYPE_RGB_ALPHA)
             png_set_bgr(png_ptr);

      PNG files store RGB pixels packed into 3 or 6 bytes. This code expands
      them into 4 or 8 bytes for windowing systems that need them in this
      format:

          if (color_type == PNG_COLOR_TYPE_RGB)
             png_set_filler(png_ptr, filler, PNG_FILLER_BEFORE);

      where "filler" is the 8-bit or 16-bit number to fill with, and the
      location is either PNG_FILLER_BEFORE or PNG_FILLER_AFTER, depending
      upon whether you want the filler before the RGB or after. When filling
      an 8-bit pixel, the least significant 8 bits of the number are used,
      if a 16-bit number is supplied.  This transformation does not affect
      images that already have full alpha channels.  To add an opaque alpha
      channel, use filler=0xffff and PNG_FILLER_AFTER which will generate
      RGBA pixels.

      Note that png_set_filler() does not change the color type.  If you
      want to do that, you can add a true alpha channel with

          if (color_type == PNG_COLOR_TYPE_RGB ||
             color_type == PNG_COLOR_TYPE_GRAY)
             png_set_add_alpha(png_ptr, filler, PNG_FILLER_AFTER);




                                   - 51 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      where "filler" contains the alpha value to assign to each pixel.  The
      png_set_add_alpha() function was added in libpng-1.2.7.

      If you are reading an image with an alpha channel, and you need the
      data as ARGB instead of the normal PNG format RGBA:

          if (color_type == PNG_COLOR_TYPE_RGB_ALPHA)
             png_set_swap_alpha(png_ptr);

      For some uses, you may want a grayscale image to be represented as
      RGB.  This code will do that conversion:

          if (color_type == PNG_COLOR_TYPE_GRAY ||
              color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
             png_set_gray_to_rgb(png_ptr);

      Conversely, you can convert an RGB or RGBA image to grayscale or
      grayscale with alpha.

          if (color_type == PNG_COLOR_TYPE_RGB ||
              color_type == PNG_COLOR_TYPE_RGB_ALPHA)
             png_set_rgb_to_gray(png_ptr, error_action,
                double red_weight, double green_weight);

          error_action = 1: silently do the conversion

          error_action = 2: issue a warning if the original
                            image has any pixel where
                            red != green or red != blue

          error_action = 3: issue an error and abort the
                            conversion if the original
                            image has any pixel where
                            red != green or red != blue

          red_weight:       weight of red component

          green_weight:     weight of green component
                            If either weight is negative, default
                            weights are used.

      In the corresponding fixed point API the red_weight and green_weight
      values are simply scaled by 100,000:

          png_set_rgb_to_gray(png_ptr, error_action,
             png_fixed_point red_weight,
             png_fixed_point green_weight);

      If you have set error_action = 1 or 2, you can later check whether the
      image really was gray, after processing the image rows, with the
      png_get_rgb_to_gray_status(png_ptr) function.  It will return a



                                   - 52 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      png_byte that is zero if the image was gray or 1 if there were any
      non-gray pixels.  Background and sBIT data will be silently converted
      to grayscale, using the green channel data for sBIT, regardless of the
      error_action setting.

      The default values come from the PNG file cHRM chunk if present;
      otherwise, the defaults correspond to the ITU-R recommendation 709,
      and also the sRGB color space, as recommended in the Charles Poynton's
      Colour FAQ, Copyright (c) 2006-11-28 Charles Poynton, in section 9:

      <http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9>

          Y = 0.2126 * R + 0.7152 * G + 0.0722 * B

      Previous versions of this document, 1998 through 2002, recommended a
      slightly different formula:

          Y = 0.212671 * R + 0.715160 * G + 0.072169 * B

      Libpng uses an integer approximation:

          Y = (6968 * R + 23434 * G + 2366 * B)/32768

      The calculation is done in a linear colorspace, if the image gamma can
      be determined.

      The png_set_background() function has been described already; it tells
      libpng to composite images with alpha or simple transparency against
      the supplied background color.  For compatibility with versions of
      libpng earlier than libpng-1.5.4 it is recommended that you call the
      function after reading the file header, even if you don't want to use
      the color in a bKGD chunk, if one exists.

      If the PNG file contains a bKGD chunk (PNG_INFO_bKGD valid), you may
      use this color, or supply another color more suitable for the current
      display (e.g., the background color from a web page).  You need to
      tell libpng how the color is represented, both the format of the
      component values in the color (the number of bits) and the gamma
      encoding of the color.  The function takes two arguments,
      background_gamma_mode and need_expand to convey this information;
      however, only two combinations are likely to be useful:

          png_color_16 my_background;
          png_color_16p image_background;

          if (png_get_bKGD(png_ptr, info_ptr, &image_background))
             png_set_background(png_ptr, image_background,
                 PNG_BACKGROUND_GAMMA_FILE, 1/*needs to be expanded*/, 1);
          else
             png_set_background(png_ptr, &my_background,
                 PNG_BACKGROUND_GAMMA_SCREEN, 0/*do not expand*/, 1);



                                   - 53 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      The second call was described above - my_background is in the format
      of the final, display, output produced by libpng.  Because you now
      know the format of the PNG it is possible to avoid the need to choose
      either 8-bit or 16-bit output and to retain palette images (the
      palette colors will be modified appropriately and the tRNS chunk
      removed.)  However, if you are doing this, take great care not to ask
      for transformations without checking first that they apply!

      In the first call the background color has the original bit depth and
      color type of the PNG file.  So, for palette images the color is
      supplied as a palette index and for low bit greyscale images the color
      is a reduced bit value in image_background->gray.

      If you didn't call png_set_gamma() before reading the file header, for
      example if you need your code to remain compatible with older versions
      of libpng prior to libpng-1.5.4, this is the place to call it.

      Do not call it if you called png_set_alpha_mode(); doing so will
      damage the settings put in place by png_set_alpha_mode().  (If
      png_set_alpha_mode() is supported then you can certainly do
      png_set_gamma() before reading the PNG header.)

      This API unconditionally sets the screen and file gamma values, so it
      will override the value in the PNG file unless it is called before the
      PNG file reading starts.  For this reason you must always call it with
      the PNG file value when you call it in this position:

         if (png_get_gAMA(png_ptr, info_ptr, &file_gamma))
            png_set_gamma(png_ptr, screen_gamma, file_gamma);

         else
            png_set_gamma(png_ptr, screen_gamma, 0.45455);

      If you need to reduce an RGB file to a paletted file, or if a paletted
      file has more entries than will fit on your screen, png_set_quantize()
      will do that.  Note that this is a simple match quantization that
      merely finds the closest color available.  This should work fairly
      well with optimized palettes, but fairly badly with linear color
      cubes.  If you pass a palette that is larger than maximum_colors, the
      file will reduce the number of colors in the palette so it will fit
      into maximum_colors.  If there is a histogram, libpng will use it to
      make more intelligent choices when reducing the palette.  If there is
      no histogram, it may not do as good a job.

         if (color_type & PNG_COLOR_MASK_COLOR)
         {
            if (png_get_valid(png_ptr, info_ptr,
                PNG_INFO_PLTE))
            {
               png_uint_16p histogram = NULL;




                                   - 54 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



               png_get_hIST(png_ptr, info_ptr,
                   &histogram);
               png_set_quantize(png_ptr, palette, num_palette,
                  max_screen_colors, histogram, 1);
            }

            else
            {
               png_color std_color_cube[MAX_SCREEN_COLORS] =
                  { ... colors ... };

               png_set_quantize(png_ptr, std_color_cube,
                  MAX_SCREEN_COLORS, MAX_SCREEN_COLORS,
                  NULL,0);
            }
         }

      PNG files describe monochrome as black being zero and white being one.
      The following code will reverse this (make black be one and white be
      zero):

         if (bit_depth == 1 && color_type == PNG_COLOR_TYPE_GRAY)
            png_set_invert_mono(png_ptr);

      This function can also be used to invert grayscale and gray-alpha
      images:

         if (color_type == PNG_COLOR_TYPE_GRAY ||
             color_type == PNG_COLOR_TYPE_GRAY_ALPHA)
            png_set_invert_mono(png_ptr);

      PNG files store 16-bit pixels in network byte order (big-endian, ie.
      most significant bits first).  This code changes the storage to the
      other way (little-endian, i.e. least significant bits first, the way
      PCs store them):

          if (bit_depth == 16)
             png_set_swap(png_ptr);

      If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
      need to change the order the pixels are packed into bytes, you can
      use:

          if (bit_depth < 8)
             png_set_packswap(png_ptr);

      Finally, you can write your own transformation function if none of the
      existing ones meets your needs.  This is done by setting a callback
      with

          png_set_read_user_transform_fn(png_ptr,



                                   - 55 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



              read_transform_fn);

      You must supply the function

          void read_transform_fn(png_structp png_ptr, png_row_infop
              row_info, png_bytep data)

      See pngtest.c for a working example.  Your function will be called
      after all of the other transformations have been processed.  Take care
      with interlaced images if you do the interlace yourself - the width of
      the row is the width in 'row_info', not the overall image width.

      If supported, libpng provides two information routines that you can
      use to find where you are in processing the image:

         png_get_current_pass_number(png_structp png_ptr);
         png_get_current_row_number(png_structp png_ptr);

      Don't try using these outside a transform callback - firstly they are
      only supported if user transforms are supported, secondly they may
      well return unexpected results unless the row is actually being
      processed at the moment they are called.

      With interlaced images the value returned is the row in the input
      sub-image image.  Use PNG_ROW_FROM_PASS_ROW(row, pass) and
      PNG_COL_FROM_PASS_COL(col, pass) to find the output pixel (x,y) given
      an interlaced sub-image pixel (row,col,pass).

      The discussion of interlace handling above contains more information
      on how to use these values.

      You can also set up a pointer to a user structure for use by your
      callback function, and you can inform libpng that your transform
      function will change the number of channels or bit depth with the
      function

          png_set_user_transform_info(png_ptr, user_ptr,
              user_depth, user_channels);

      The user's application, not libpng, is responsible for allocating and
      freeing any memory required for the user structure.

      You can retrieve the pointer via the function
      png_get_user_transform_ptr().  For example:

          voidp read_user_transform_ptr =
              png_get_user_transform_ptr(png_ptr);

      The last thing to handle is interlacing; this is covered in detail
      below, but you must call the function here if you want libpng to
      handle expansion of the interlaced image.



                                   - 56 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          number_of_passes = png_set_interlace_handling(png_ptr);

      After setting the transformations, libpng can update your png_info
      structure to reflect any transformations you've requested with this
      call.

          png_read_update_info(png_ptr, info_ptr);

      This is most useful to update the info structure's rowbytes field so
      you can use it to allocate your image memory.  This function will also
      update your palette with the correct screen_gamma and background if
      these have been given with the calls above.  You may only call
      png_read_update_info() once with a particular info_ptr.

      After you call png_read_update_info(), you can allocate any memory you
      need to hold the image.  The row data is simply raw byte data for all
      forms of images.  As the actual allocation varies among applications,
      no example will be given.  If you are allocating one large chunk, you
      will need to build an array of pointers to each row, as it will be
      needed for some of the functions below.

      Remember: Before you call png_read_update_info(), the png_get_*()
      functions return the values corresponding to the original PNG image.
      After you call png_read_update_info the values refer to the image that
      libpng will output.  Consequently you must call all the png_set_
      functions before you call png_read_update_info().  This is
      particularly important for png_set_interlace_handling() - if you are
      going to call png_read_update_info() you must call
      png_set_interlace_handling() before it unless you want to receive
      interlaced output.


    Reading image data
      After you've allocated memory, you can read the image data.  The
      simplest way to do this is in one function call.  If you are
      allocating enough memory to hold the whole image, you can just call
      png_read_image() and libpng will read in all the image data and put it
      in the memory area supplied.  You will need to pass in an array of
      pointers to each row.

      This function automatically handles interlacing, so you don't need to
      call png_set_interlace_handling() (unless you call
      png_read_update_info()) or call this function multiple times, or any
      of that other stuff necessary with png_read_rows().

         png_read_image(png_ptr, row_pointers);

      where row_pointers is:

         png_bytep row_pointers[height];




                                   - 57 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      You can point to void or char or whatever you use for pixels.

      If you don't want to read in the whole image at once, you can use
      png_read_rows() instead.  If there is no interlacing (check
      interlace_type == PNG_INTERLACE_NONE), this is simple:

          png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);

      where row_pointers is the same as in the png_read_image() call.

      If you are doing this just one row at a time, you can do this with a
      single row_pointer instead of an array of row_pointers:

          png_bytep row_pointer = row;
          png_read_row(png_ptr, row_pointer, NULL);

      If the file is interlaced (interlace_type != 0 in the IHDR chunk),
      things get somewhat harder.  The only current (PNG Specification
      version 1.2) interlacing type for PNG is (interlace_type ==
      PNG_INTERLACE_ADAM7); a somewhat complicated 2D interlace scheme,
      known as Adam7, that breaks down an image into seven smaller images of
      varying size, based on an 8x8 grid.  This number is defined (from
      libpng 1.5) as PNG_INTERLACE_ADAM7_PASSES in png.h

      libpng can fill out those images or it can give them to you "as is".
      It is almost always better to have libpng handle the interlacing for
      you.  If you want the images filled out, there are two ways to do
      that.  The one mentioned in the PNG specification is to expand each
      pixel to cover those pixels that have not been read yet (the
      "rectangle" method).  This results in a blocky image for the first
      pass, which gradually smooths out as more pixels are read.  The other
      method is the "sparkle" method, where pixels are drawn only in their
      final locations, with the rest of the image remaining whatever colors
      they were initialized to before the start of the read.  The first
      method usually looks better, but tends to be slower, as there are more
      pixels to put in the rows.

      If, as is likely, you want libpng to expand the images, call this
      before calling png_start_read_image() or png_read_update_info():

          if (interlace_type == PNG_INTERLACE_ADAM7)
             number_of_passes
                 = png_set_interlace_handling(png_ptr);

      This will return the number of passes needed.  Currently, this is
      seven, but may change if another interlace type is added.  This
      function can be called even if the file is not interlaced, where it
      will return one pass.  You then need to read the whole image
      'number_of_passes' times.  Each time will distribute the pixels from
      the current pass to the correct place in the output image, so you need



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      to supply the same rows to png_read_rows in each pass.

      If you are not going to display the image after each pass, but are
      going to wait until the entire image is read in, use the sparkle
      effect.  This effect is faster and the end result of either method is
      exactly the same.  If you are planning on displaying the image after
      each pass, the "rectangle" effect is generally considered the better
      looking one.

      If you only want the "sparkle" effect, just call png_read_row() or
      png_read_rows() as normal, with the third parameter NULL.  Make sure
      you make pass over the image number_of_passes times, and you don't
      change the data in the rows between calls.  You can change the
      locations of the data, just not the data.  Each pass only writes the
      pixels appropriate for that pass, and assumes the data from previous
      passes is still valid.

          png_read_rows(png_ptr, row_pointers, NULL,
              number_of_rows);
          or
          png_read_row(png_ptr, row_pointers, NULL);

      If you only want the first effect (the rectangles), do the same as
      before except pass the row buffer in the third parameter, and leave
      the second parameter NULL.

          png_read_rows(png_ptr, NULL, row_pointers,
              number_of_rows);
          or
          png_read_row(png_ptr, NULL, row_pointers);

      If you don't want libpng to handle the interlacing details, just call
      png_read_rows() PNG_INTERLACE_ADAM7_PASSES times to read in all the
      images.  Each of the images is a valid image by itself; however, you
      will almost certainly need to distribute the pixels from each sub-
      image to the correct place.  This is where everything gets very
      tricky.

      If you want to retrieve the separate images you must pass the correct
      number of rows to each successive call of png_read_rows().  The
      calculation gets pretty complicated for small images, where some sub-
      images may not even exist because either their width or height ends up
      zero.  libpng provides two macros to help you in 1.5 and later
      versions:

         png_uint_32 width = PNG_PASS_COLS(image_width, pass_number);
         png_uint_32 height = PNG_PASS_ROWS(image_height, pass_number);

      Respectively these tell you the width and height of the sub-image
      corresponding to the numbered pass.  'pass' is in in the range 0 to 6
      - this can be confusing because the specification refers to the same



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      passes as 1 to 7!  Be careful, you must check both the width and
      height before calling png_read_rows() and not call it for that pass if
      either is zero.

      You can, of course, read each sub-image row by row.  If you want to
      produce optimal code to make a pixel-by-pixel transformation of an
      interlaced image this is the best approach; read each row of each
      pass, transform it, and write it out to a new interlaced image.

      If you want to de-interlace the image yourself libpng provides further
      macros to help that tell you where to place the pixels in the output
      image.  Because the interlacing scheme is rectangular - sub-image
      pixels are always arranged on a rectangular grid - all you need to
      know for each pass is the starting column and row in the output image
      of the first pixel plus the spacing between each pixel.  As of libpng
      1.5 there are four macros to retrieve this information:

         png_uint_32 x = PNG_PASS_START_COL(pass);
         png_uint_32 y = PNG_PASS_START_ROW(pass);
         png_uint_32 xStep = 1U << PNG_PASS_COL_SHIFT(pass);
         png_uint_32 yStep = 1U << PNG_PASS_ROW_SHIFT(pass);

      These allow you to write the obvious loop:

         png_uint_32 input_y = 0;
         png_uint_32 output_y = PNG_PASS_START_ROW(pass);

         while (output_y < output_image_height)
         {
            png_uint_32 input_x = 0;
            png_uint_32 output_x = PNG_PASS_START_COL(pass);

            while (output_x < output_image_width)
            {
               image[output_y][output_x] =
                   subimage[pass][input_y][input_x++];

               output_x += xStep;
            }

            ++input_y;
            output_y += yStep;
         }

      Notice that the steps between successive output rows and columns are
      returned as shifts.  This is possible because the pixels in the
      subimages are always a power of 2 apart - 1, 2, 4 or 8 pixels - in the
      original image.  In practice you may need to directly calculate the
      output coordinate given an input coordinate.  libpng provides two
      further macros for this purpose:




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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



         png_uint_32 output_x = PNG_COL_FROM_PASS_COL(input_x, pass);
         png_uint_32 output_y = PNG_ROW_FROM_PASS_ROW(input_y, pass);

      Finally a pair of macros are provided to tell you if a particular
      image row or column appears in a given pass:

         int col_in_pass = PNG_COL_IN_INTERLACE_PASS(output_x, pass);
         int row_in_pass = PNG_ROW_IN_INTERLACE_PASS(output_y, pass);

      Bear in mind that you will probably also need to check the width and
      height of the pass in addition to the above to be sure the pass even
      exists!

      With any luck you are convinced by now that you don't want to do your
      own interlace handling.  In reality normally the only good reason for
      doing this is if you are processing PNG files on a pixel-by-pixel
      basis and don't want to load the whole file into memory when it is
      interlaced.

      libpng includes a test program, pngvalid, that illustrates reading and
      writing of interlaced images.  If you can't get interlacing to work in
      your code and don't want to leave it to libpng (the recommended
      approach), see how pngvalid.c does it.


    Finishing a sequential read
      After you are finished reading the image through the low-level
      interface, you can finish reading the file.

      If you want to use a different crc action for handling CRC errors in
      chunks after the image data, you can call png_set_crc_action() again
      at this point.

      If you are interested in comments or time, which may be stored either
      before or after the image data, you should pass the separate png_info
      struct if you want to keep the comments from before and after the
      image separate.

          png_infop end_info = png_create_info_struct(png_ptr);

          if (!end_info)
          {
             png_destroy_read_struct(&png_ptr, &info_ptr,
                 (png_infopp)NULL);
             return (ERROR);
          }

         png_read_end(png_ptr, end_info);

      If you are not interested, you should still call png_read_end() but
      you can pass NULL, avoiding the need to create an end_info structure.



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      If you do this, libpng will not process any chunks after IDAT other
      than skipping over them and perhaps (depending on whether you have
      called png_set_crc_action) checking their CRCs while looking for the
      IEND chunk.

         png_read_end(png_ptr, (png_infop)NULL);

      If you don't call png_read_end(), then your file pointer will be left
      pointing to the first chunk after the last IDAT, which is probably not
      what you want if you expect to read something beyond the end of the
      PNG datastream.

      When you are done, you can free all memory allocated by libpng like
      this:

         png_destroy_read_struct(&png_ptr, &info_ptr,
             &end_info);

      or, if you didn't create an end_info structure,

         png_destroy_read_struct(&png_ptr, &info_ptr,
             (png_infopp)NULL);

      It is also possible to individually free the info_ptr members that
      point to libpng-allocated storage with the following function:

          png_free_data(png_ptr, info_ptr, mask, seq)

          mask - identifies data to be freed, a mask
                 containing the bitwise OR of one or
                 more of
                   PNG_FREE_PLTE, PNG_FREE_TRNS,
                   PNG_FREE_HIST, PNG_FREE_ICCP,
                   PNG_FREE_PCAL, PNG_FREE_ROWS,
                   PNG_FREE_SCAL, PNG_FREE_SPLT,
                   PNG_FREE_TEXT, PNG_FREE_UNKN,
                 or simply PNG_FREE_ALL

          seq  - sequence number of item to be freed
                 (-1 for all items)

      This function may be safely called when the relevant storage has
      already been freed, or has not yet been allocated, or was allocated by
      the user and not by libpng,  and will in those cases do nothing.  The
      "seq" parameter is ignored if only one item of the selected data type,
      such as PLTE, is allowed.  If "seq" is not -1, and multiple items are
      allowed for the data type identified in the mask, such as text or
      sPLT, only the n'th item in the structure is freed, where n is "seq".

      The default behavior is only to free data that was allocated
      internally by libpng.  This can be changed, so that libpng will not



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      free the data, or so that it will free data that was allocated by the
      user with png_malloc() or png_calloc() and passed in via a png_set_*()
      function, with

          png_data_freer(png_ptr, info_ptr, freer, mask)

          freer  - one of
                     PNG_DESTROY_WILL_FREE_DATA
                     PNG_SET_WILL_FREE_DATA
                     PNG_USER_WILL_FREE_DATA

          mask   - which data elements are affected
                   same choices as in png_free_data()

      This function only affects data that has already been allocated.  You
      can call this function after reading the PNG data but before calling
      any png_set_*() functions, to control whether the user or the
      png_set_*() function is responsible for freeing any existing data that
      might be present, and again after the png_set_*() functions to control
      whether the user or png_destroy_*() is supposed to free the data.
      When the user assumes responsibility for libpng-allocated data, the
      application must use png_free() to free it, and when the user
      transfers responsibility to libpng for data that the user has
      allocated, the user must have used png_malloc() or png_calloc() to
      allocate it.

      If you allocated your row_pointers in a single block, as suggested
      above in the description of the high level read interface, you must
      not transfer responsibility for freeing it to the png_set_rows or
      png_read_destroy function, because they would also try to free the
      individual row_pointers[i].

      If you allocated text_ptr.text, text_ptr.lang, and
      text_ptr.translated_keyword separately, do not transfer responsibility
      for freeing text_ptr to libpng, because when libpng fills a png_text
      structure it combines these members with the key member, and
      png_free_data() will free only text_ptr.key.  Similarly, if you
      transfer responsibility for free'ing text_ptr from libpng to your
      application, your application must not separately free those members.

      The png_free_data() function will turn off the "valid" flag for
      anything it frees.  If you need to turn the flag off for a chunk that
      was freed by your application instead of by libpng, you can use

          png_set_invalid(png_ptr, info_ptr, mask);

          mask - identifies the chunks to be made invalid,
                 containing the bitwise OR of one or
                 more of
                   PNG_INFO_gAMA, PNG_INFO_sBIT,
                   PNG_INFO_cHRM, PNG_INFO_PLTE,



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



                   PNG_INFO_tRNS, PNG_INFO_bKGD,
                   PNG_INFO_hIST, PNG_INFO_pHYs,
                   PNG_INFO_oFFs, PNG_INFO_tIME,
                   PNG_INFO_pCAL, PNG_INFO_sRGB,
                   PNG_INFO_iCCP, PNG_INFO_sPLT,
                   PNG_INFO_sCAL, PNG_INFO_IDAT

      For a more compact example of reading a PNG image, see the file
      example.c.


    Reading PNG files progressively
      The progressive reader is slightly different from the non-progressive
      reader.  Instead of calling png_read_info(), png_read_rows(), and
      png_read_end(), you make one call to png_process_data(), which calls
      callbacks when it has the info, a row, or the end of the image.  You
      set up these callbacks with png_set_progressive_read_fn().  You don't
      have to worry about the input/output functions of libpng, as you are
      giving the library the data directly in png_process_data().  I will
      assume that you have read the section on reading PNG files above, so I
      will only highlight the differences (although I will show all of the
      code).

      png_structp png_ptr; png_infop info_ptr;

       /*  An example code fragment of how you would
           initialize the progressive reader in your
           application. */
       int
       initialize_png_reader()
       {
          png_ptr = png_create_read_struct
              (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
               user_error_fn, user_warning_fn);

          if (!png_ptr)
              return (ERROR);

          info_ptr = png_create_info_struct(png_ptr);

          if (!info_ptr)
          {
             png_destroy_read_struct(&png_ptr,
                (png_infopp)NULL, (png_infopp)NULL);
             return (ERROR);
          }

          if (setjmp(png_jmpbuf(png_ptr)))
          {
             png_destroy_read_struct(&png_ptr, &info_ptr,
                (png_infopp)NULL);



                                   - 64 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



             return (ERROR);
          }

          /* This one's new.  You can provide functions
             to be called when the header info is valid,
             when each row is completed, and when the image
             is finished.  If you aren't using all functions,
             you can specify NULL parameters.  Even when all
             three functions are NULL, you need to call
             png_set_progressive_read_fn().  You can use
             any struct as the user_ptr (cast to a void pointer
             for the function call), and retrieve the pointer
             from inside the callbacks using the function

                png_get_progressive_ptr(png_ptr);

             which will return a void pointer, which you have
             to cast appropriately.
           */
          png_set_progressive_read_fn(png_ptr, (void *)user_ptr,
              info_callback, row_callback, end_callback);

          return 0;
       }

       /* A code fragment that you call as you receive blocks
         of data */
       int
       process_data(png_bytep buffer, png_uint_32 length)
       {
          if (setjmp(png_jmpbuf(png_ptr)))
          {
             png_destroy_read_struct(&png_ptr, &info_ptr,
                 (png_infopp)NULL);
             return (ERROR);
          }

          /* This one's new also.  Simply give it a chunk
             of data from the file stream (in order, of
             course).  On machines with segmented memory
             models machines, don't give it any more than
             64K.  The library seems to run fine with sizes
             of 4K. Although you can give it much less if
             necessary (I assume you can give it chunks of
             1 byte, I haven't tried less than 256 bytes
             yet).  When this function returns, you may
             want to display any rows that were generated
             in the row callback if you don't already do
             so there.
           */
          png_process_data(png_ptr, info_ptr, buffer, length);



                                   - 65 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          /* At this point you can call png_process_data_skip if
             you want to handle data the library will skip yourself;
             it simply returns the number of bytes to skip (and stops
             libpng skipping that number of bytes on the next
             png_process_data call).
          return 0;
       }

       /* This function is called (as set by
          png_set_progressive_read_fn() above) when enough data
          has been supplied so all of the header has been
          read.
       */
       void
       info_callback(png_structp png_ptr, png_infop info)
       {
          /* Do any setup here, including setting any of
             the transformations mentioned in the Reading
             PNG files section.  For now, you _must_ call
             either png_start_read_image() or
             png_read_update_info() after all the
             transformations are set (even if you don't set
             any).  You may start getting rows before
             png_process_data() returns, so this is your
             last chance to prepare for that.

             This is where you turn on interlace handling,
             assuming you don't want to do it yourself.

             If you need to you can stop the processing of
             your original input data at this point by calling
             png_process_data_pause.  This returns the number
             of unprocessed bytes from the last png_process_data
             call - it is up to you to ensure that the next call
             sees these bytes again.  If you don't want to bother
             with this you can get libpng to cache the unread
             bytes by setting the 'save' parameter (see png.h) but
             then libpng will have to copy the data internally.
           */
       }

       /* This function is called when each row of image
          data is complete */
       void
       row_callback(png_structp png_ptr, png_bytep new_row,
          png_uint_32 row_num, int pass)
       {
          /* If the image is interlaced, and you turned
             on the interlace handler, this function will
             be called for every row in every pass.  Some
             of these rows will not be changed from the



                                   - 66 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



             previous pass.  When the row is not changed,
             the new_row variable will be NULL.  The rows
             and passes are called in order, so you don't
             really need the row_num and pass, but I'm
             supplying them because it may make your life
             easier.

             If you did not turn on interlace handling then
             the callback is called for each row of each
             sub-image when the image is interlaced.  In this
             case 'row_num' is the row in the sub-image, not
             the row in the output image as it is in all other
             cases.

             For the non-NULL rows of interlaced images when
             you have switched on libpng interlace handling,
             you must call png_progressive_combine_row()
             passing in the row and the old row.  You can
             call this function for NULL rows (it will just
             return) and for non-interlaced images (it just
             does the memcpy for you) if it will make the
             code easier.  Thus, you can just do this for
             all cases if you switch on interlace handling;
           */

              png_progressive_combine_row(png_ptr, old_row,
                new_row);

          /* where old_row is what was displayed
             previously for the row.  Note that the first
             pass (pass == 0, really) will completely cover
             the old row, so the rows do not have to be
             initialized.  After the first pass (and only
             for interlaced images), you will have to pass
             the current row, and the function will combine
             the old row and the new row.

             You can also call png_process_data_pause in this
             callback - see above.
          */
       }

       void
       end_callback(png_structp png_ptr, png_infop info)
       {
          /* This function is called after the whole image
             has been read, including any chunks after the
             image (up to and including the IEND).  You
             will usually have the same info chunk as you
             had in the header, although some data may have
             been added to the comments and time fields.



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



             Most people won't do much here, perhaps setting
             a flag that marks the image as finished.
           */
       }




 IV. Writing
      Much of this is very similar to reading.  However, everything of
      importance is repeated here, so you won't have to constantly look back
      up in the reading section to understand writing.


    Setup
      You will want to do the I/O initialization before you get into libpng,
      so if it doesn't work, you don't have anything to undo. If you are not
      using the standard I/O functions, you will need to replace them with
      custom writing functions.  See the discussion under Customizing
      libpng.

          FILE *fp = fopen(file_name, "wb");

          if (!fp)
             return (ERROR);

      Next, png_struct and png_info need to be allocated and initialized.
      As these can be both relatively large, you may not want to store these
      on the stack, unless you have stack space to spare.  Of course, you
      will want to check if they return NULL.  If you are also reading, you
      won't want to name your read structure and your write structure both
      "png_ptr"; you can call them anything you like, such as "read_ptr" and
      "write_ptr".  Look at pngtest.c, for example.

          png_structp png_ptr = png_create_write_struct
             (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
              user_error_fn, user_warning_fn);

          if (!png_ptr)
             return (ERROR);

          png_infop info_ptr = png_create_info_struct(png_ptr);
          if (!info_ptr)
          {
             png_destroy_write_struct(&png_ptr,
                 (png_infopp)NULL);
             return (ERROR);
          }

      If you want to use your own memory allocation routines, define
      PNG_USER_MEM_SUPPORTED and use png_create_write_struct_2() instead of



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      png_create_write_struct():

          png_structp png_ptr = png_create_write_struct_2
             (PNG_LIBPNG_VER_STRING, (png_voidp)user_error_ptr,
              user_error_fn, user_warning_fn, (png_voidp)
              user_mem_ptr, user_malloc_fn, user_free_fn);

      After you have these structures, you will need to set up the error
      handling.  When libpng encounters an error, it expects to longjmp()
      back to your routine.  Therefore, you will need to call setjmp() and
      pass the png_jmpbuf(png_ptr).  If you write the file from different
      routines, you will need to update the png_jmpbuf(png_ptr) every time
      you enter a new routine that will call a png_*() function.  See your
      documentation of setjmp/longjmp for your compiler for more information
      on setjmp/longjmp.  See the discussion on libpng error handling in the
      Customizing Libpng section below for more information on the libpng
      error handling.

          if (setjmp(png_jmpbuf(png_ptr)))
          {
          png_destroy_write_struct(&png_ptr, &info_ptr);
             fclose(fp);
             return (ERROR);
          }
          ...
          return;

      If you would rather avoid the complexity of setjmp/longjmp issues, you
      can compile libpng with PNG_NO_SETJMP, in which case errors will
      result in a call to PNG_ABORT() which defaults to abort().

      You can #define PNG_ABORT() to a function that does something more
      useful than abort(), as long as your function does not return.

      Checking for invalid palette index on write was added at libpng
      1.5.10.  If a pixel contains an invalid (out-of-range) index libpng
      issues a benign error.  This is enabled by default because this
      condition is an error according to the PNG specification, Clause
      11.3.2, but the error can be ignored in each png_ptr with

         png_set_check_for_invalid_index(png_ptr, 0);

      If the error is ignored, or if png_benign_error() treats it as a
      warning, any invalid pixels are written as-is by the encoder,
      resulting in an invalid PNG datastream as output.  In this case the
      application is responsible for ensuring that the pixel indexes are in
      range when it writes a PLTE chunk with fewer entries than the bit
      depth would allow.

      Now you need to set up the output code.  The default for libpng is to
      use the C function fwrite().  If you use this, you will need to pass a



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      valid FILE * in the function png_init_io().  Be sure that the file is
      opened in binary mode.  Again, if you wish to handle writing data in
      another way, see the discussion on libpng I/O handling in the
      Customizing Libpng section below.

          png_init_io(png_ptr, fp);

      If you are embedding your PNG into a datastream such as MNG, and don't
      want libpng to write the 8-byte signature, or if you have already
      written the signature in your application, use

          png_set_sig_bytes(png_ptr, 8);

      to inform libpng that it should not write a signature.


    Write callbacks
      At this point, you can set up a callback function that will be called
      after each row has been written, which you can use to control a
      progress meter or the like.  It's demonstrated in pngtest.c.  You must
      supply a function

          void write_row_callback(png_structp png_ptr, png_uint_32 row,
             int pass);
          {
            /* put your code here */
          }

      (You can give it another name that you like instead of
      "write_row_callback")

      To inform libpng about your function, use

          png_set_write_status_fn(png_ptr, write_row_callback);

      When this function is called the row has already been completely
      processed and it has also been written out.  The 'row' and 'pass'
      refer to the next row to be handled.  For the non-interlaced case the
      row that was just handled is simply one less than the passed in row
      number, and pass will always be 0.  For the interlaced case the same
      applies unless the row value is 0, in which case the row just handled
      was the last one from one of the preceding passes.  Because
      interlacing may skip a pass you cannot be sure that the preceding pass
      is just 'pass-1', if you really need to know what the last pass is
      record (row,pass) from the callback and use the last recorded value
      each time.

      As with the user transform you can find the output row using the
      PNG_ROW_FROM_PASS_ROW macro.

      You now have the option of modifying how the compression library will



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      run.  The following functions are mainly for testing, but may be
      useful in some cases, like if you need to write PNG files extremely
      fast and are willing to give up some compression, or if you want to
      get the maximum possible compression at the expense of slower writing.
      If you have no special needs in this area, let the library do what it
      wants by not calling this function at all, as it has been tuned to
      deliver a good speed/compression ratio. The second parameter to
      png_set_filter() is the filter method, for which the only valid values
      are 0 (as of the July 1999 PNG specification, version 1.2) or 64 (if
      you are writing a PNG datastream that is to be embedded in a MNG
      datastream).  The third parameter is a flag that indicates which
      filter type(s) are to be tested for each scanline.  See the PNG
      specification for details on the specific filter types.


          /* turn on or off filtering, and/or choose
             specific filters.  You can use either a single
             PNG_FILTER_VALUE_NAME or the bitwise OR of one
             or more PNG_FILTER_NAME masks.
           */
          png_set_filter(png_ptr, 0,
             PNG_FILTER_NONE  | PNG_FILTER_VALUE_NONE |
             PNG_FILTER_SUB   | PNG_FILTER_VALUE_SUB  |
             PNG_FILTER_UP    | PNG_FILTER_VALUE_UP   |
             PNG_FILTER_AVG   | PNG_FILTER_VALUE_AVG  |
             PNG_FILTER_PAETH | PNG_FILTER_VALUE_PAETH|
             PNG_ALL_FILTERS  | PNG_FAST_FILTERS);

      If an application wants to start and stop using particular filters
      during compression, it should start out with all of the filters (to
      ensure that the previous row of pixels will be stored in case it's
      needed later), and then add and remove them after the start of
      compression.

      If you are writing a PNG datastream that is to be embedded in a MNG
      datastream, the second parameter can be either 0 or 64.

      The png_set_compression_*() functions interface to the zlib
      compression library, and should mostly be ignored unless you really
      know what you are doing.  The only generally useful call is
      png_set_compression_level() which changes how much time zlib spends on
      trying to compress the image data.  See the Compression Library
      (zlib.h and algorithm.txt, distributed with zlib) for details on the
      compression levels.

          #include zlib.h

          /* Set the zlib compression level */
          png_set_compression_level(png_ptr,
              Z_BEST_COMPRESSION);




                                   - 71 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          /* Set other zlib parameters for compressing IDAT */
          png_set_compression_mem_level(png_ptr, 8);
          png_set_compression_strategy(png_ptr,
              Z_DEFAULT_STRATEGY);
          png_set_compression_window_bits(png_ptr, 15);
          png_set_compression_method(png_ptr, 8);
          png_set_compression_buffer_size(png_ptr, 8192)

          /* Set zlib parameters for text compression
           * If you don't call these, the parameters
           * fall back on those defined for IDAT chunks
           */
          png_set_text_compression_mem_level(png_ptr, 8);
          png_set_text_compression_strategy(png_ptr,
              Z_DEFAULT_STRATEGY);
          png_set_text_compression_window_bits(png_ptr, 15);
          png_set_text_compression_method(png_ptr, 8);


    Setting the contents of info for
      You now need to fill in the png_info structure with all the data you
      wish to write before the actual image.  Note that the only thing you
      are allowed to write after the image is the text chunks and the time
      chunk (as of PNG Specification 1.2, anyway).  See png_write_end() and
      the latest PNG specification for more information on that.  If you
      wish to write them before the image, fill them in now, and flag that
      data as being valid.  If you want to wait until after the data, don't
      fill them until png_write_end().  For all the fields in png_info and
      their data types, see png.h.  For explanations of what the fields
      contain, see the PNG specification.

      Some of the more important parts of the png_info are:

          png_set_IHDR(png_ptr, info_ptr, width, height,
             bit_depth, color_type, interlace_type,
             compression_type, filter_method)

          width          - holds the width of the image
                           in pixels (up to 2^31).

          height         - holds the height of the image
                           in pixels (up to 2^31).

          bit_depth      - holds the bit depth of one of the
                           image channels.
                           (valid values are 1, 2, 4, 8, 16
                           and depend also on the
                           color_type.  See also significant
                           bits (sBIT) below).

          color_type     - describes which color/alpha



                                   - 72 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



                           channels are present.
                           PNG_COLOR_TYPE_GRAY
                              (bit depths 1, 2, 4, 8, 16)
                           PNG_COLOR_TYPE_GRAY_ALPHA
                              (bit depths 8, 16)
                           PNG_COLOR_TYPE_PALETTE
                              (bit depths 1, 2, 4, 8)
                           PNG_COLOR_TYPE_RGB
                              (bit_depths 8, 16)
                           PNG_COLOR_TYPE_RGB_ALPHA
                              (bit_depths 8, 16)

                           PNG_COLOR_MASK_PALETTE
                           PNG_COLOR_MASK_COLOR
                           PNG_COLOR_MASK_ALPHA

          interlace_type - PNG_INTERLACE_NONE or
                           PNG_INTERLACE_ADAM7

          compression_type - (must be
                           PNG_COMPRESSION_TYPE_DEFAULT)

          filter_method  - (must be PNG_FILTER_TYPE_DEFAULT
                           or, if you are writing a PNG to
                           be embedded in a MNG datastream,
                           can also be
                           PNG_INTRAPIXEL_DIFFERENCING)

      If you call png_set_IHDR(), the call must appear before any of the
      other png_set_*() functions, because they might require access to some
      of the IHDR settings.  The remaining png_set_*() functions can be
      called in any order.

      If you wish, you can reset the compression_type, interlace_type, or
      filter_method later by calling png_set_IHDR() again; if you do this,
      the width, height, bit_depth, and color_type must be the same in each
      call.

          png_set_PLTE(png_ptr, info_ptr, palette,
             num_palette);

          palette        - the palette for the file
                           (array of png_color)
          num_palette    - number of entries in the palette


          png_set_gAMA(png_ptr, info_ptr, file_gamma);
          png_set_gAMA_fixed(png_ptr, info_ptr, int_file_gamma);

          file_gamma     - the gamma at which the image was
                           created (PNG_INFO_gAMA)



                                   - 73 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          int_file_gamma - 100,000 times the gamma at which
                           the image was created

          png_set_cHRM(png_ptr, info_ptr,  white_x, white_y, red_x, red_y,
                           green_x, green_y, blue_x, blue_y)
          png_set_cHRM_XYZ(png_ptr, info_ptr, red_X, red_Y, red_Z, green_X,
                           green_Y, green_Z, blue_X, blue_Y, blue_Z)
          png_set_cHRM_fixed(png_ptr, info_ptr, int_white_x, int_white_y,
                           int_red_x, int_red_y, int_green_x, int_green_y,
                           int_blue_x, int_blue_y)
          png_set_cHRM_XYZ_fixed(png_ptr, info_ptr, int_red_X, int_red_Y,
                           int_red_Z, int_green_X, int_green_Y, int_green_Z,
                           int_blue_X, int_blue_Y, int_blue_Z)

          {white,red,green,blue}_{x,y}
                           A color space encoding specified using the
      chromaticities
                           of the end points and the white point.

          {red,green,blue}_{X,Y,Z}
                           A color space encoding specified using the
      encoding end
                           points - the CIE tristimulus specification of the
      intended
                           color of the red, green and blue channels in the
      PNG RGB
                           data.  The white point is simply the sum of the
      three end
                           points.

          png_set_sRGB(png_ptr, info_ptr, srgb_intent);

          srgb_intent    - the rendering intent
                           (PNG_INFO_sRGB) The presence of
                           the sRGB chunk means that the pixel
                           data is in the sRGB color space.
                           This chunk also implies specific
                           values of gAMA and cHRM.  Rendering
                           intent is the CSS-1 property that
                           has been defined by the International
                           Color Consortium
                           (http://www.color.org).
                           It can be one of
                           PNG_sRGB_INTENT_SATURATION,
                           PNG_sRGB_INTENT_PERCEPTUAL,
                           PNG_sRGB_INTENT_ABSOLUTE, or
                           PNG_sRGB_INTENT_RELATIVE.


          png_set_sRGB_gAMA_and_cHRM(png_ptr, info_ptr,
             srgb_intent);



                                   - 74 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          srgb_intent    - the rendering intent
                           (PNG_INFO_sRGB) The presence of the
                           sRGB chunk means that the pixel
                           data is in the sRGB color space.
                           This function also causes gAMA and
                           cHRM chunks with the specific values
                           that are consistent with sRGB to be
                           written.

          png_set_iCCP(png_ptr, info_ptr, name, compression_type,
                             profile, proflen);

          name             - The profile name.

          compression_type - The compression type; always
                             PNG_COMPRESSION_TYPE_BASE for PNG 1.0.
                             You may give NULL to this argument to
                             ignore it.

          profile          - International Color Consortium color
                             profile data. May contain NULs.

          proflen          - length of profile data in bytes.

          png_set_sBIT(png_ptr, info_ptr, sig_bit);

          sig_bit        - the number of significant bits for
                           (PNG_INFO_sBIT) each of the gray, red,
                           green, and blue channels, whichever are
                           appropriate for the given color type
                           (png_color_16)

          png_set_tRNS(png_ptr, info_ptr, trans_alpha,
             num_trans, trans_color);

          trans_alpha    - array of alpha (transparency)
                           entries for palette (PNG_INFO_tRNS)

          num_trans      - number of transparent entries
                           (PNG_INFO_tRNS)

          trans_color    - graylevel or color sample values
                           (in order red, green, blue) of the
                           single transparent color for
                           non-paletted images (PNG_INFO_tRNS)

          png_set_hIST(png_ptr, info_ptr, hist);

          hist           - histogram of palette (array of
                           png_uint_16) (PNG_INFO_hIST)




                                   - 75 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          png_set_tIME(png_ptr, info_ptr, mod_time);

          mod_time       - time image was last modified
                           (PNG_VALID_tIME)

          png_set_bKGD(png_ptr, info_ptr, background);

          background     - background color (of type
                           png_color_16p) (PNG_VALID_bKGD)

          png_set_text(png_ptr, info_ptr, text_ptr, num_text);

          text_ptr       - array of png_text holding image
                           comments

          text_ptr[i].compression - type of compression used
                       on "text" PNG_TEXT_COMPRESSION_NONE
                                 PNG_TEXT_COMPRESSION_zTXt
                                 PNG_ITXT_COMPRESSION_NONE
                                 PNG_ITXT_COMPRESSION_zTXt
          text_ptr[i].key   - keyword for comment.  Must contain
                       1-79 characters.
          text_ptr[i].text  - text comments for current
                               keyword.  Can be NULL or empty.
          text_ptr[i].text_length - length of text string,
                       after decompression, 0 for iTXt
          text_ptr[i].itxt_length - length of itxt string,
                       after decompression, 0 for tEXt/zTXt
          text_ptr[i].lang  - language of comment (NULL or
                               empty for unknown).
          text_ptr[i].translated_keyword  - keyword in UTF-8 (NULL
                               or empty for unknown).

          Note that the itxt_length, lang, and lang_key
          members of the text_ptr structure only exist when the
          library is built with iTXt chunk support.  Prior to
          libpng-1.4.0 the library was built by default without
          iTXt support. Also note that when iTXt is supported,
          they contain NULL pointers when the "compression"
          field contains PNG_TEXT_COMPRESSION_NONE or
          PNG_TEXT_COMPRESSION_zTXt.

          num_text       - number of comments

          png_set_sPLT(png_ptr, info_ptr, &palette_ptr,
             num_spalettes);

          palette_ptr    - array of png_sPLT_struct structures
                           to be added to the list of palettes
                           in the info structure.
          num_spalettes  - number of palette structures to be



                                   - 76 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



                           added.

          png_set_oFFs(png_ptr, info_ptr, offset_x, offset_y,
              unit_type);

          offset_x  - positive offset from the left
                           edge of the screen

          offset_y  - positive offset from the top
                           edge of the screen

          unit_type - PNG_OFFSET_PIXEL, PNG_OFFSET_MICROMETER

          png_set_pHYs(png_ptr, info_ptr, res_x, res_y,
              unit_type);

          res_x       - pixels/unit physical resolution
                        in x direction

          res_y       - pixels/unit physical resolution
                        in y direction

          unit_type   - PNG_RESOLUTION_UNKNOWN,
                        PNG_RESOLUTION_METER

          png_set_sCAL(png_ptr, info_ptr, unit, width, height)

          unit        - physical scale units (an integer)

          width       - width of a pixel in physical scale units

          height      - height of a pixel in physical scale units
                        (width and height are doubles)

          png_set_sCAL_s(png_ptr, info_ptr, unit, width, height)

          unit        - physical scale units (an integer)

          width       - width of a pixel in physical scale units
                        expressed as a string

          height      - height of a pixel in physical scale units
                       (width and height are strings like "2.54")

          png_set_unknown_chunks(png_ptr, info_ptr, &unknowns,
             num_unknowns)

          unknowns          - array of png_unknown_chunk
                              structures holding unknown chunks
          unknowns[i].name  - name of unknown chunk
          unknowns[i].data  - data of unknown chunk



                                   - 77 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          unknowns[i].size  - size of unknown chunk's data
          unknowns[i].location - position to write chunk in file
                                 0: do not write chunk
                                 PNG_HAVE_IHDR: before PLTE
                                 PNG_HAVE_PLTE: before IDAT
                                 PNG_AFTER_IDAT: after IDAT

      The "location" member is set automatically according to what part of
      the output file has already been written.  You can change its value
      after calling png_set_unknown_chunks() as demonstrated in pngtest.c.
      Within each of the "locations", the chunks are sequenced according to
      their position in the structure (that is, the value of "i", which is
      the order in which the chunk was either read from the input file or
      defined with png_set_unknown_chunks).

      A quick word about text and num_text.  text is an array of png_text
      structures.  num_text is the number of valid structures in the array.
      Each png_text structure holds a language code, a keyword, a text
      value, and a compression type.

      The compression types have the same valid numbers as the compression
      types of the image data.  Currently, the only valid number is zero.
      However, you can store text either compressed or uncompressed, unlike
      images, which always have to be compressed.  So if you don't want the
      text compressed, set the compression type to
      PNG_TEXT_COMPRESSION_NONE.  Because tEXt and zTXt chunks don't have a
      language field, if you specify PNG_TEXT_COMPRESSION_NONE or
      PNG_TEXT_COMPRESSION_zTXt any language code or translated keyword will
      not be written out.

      Until text gets around a few hundred bytes, it is not worth
      compressing it.  After the text has been written out to the file, the
      compression type is set to PNG_TEXT_COMPRESSION_NONE_WR or
      PNG_TEXT_COMPRESSION_zTXt_WR, so that it isn't written out again at
      the end (in case you are calling png_write_end() with the same
      struct).

      The keywords that are given in the PNG Specification are:

          Title            Short (one line) title or
                           caption for image

          Author           Name of image's creator

          Description      Description of image (possibly long)

          Copyright        Copyright notice

          Creation Time    Time of original image creation
                           (usually RFC 1123 format, see below)




                                   - 78 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          Software         Software used to create the image

          Disclaimer       Legal disclaimer

          Warning          Warning of nature of content

          Source           Device used to create the image

          Comment          Miscellaneous comment; conversion
                           from other image format

      The keyword-text pairs work like this.  Keywords should be short
      simple descriptions of what the comment is about.  Some typical
      keywords are found in the PNG specification, as is some
      recommendations on keywords.  You can repeat keywords in a file.  You
      can even write some text before the image and some after.  For
      example, you may want to put a description of the image before the
      image, but leave the disclaimer until after, so viewers working over
      modem connections don't have to wait for the disclaimer to go over the
      modem before they start seeing the image.  Finally, keywords should be
      full words, not abbreviations.  Keywords and text are in the ISO
      8859-1 (Latin-1) character set (a superset of regular ASCII) and can
      not contain NUL characters, and should not contain control or other
      unprintable characters.  To make the comments widely readable, stick
      with basic ASCII, and avoid machine specific character set extensions
      like the IBM-PC character set.  The keyword must be present, but you
      can leave off the text string on non-compressed pairs.  Compressed
      pairs must have a text string, as only the text string is compressed
      anyway, so the compression would be meaningless.

      PNG supports modification time via the png_time structure.  Two
      conversion routines are provided, png_convert_from_time_t() for time_t
      and png_convert_from_struct_tm() for struct tm.  The time_t routine
      uses gmtime().  You don't have to use either of these, but if you wish
      to fill in the png_time structure directly, you should provide the
      time in universal time (GMT) if possible instead of your local time.
      Note that the year number is the full year (e.g. 1998, rather than 98
      - PNG is year 2000 compliant!), and that months start with 1.

      If you want to store the time of the original image creation, you
      should use a plain tEXt chunk with the "Creation Time" keyword.  This
      is necessary because the "creation time" of a PNG image is somewhat
      vague, depending on whether you mean the PNG file, the time the image
      was created in a non-PNG format, a still photo from which the image
      was scanned, or possibly the subject matter itself.  In order to
      facilitate machine-readable dates, it is recommended that the
      "Creation Time" tEXt chunk use RFC 1123 format dates (e.g. "22 May
      1997 18:07:10 GMT"), although this isn't a requirement.  Unlike the
      tIME chunk, the "Creation Time" tEXt chunk is not expected to be
      automatically changed by the software.  To facilitate the use of RFC
      1123 dates, a function png_convert_to_rfc1123_buffer(buffer,



                                   - 79 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      png_timep) is provided to convert from PNG time to an RFC 1123 format
      string.  The caller must provide a writeable buffer of at least 29
      bytes.


    Writing unknown chunks
      You can use the png_set_unknown_chunks function to queue up private
      chunks for writing.  You give it a chunk name, location, raw data, and
      a size.  You also must use png_set_keep_unknown_chunks() to ensure
      that libpng will handle them.  That's all there is to it.  The chunks
      will be written by the next following png_write_info_before_PLTE,
      png_write_info, or png_write_end function, depending upon the
      specified location.  Any chunks previously read into the info
      structure's unknown-chunk list will also be written out in a sequence
      that satisfies the PNG specification's ordering rules.

      Here is an example of writing two private chunks, prVt and miNE:

          #ifdef PNG_WRITE_UNKNOWN_CHUNKS_SUPPORTED
          /* Set unknown chunk data */
          png_unknown_chunk unk_chunk[2];
          strcpy((char *) unk_chunk[0].name, "prVt";
          unk_chunk[0].data = (unsigned char *) "PRIVATE DATA";
          unk_chunk[0].size = strlen(unk_chunk[0].data)+1;
          unk_chunk[0].location = PNG_HAVE_IHDR;
          strcpy((char *) unk_chunk[1].name, "miNE";
          unk_chunk[1].data = (unsigned char *) "MY CHUNK DATA";
          unk_chunk[1].size = strlen(unk_chunk[0].data)+1;
          unk_chunk[1].location = PNG_AFTER_IDAT;
          png_set_unknown_chunks(write_ptr, write_info_ptr,
              unk_chunk, 2);
          /* Needed because miNE is not safe-to-copy */
          png_set_keep_unknown_chunks(png, PNG_HANDLE_CHUNK_ALWAYS,
             (png_bytep) "miNE", 1);
          # if PNG_LIBPNG_VER < 10600
            /* Deal with unknown chunk location bug in 1.5.x and earlier */
            png_set_unknown_chunk_location(png, info, 0, PNG_HAVE_IHDR);
            png_set_unknown_chunk_location(png, info, 1, PNG_AFTER_IDAT);
          # endif
          # if PNG_LIBPNG_VER < 10500
            /* PNG_AFTER_IDAT writes two copies of the chunk prior to
      libpng-1.5.0,
             * one before IDAT and another after IDAT, so don't use it; only
      use
             * PNG_HAVE_IHDR location.  This call resets the location
      previously
             * set by assignment and png_set_unknown_chunk_location() for
      chunk 1.
             */
            png_set_unknown_chunk_location(png, info, 1, PNG_HAVE_IHDR);
          # endif



                                   - 80 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          #endif


    The high-level write interface
      At this point there are two ways to proceed; through the high-level
      write interface, or through a sequence of low-level write operations.
      You can use the high-level interface if your image data is present in
      the info structure.  All defined output transformations are permitted,
      enabled by the following masks.

          PNG_TRANSFORM_IDENTITY      No transformation
          PNG_TRANSFORM_PACKING       Pack 1, 2 and 4-bit samples
          PNG_TRANSFORM_PACKSWAP      Change order of packed
                                      pixels to LSB first
          PNG_TRANSFORM_INVERT_MONO   Invert monochrome images
          PNG_TRANSFORM_SHIFT         Normalize pixels to the
                                      sBIT depth
          PNG_TRANSFORM_BGR           Flip RGB to BGR, RGBA
                                      to BGRA
          PNG_TRANSFORM_SWAP_ALPHA    Flip RGBA to ARGB or GA
                                      to AG
          PNG_TRANSFORM_INVERT_ALPHA  Change alpha from opacity
                                      to transparency
          PNG_TRANSFORM_SWAP_ENDIAN   Byte-swap 16-bit samples
          PNG_TRANSFORM_STRIP_FILLER        Strip out filler
                                            bytes (deprecated).
          PNG_TRANSFORM_STRIP_FILLER_BEFORE Strip out leading
                                            filler bytes
          PNG_TRANSFORM_STRIP_FILLER_AFTER  Strip out trailing
                                            filler bytes

      If you have valid image data in the info structure (you can use
      png_set_rows() to put image data in the info structure), simply do
      this:

          png_write_png(png_ptr, info_ptr, png_transforms, NULL)

      where png_transforms is an integer containing the bitwise OR of some
      set of transformation flags.  This call is equivalent to
      png_write_info(), followed the set of transformations indicated by the
      transform mask, then png_write_image(), and finally png_write_end().

      (The final parameter of this call is not yet used.  Someday it might
      point to transformation parameters required by some future output
      transform.)

      You must use png_transforms and not call any png_set_transform()
      functions when you use png_write_png().






                                   - 81 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



    The low-level write interface
      If you are going the low-level route instead, you are now ready to
      write all the file information up to the actual image data.  You do
      this with a call to png_write_info().

          png_write_info(png_ptr, info_ptr);

      Note that there is one transformation you may need to do before
      png_write_info().  In PNG files, the alpha channel in an image is the
      level of opacity.  If your data is supplied as a level of
      transparency, you can invert the alpha channel before you write it, so
      that 0 is fully transparent and 255 (in 8-bit or paletted images) or
      65535 (in 16-bit images) is fully opaque, with

          png_set_invert_alpha(png_ptr);

      This must appear before png_write_info() instead of later with the
      other transformations because in the case of paletted images the tRNS
      chunk data has to be inverted before the tRNS chunk is written.  If
      your image is not a paletted image, the tRNS data (which in such cases
      represents a single color to be rendered as transparent) won't need to
      be changed, and you can safely do this transformation after your
      png_write_info() call.

      If you need to write a private chunk that you want to appear before
      the PLTE chunk when PLTE is present, you can write the PNG info in two
      steps, and insert code to write your own chunk between them:

          png_write_info_before_PLTE(png_ptr, info_ptr);
          png_set_unknown_chunks(png_ptr, info_ptr, ...);
          png_write_info(png_ptr, info_ptr);

      After you've written the file information, you can set up the library
      to handle any special transformations of the image data.  The various
      ways to transform the data will be described in the order that they
      should occur.  This is important, as some of these change the color
      type and/or bit depth of the data, and some others only work on
      certain color types and bit depths.  Even though each transformation
      checks to see if it has data that it can do something with, you should
      make sure to only enable a transformation if it will be valid for the
      data.  For example, don't swap red and blue on grayscale data.

      PNG files store RGB pixels packed into 3 or 6 bytes.  This code tells
      the library to strip input data that has 4 or 8 bytes per pixel down
      to 3 or 6 bytes (or strip 2 or 4-byte grayscale+filler data to 1 or 2
      bytes per pixel).

          png_set_filler(png_ptr, 0, PNG_FILLER_BEFORE);

      where the 0 is unused, and the location is either PNG_FILLER_BEFORE or
      PNG_FILLER_AFTER, depending upon whether the filler byte in the pixel



                                   - 82 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      is stored XRGB or RGBX.

      PNG files pack pixels of bit depths 1, 2, and 4 into bytes as small as
      they can, resulting in, for example, 8 pixels per byte for 1 bit
      files.  If the data is supplied at 1 pixel per byte, use this code,
      which will correctly pack the pixels into a single byte:

          png_set_packing(png_ptr);

      PNG files reduce possible bit depths to 1, 2, 4, 8, and 16.  If your
      data is of another bit depth, you can write an sBIT chunk into the
      file so that decoders can recover the original data if desired.

          /* Set the true bit depth of the image data */
          if (color_type & PNG_COLOR_MASK_COLOR)
          {
             sig_bit.red = true_bit_depth;
             sig_bit.green = true_bit_depth;
             sig_bit.blue = true_bit_depth;
          }

          else
          {
             sig_bit.gray = true_bit_depth;
          }

          if (color_type & PNG_COLOR_MASK_ALPHA)
          {
             sig_bit.alpha = true_bit_depth;
          }

          png_set_sBIT(png_ptr, info_ptr, &sig_bit);

      If the data is stored in the row buffer in a bit depth other than one
      supported by PNG (e.g. 3 bit data in the range 0-7 for a 4-bit PNG),
      this will scale the values to appear to be the correct bit depth as is
      required by PNG.

          png_set_shift(png_ptr, &sig_bit);

      PNG files store 16-bit pixels in network byte order (big-endian, ie.
      most significant bits first).  This code would be used if they are
      supplied the other way (little-endian, i.e. least significant bits
      first, the way PCs store them):

          if (bit_depth > 8)
             png_set_swap(png_ptr);

      If you are using packed-pixel images (1, 2, or 4 bits/pixel), and you
      need to change the order the pixels are packed into bytes, you can
      use:



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          if (bit_depth < 8)
             png_set_packswap(png_ptr);

      PNG files store 3 color pixels in red, green, blue order.  This code
      would be used if they are supplied as blue, green, red:

          png_set_bgr(png_ptr);

      PNG files describe monochrome as black being zero and white being one.
      This code would be used if the pixels are supplied with this reversed
      (black being one and white being zero):

          png_set_invert_mono(png_ptr);

      Finally, you can write your own transformation function if none of the
      existing ones meets your needs.  This is done by setting a callback
      with

          png_set_write_user_transform_fn(png_ptr,
             write_transform_fn);

      You must supply the function

          void write_transform_fn(png_structp png_ptr, png_row_infop
             row_info, png_bytep data)

      See pngtest.c for a working example.  Your function will be called
      before any of the other transformations are processed.  If supported
      libpng also supplies an information routine that may be called from
      your callback:

         png_get_current_row_number(png_ptr);
         png_get_current_pass_number(png_ptr);

      This returns the current row passed to the transform.  With interlaced
      images the value returned is the row in the input sub-image image.
      Use PNG_ROW_FROM_PASS_ROW(row, pass) and PNG_COL_FROM_PASS_COL(col,
      pass) to find the output pixel (x,y) given an interlaced sub-image
      pixel (row,col,pass).

      The discussion of interlace handling above contains more information
      on how to use these values.

      You can also set up a pointer to a user structure for use by your
      callback function.

          png_set_user_transform_info(png_ptr, user_ptr, 0, 0);

      The user_channels and user_depth parameters of this function are
      ignored when writing; you can set them to zero as shown.




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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      You can retrieve the pointer via the function
      png_get_user_transform_ptr().  For example:

          voidp write_user_transform_ptr =
             png_get_user_transform_ptr(png_ptr);

      It is possible to have libpng flush any pending output, either
      manually, or automatically after a certain number of lines have been
      written.  To flush the output stream a single time call:

          png_write_flush(png_ptr);

      and to have libpng flush the output stream periodically after a
      certain number of scanlines have been written, call:

          png_set_flush(png_ptr, nrows);

      Note that the distance between rows is from the last time
      png_write_flush() was called, or the first row of the image if it has
      never been called.  So if you write 50 lines, and then png_set_flush
      25, it will flush the output on the next scanline, and every 25 lines
      thereafter, unless png_write_flush() is called before 25 more lines
      have been written.  If nrows is too small (less than about 10 lines
      for a 640 pixel wide RGB image) the image compression may decrease
      noticeably (although this may be acceptable for real-time
      applications).  Infrequent flushing will only degrade the compression
      performance by a few percent over images that do not use flushing.


    Writing the image data
      That's it for the transformations.  Now you can write the image data.
      The simplest way to do this is in one function call.  If you have the
      whole image in memory, you can just call png_write_image() and libpng
      will write the image.  You will need to pass in an array of pointers
      to each row.  This function automatically handles interlacing, so you
      don't need to call png_set_interlace_handling() or call this function
      multiple times, or any of that other stuff necessary with
      png_write_rows().

          png_write_image(png_ptr, row_pointers);

      where row_pointers is:

          png_byte *row_pointers[height];

      You can point to void or char or whatever you use for pixels.

      If you don't want to write the whole image at once, you can use
      png_write_rows() instead.  If the file is not interlaced, this is
      simple:




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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          png_write_rows(png_ptr, row_pointers,
             number_of_rows);

      row_pointers is the same as in the png_write_image() call.

      If you are just writing one row at a time, you can do this with a
      single row_pointer instead of an array of row_pointers:

          png_bytep row_pointer = row;

          png_write_row(png_ptr, row_pointer);

      When the file is interlaced, things can get a good deal more
      complicated.  The only currently (as of the PNG Specification version
      1.2, dated July 1999) defined interlacing scheme for PNG files is the
      "Adam7" interlace scheme, that breaks down an image into seven smaller
      images of varying size.  libpng will build these images for you, or
      you can do them yourself.  If you want to build them yourself, see the
      PNG specification for details of which pixels to write when.

      If you don't want libpng to handle the interlacing details, just use
      png_set_interlace_handling() and call png_write_rows() the correct
      number of times to write all the sub-images
      (png_set_interlace_handling() returns the number of sub-images.)

      If you want libpng to build the sub-images, call this before you start
      writing any rows:

          number_of_passes = png_set_interlace_handling(png_ptr);

      This will return the number of passes needed.  Currently, this is
      seven, but may change if another interlace type is added.

      Then write the complete image number_of_passes times.

          png_write_rows(png_ptr, row_pointers, number_of_rows);

      Think carefully before you write an interlaced image.  Typically code
      that reads such images reads all the image data into memory,
      uncompressed, before doing any processing.  Only code that can display
      an image on the fly can take advantage of the interlacing and even
      then the image has to be exactly the correct size for the output
      device, because scaling an image requires adjacent pixels and these
      are not available until all the passes have been read.

      If you do write an interlaced image you will hardly ever need to
      handle the interlacing yourself.  Call png_set_interlace_handling()
      and use the approach described above.

      The only time it is conceivable that you will really need to write an
      interlaced image pass-by-pass is when you have read one pass by pass



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      and made some pixel-by-pixel transformation to it, as described in the
      read code above.  In this case use the PNG_PASS_ROWS and PNG_PASS_COLS
      macros to determine the size of each sub-image in turn and simply
      write the rows you obtained from the read code.


    Finishing a sequential write
      After you are finished writing the image, you should finish writing
      the file.  If you are interested in writing comments or time, you
      should pass an appropriately filled png_info pointer.  If you are not
      interested, you can pass NULL.

          png_write_end(png_ptr, info_ptr);

      When you are done, you can free all memory used by libpng like this:

          png_destroy_write_struct(&png_ptr, &info_ptr);

      It is also possible to individually free the info_ptr members that
      point to libpng-allocated storage with the following function:

          png_free_data(png_ptr, info_ptr, mask, seq)

          mask  - identifies data to be freed, a mask
                  containing the bitwise OR of one or
                  more of
                    PNG_FREE_PLTE, PNG_FREE_TRNS,
                    PNG_FREE_HIST, PNG_FREE_ICCP,
                    PNG_FREE_PCAL, PNG_FREE_ROWS,
                    PNG_FREE_SCAL, PNG_FREE_SPLT,
                    PNG_FREE_TEXT, PNG_FREE_UNKN,
                  or simply PNG_FREE_ALL

          seq   - sequence number of item to be freed
                  (-1 for all items)

      This function may be safely called when the relevant storage has
      already been freed, or has not yet been allocated, or was allocated by
      the user  and not by libpng,  and will in those cases do nothing.  The
      "seq" parameter is ignored if only one item of the selected data type,
      such as PLTE, is allowed.  If "seq" is not -1, and multiple items are
      allowed for the data type identified in the mask, such as text or
      sPLT, only the n'th item in the structure is freed, where n is "seq".

      If you allocated data such as a palette that you passed in to libpng
      with png_set_*, you must not free it until just before the call to
      png_destroy_write_struct().

      The default behavior is only to free data that was allocated
      internally by libpng.  This can be changed, so that libpng will not
      free the data, or so that it will free data that was allocated by the



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      user with png_malloc() or png_calloc() and passed in via a png_set_*()
      function, with

          png_data_freer(png_ptr, info_ptr, freer, mask)

          freer  - one of
                     PNG_DESTROY_WILL_FREE_DATA
                     PNG_SET_WILL_FREE_DATA
                     PNG_USER_WILL_FREE_DATA

          mask   - which data elements are affected
                   same choices as in png_free_data()

      For example, to transfer responsibility for some data from a read
      structure to a write structure, you could use

          png_data_freer(read_ptr, read_info_ptr,
             PNG_USER_WILL_FREE_DATA,
             PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

          png_data_freer(write_ptr, write_info_ptr,
             PNG_DESTROY_WILL_FREE_DATA,
             PNG_FREE_PLTE|PNG_FREE_tRNS|PNG_FREE_hIST)

      thereby briefly reassigning responsibility for freeing to the user but
      immediately afterwards reassigning it once more to the write_destroy
      function.  Having done this, it would then be safe to destroy the read
      structure and continue to use the PLTE, tRNS, and hIST data in the
      write structure.

      This function only affects data that has already been allocated.  You
      can call this function before calling after the png_set_*() functions
      to control whether the user or png_destroy_*() is supposed to free the
      data.  When the user assumes responsibility for libpng-allocated data,
      the application must use png_free() to free it, and when the user
      transfers responsibility to libpng for data that the user has
      allocated, the user must have used png_malloc() or png_calloc() to
      allocate it.

      If you allocated text_ptr.text, text_ptr.lang, and
      text_ptr.translated_keyword separately, do not transfer responsibility
      for freeing text_ptr to libpng, because when libpng fills a png_text
      structure it combines these members with the key member, and
      png_free_data() will free only text_ptr.key.  Similarly, if you
      transfer responsibility for free'ing text_ptr from libpng to your
      application, your application must not separately free those members.
      For a more compact example of writing a PNG image, see the file
      example.c.






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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



 V. Simplified API
      The simplified API, which became available in libpng-1.6.0, hides the
      details of both libpng and the PNG file format itself.  It allows PNG
      files to be read into a very limited number of in-memory bitmap
      formats or to be written from the same formats.  If these formats do
      not accommodate your needs then you can, and should, use the more
      sophisticated APIs above - these support a wide variety of in-memory
      formats and a wide variety of sophisticated transformations to those
      formats as well as a wide variety of APIs to manipulate ancilliary
      information.

      To read a PNG file using the simplified API:

        1) Declare a 'png_image' structure (see below) on the stack, set the
           version field to PNG_IMAGE_VERSION and the 'opaque' pointer to
      NULL
           (this is REQUIRED, your program may crash if you don't do it.)

        2) Call the appropriate png_image_begin_read... function.

        3) Set the png_image 'format' member to the required sample format.

        4) Allocate a buffer for the image and, if required, the color-map.

        5) Call png_image_finish_read to read the image and, if required,
      the
           color-map into your buffers.

      There are no restrictions on the format of the PNG input itself; all
      valid color types, bit depths, and interlace methods are acceptable,
      and the input image is transformed as necessary to the requested in-
      memory format during the png_image_finish_read() step.  The only
      caveat is that if you request a color-mapped image from a PNG that is
      full-color or makes complex use of an alpha channel the transformation
      is extremely lossy and the result may look terrible.

      To write a PNG file using the simplified API:

        1) Declare a 'png_image' structure on the stack and memset()
           it to all zero.

        2) Initialize the members of the structure that describe the
           image, setting the 'format' member to the format of the
           image samples.

        3) Call the appropriate png_image_write... function with a
           pointer to the image and, if necessary, the color-map to write
           the PNG data.

      png_image is a structure that describes the in-memory format of an
      image when it is being read or defines the in-memory format of an



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      image that you need to write.  The "png_image" structure contains the
      following members:

         png_controlp opaque  Initialize to NULL, free with png_image_free
         png_uint_32  version Set to PNG_IMAGE_VERSION
         png_uint_32  width   Image width in pixels (columns)
         png_uint_32  height  Image height in pixels (rows)
         png_uint_32  format  Image format as defined below
         png_uint_32  flags   A bit mask containing informational flags
         png_uint_32  colormap_entries; Number of entries in the color-map
         png_uint_32  warning_or_error;
         char         message[64];

      In the event of an error or warning the "warning_or_error" field will
      be set to a non-zero value and the 'message' field will contain a ' '
      terminated string with the libpng error or warning message.  If both
      warnings and an error were encountered, only the error is recorded.
      If there are multiple warnings, only the first one is recorded.

      The upper 30 bits of the "warning_or_error" value are reserved; the
      low two bits contain a two bit code such that a value more than 1
      indicates a failure in the API just called:

         0 - no warning or error
         1 - warning
         2 - error
         3 - error preceded by warning

      The pixels (samples) of the image have one to four channels whose
      components have original values in the range 0 to 1.0:

        1: A single gray or luminance channel (G).
        2: A gray/luminance channel and an alpha channel (GA).
        3: Three red, green, blue color channels (RGB).
        4: Three color channels and an alpha channel (RGBA).

      The channels are encoded in one of two ways:

        a) As a small integer, value 0..255, contained in a single byte.
      For the alpha channel the original value is simply value/255.  For the
      color or luminance channels the value is encoded according to the sRGB
      specification and matches the 8-bit format expected by typical display
      devices.

      The color/gray channels are not scaled (pre-multiplied) by the alpha
      channel and are suitable for passing to color management software.

        b) As a value in the range 0..65535, contained in a 2-byte integer,
      in the native byte order of the platform on which the application is
      running.  All channels can be converted to the original value by
      dividing by 65535; all channels are linear.  Color channels use the



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      RGB encoding (RGB end-points) of the sRGB specification.  This
      encoding is identified by the PNG_FORMAT_FLAG_LINEAR flag below.

      When the simplified API needs to convert between sRGB and linear
      colorspaces, the actual sRGB transfer curve defined in the sRGB
      specification (see the article at http://en.wikipedia.org/wiki/SRGB)
      is used, not the gamma=1/2.2 approximation used elsewhere in libpng.

      When an alpha channel is present it is expected to denote pixel
      coverage of the color or luminance channels and is returned as an
      associated alpha channel: the color/gray channels are scaled (pre-
      multiplied) by the alpha value.

      The samples are either contained directly in the image data, between 1
      and 8 bytes per pixel according to the encoding, or are held in a
      color-map indexed by bytes in the image data.  In the case of a
      color-map the color-map entries are individual samples, encoded as
      above, and the image data has one byte per pixel to select the
      relevant sample from the color-map.

      PNG_FORMAT_*

      The #defines to be used in png_image::format.  Each #define identifies
      a particular layout of channel data and, if present, alpha values.
      There are separate defines for each of the two component encodings.

      A format is built up using single bit flag values.  All combinations
      are valid.  Formats can be built up from the flag values or you can
      use one of the predefined values below.  When testing formats always
      use the FORMAT_FLAG macros to test for individual features - future
      versions of the library may add new flags.

      When reading or writing color-mapped images the format should be set
      to the format of the entries in the color-map then
      png_image_{read,write}_colormap called to read or write the color-map
      and set the format correctly for the image data.  Do not set the
      PNG_FORMAT_FLAG_COLORMAP bit directly!

      NOTE: libpng can be built with particular features disabled. If you
      see compiler errors because the definition of one of the following
      flags has been compiled out it is because libpng does not have the
      required support.  It is possible, however, for the libpng
      configuration to enable the format on just read or just write; in that
      case you may see an error at run time.  You can guard against this by
      checking for the definition of the appropriate "_SUPPORTED" macro, one
      of:

         PNG_SIMPLIFIED_{READ,WRITE}_{BGR,AFIRST}_SUPPORTED

         PNG_FORMAT_FLAG_ALPHA    format with an alpha channel
         PNG_FORMAT_FLAG_COLOR    color format: otherwise grayscale



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



         PNG_FORMAT_FLAG_LINEAR   2-byte channels else 1-byte
         PNG_FORMAT_FLAG_COLORMAP image data is color-mapped
         PNG_FORMAT_FLAG_BGR      BGR colors, else order is RGB
         PNG_FORMAT_FLAG_AFIRST   alpha channel comes first

      Supported formats are as follows.  Future versions of libpng may
      support more formats; for compatibility with older versions simply
      check if the format macro is defined using #ifdef.  These defines
      describe the in-memory layout of the components of the pixels of the
      image.

      First the single byte (sRGB) formats:

         PNG_FORMAT_GRAY
         PNG_FORMAT_GA
         PNG_FORMAT_AG
         PNG_FORMAT_RGB
         PNG_FORMAT_BGR
         PNG_FORMAT_RGBA
         PNG_FORMAT_ARGB
         PNG_FORMAT_BGRA
         PNG_FORMAT_ABGR

      Then the linear 2-byte formats.  When naming these "Y" is used to
      indicate a luminance (gray) channel.  The component order within the
      pixel is always the same - there is no provision for swapping the
      order of the components in the linear format.  The components are 16-
      bit integers in the native byte order for your platform, and there is
      no provision for swapping the bytes to a different endian condition.

         PNG_FORMAT_LINEAR_Y
         PNG_FORMAT_LINEAR_Y_ALPHA
         PNG_FORMAT_LINEAR_RGB
         PNG_FORMAT_LINEAR_RGB_ALPHA

      With color-mapped formats the image data is one byte for each pixel.
      The byte is an index into the color-map which is formatted as above.
      To obtain a color-mapped format it is sufficient just to add the
      PNG_FOMAT_FLAG_COLORMAP to one of the above definitions, or you can
      use one of the definitions below.

         PNG_FORMAT_RGB_COLORMAP
         PNG_FORMAT_BGR_COLORMAP
         PNG_FORMAT_RGBA_COLORMAP
         PNG_FORMAT_ARGB_COLORMAP
         PNG_FORMAT_BGRA_COLORMAP
         PNG_FORMAT_ABGR_COLORMAP

      PNG_IMAGE macros

      These are convenience macros to derive information from a png_image



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      structure.  The PNG_IMAGE_SAMPLE_ macros return values appropriate to
      the actual image sample values - either the entries in the color-map
      or the pixels in the image.  The PNG_IMAGE_PIXEL_ macros return
      corresponding values for the pixels and will always return 1 for
      color-mapped formats.  The remaining macros return information about
      the rows in the image and the complete image.

      NOTE: All the macros that take a png_image::format parameter are
      compile time constants if the format parameter is, itself, a constant.
      Therefore these macros can be used in array declarations and case
      labels where required.  Similarly the macros are also pre-processor
      constants (sizeof is not used) so they can be used in #if tests.

        PNG_IMAGE_SAMPLE_CHANNELS(fmt)
          Returns the total number of channels in a given format: 1..4

        PNG_IMAGE_SAMPLE_COMPONENT_SIZE(fmt)
          Returns the size in bytes of a single component of a pixel or
      color-map
          entry (as appropriate) in the image: 1 or 2.

        PNG_IMAGE_SAMPLE_SIZE(fmt)
          This is the size of the sample data for one sample.  If the image
      is
          color-mapped it is the size of one color-map entry (and image
      pixels are
          one byte in size), otherwise it is the size of one image pixel.

        PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(fmt)
          The maximum size of the color-map required by the format expressed
      in a
          count of components.  This can be used to compile-time allocate a
          color-map:

          png_uint_16
      colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(linear_fmt)];

          png_byte
      colormap[PNG_IMAGE_MAXIMUM_COLORMAP_COMPONENTS(sRGB_fmt)];

          Alternatively use the PNG_IMAGE_COLORMAP_SIZE macro below to use
      the
          information from one of the png_image_begin_read_ APIs and
      dynamically
          allocate the required memory.

        PNG_IMAGE_COLORMAP_SIZE(fmt)
         The size of the color-map required by the format; this is the size
      of the
         color-map buffer passed to the png_image_{read,write}_colormap
      APIs. It is



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



         a fixed number determined by the format so can easily be allocated
      on the
         stack if necessary.

      Corresponding information about the pixels

        PNG_IMAGE_PIXEL_CHANNELS(fmt)
         The number of separate channels (components) in a pixel; 1 for a
         color-mapped image.

        PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)   The size, in bytes, of each
      component in a pixel; 1 for a color-mapped
         image.

        PNG_IMAGE_PIXEL_SIZE(fmt)
         The size, in bytes, of a complete pixel; 1 for a color-mapped
      image.

      Information about the whole row, or whole image

        PNG_IMAGE_ROW_STRIDE(image)
         Returns the total number of components in a single row of the
      image; this
         is the minimum 'row stride', the minimum count of components
      between each
         row.  For a color-mapped image this is the minimum number of bytes
      in a
         row.

         If you need the stride measured in bytes, row_stride_bytes is
         PNG_IMAGE_ROW_STRIDE(image) * PNG_IMAGE_PIXEL_COMPONENT_SIZE(fmt)
         plus any padding bytes that your application might need, for
      example
         to start the next row on a 4-byte boundary.

        PNG_IMAGE_BUFFER_SIZE(image, row_stride)
         Return the size, in bytes, of an image buffer given a png_image and
      a row
         stride - the number of components to leave space for in each row.

        PNG_IMAGE_SIZE(image)
         Return the size, in bytes, of the image in memory given just a
      png_image;
         the row stride is the minimum stride required for the image.

        PNG_IMAGE_COLORMAP_SIZE(image)
         Return the size, in bytes, of the color-map of this image.  If the
      image
         format is not a color-map format this will return a size sufficient
      for
         256 entries in the given format; check PNG_FORMAT_FLAG_COLORMAP if



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



         you don't want to allocate a color-map in this case.

      PNG_IMAGE_FLAG_*

      Flags containing additional information about the image are held in
      the 'flags' field of png_image.

        PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB == 0x01
          This indicates the the RGB values of the in-memory bitmap do not
          correspond to the red, green and blue end-points defined by sRGB.

        PNG_IMAGE_FLAG_FAST == 0x02
         On write emphasise speed over compression; the resultant PNG file
      will be
         larger but will be produced significantly faster, particular for
      large
         images.  Do not use this option for images which will be
      distributed, only
         used it when producing intermediate files that will be read back in
         repeatedly.  For a typical 24-bit image the option will double the
      read
         speed at the cost of increasing the image size by 25%, however for
      many
         more compressible images the PNG file can be 10 times larger with
      only a
         slight speed gain.

        PNG_IMAGE_FLAG_16BIT_sRGB == 0x04
          On read if the image is a 16-bit per component image and there is
      no gAMA
          or sRGB chunk assume that the components are sRGB encoded.  Notice
      that
          images output by the simplified API always have gamma information;
      setting
          this flag only affects the interpretation of 16-bit images from an
          external source.  It is recommended that the application expose
      this flag
          to the user; the user can normally easily recognize the difference
      between
          linear and sRGB encoding.  This flag has no effect on write - the
      data
          passed to the write APIs must have the correct encoding (as
      defined
          above.)

          If the flag is not set (the default) input 16-bit per component
      data is
          assumed to be linear.

          NOTE: the flag can only be set after the png_image_begin_read_
      call,



                                   - 95 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          because that call initializes the 'flags' field.

      READ APIs

         The png_image passed to the read APIs must have been initialized by
      setting
         the png_controlp field 'opaque' to NULL (or, better, memset the
      whole thing.)

         int png_image_begin_read_from_file( png_imagep image,
           const char *file_name)

           The named file is opened for read and the image header
           is filled in from the PNG header in the file.

         int png_image_begin_read_from_stdio (png_imagep image,
           FILE* file)

            The PNG header is read from the stdio FILE object.

         int png_image_begin_read_from_memory(png_imagep image,
            png_const_voidp memory, png_size_t size)

            The PNG header is read from the given memory buffer.

         int png_image_finish_read(png_imagep image,
            png_colorp background, void *buffer,
            png_int_32 row_stride, void *colormap));

            Finish reading the image into the supplied buffer and
            clean up the png_image structure.

            row_stride is the step, in png_byte or png_uint_16 units
            as appropriate, between adjacent rows.  A positive stride
            indicates that the top-most row is first in the buffer -
            the normal top-down arrangement.  A negative stride
            indicates that the bottom-most row is first in the buffer.

            background need only be supplied if an alpha channel must
            be removed from a png_byte format and the removal is to be
            done by compositing on a solid color; otherwise it may be
            NULL and any composition will be done directly onto the
            buffer.  The value is an sRGB color to use for the
            background, for grayscale output the green channel is used.

            For linear output removing the alpha channel is always done
            by compositing on black.

         void png_image_free(png_imagep image)

            Free any data allocated by libpng in image->opaque,



                                   - 96 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



            setting the pointer to NULL.  May be called at any time
            after the structure is initialized.

      When the simplified API needs to convert between sRGB and linear
      colorspaces, the actual sRGB transfer curve defined in the sRGB
      specification (see the article at http://en.wikipedia.org/wiki/SRGB)
      is used, not the gamma=1/2.2 approximation used elsewhere in libpng.

      WRITE APIS

      For write you must initialize a png_image structure to describe the
      image to be written:

         version: must be set to PNG_IMAGE_VERSION
         opaque: must be initialized to NULL
         width: image width in pixels
         height: image height in rows
         format: the format of the data you wish to write
         flags: set to 0 unless one of the defined flags applies; set
            PNG_IMAGE_FLAG_COLORSPACE_NOT_sRGB for color format images
            where the RGB values do not correspond to the colors in sRGB.
         colormap_entries: set to the number of entries in the color-map (0
      to 256)

         int png_image_write_to_file, (png_imagep image,
            const char *file, int convert_to_8bit, const void *buffer,
            png_int_32 row_stride, const void *colormap));

            Write the image to the named file.

         int png_image_write_to_memory (png_imagep image, void *memory,
            png_alloc_size_t * PNG_RESTRICT memory_bytes,
            int convert_to_8_bit, const void *buffer, ptrdiff_t row_stride,
            const void *colormap));

            Write the image to memory.

         int png_image_write_to_stdio(png_imagep image, FILE *file,
            int convert_to_8_bit, const void *buffer,
            png_int_32 row_stride, const void *colormap)

            Write the image to the given (FILE*).

      With all write APIs if image is in one of the linear formats with
      (png_uint_16) data then setting convert_to_8_bit will cause the output
      to be a (png_byte) PNG gamma encoded according to the sRGB
      specification, otherwise a 16-bit linear encoded PNG file is written.

      With all APIs row_stride is handled as in the read APIs - it is the
      spacing from one row to the next in component sized units (float) and
      if negative indicates a bottom-up row layout in the buffer.  If you



                                   - 97 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      pass zero, libpng will calculate the row_stride for you from the width
      and number of channels.

      Note that the write API does not support interlacing, sub-8-bit
      pixels, indexed (paletted) images, or most ancillary chunks.


 VI. Modifying/Customizing libpng
      There are two issues here.  The first is changing how libpng does
      standard things like memory allocation, input/output, and error
      handling.  The second deals with more complicated things like adding
      new chunks, adding new transformations, and generally changing how
      libpng works.  Both of those are compile-time issues; that is, they
      are generally determined at the time the code is written, and there is
      rarely a need to provide the user with a means of changing them.

      Memory allocation, input/output, and error handling

      All of the memory allocation, input/output, and error handling in
      libpng goes through callbacks that are user-settable.  The default
      routines are in pngmem.c, pngrio.c, pngwio.c, and pngerror.c,
      respectively.  To change these functions, call the appropriate
      png_set_*_fn() function.

      Memory allocation is done through the functions png_malloc(),
      png_calloc(), and png_free().  The png_malloc() and png_free()
      functions currently just call the standard C functions and
      png_calloc() calls png_malloc() and then clears the newly allocated
      memory to zero; note that png_calloc(png_ptr, size) is not the same as
      the calloc(number, size) function provided by stdlib.h.  There is
      limited support for certain systems with segmented memory
      architectures and the types of pointers declared by png.h match this;
      you will have to use appropriate pointers in your application.  If you
      prefer to use a different method of allocating and freeing data, you
      can use png_create_read_struct_2() or png_create_write_struct_2() to
      register your own functions as described above.  These functions also
      provide a void pointer that can be retrieved via

          mem_ptr=png_get_mem_ptr(png_ptr);

      Your replacement memory functions must have prototypes as follows:

          png_voidp malloc_fn(png_structp png_ptr,
             png_alloc_size_t size);

          void free_fn(png_structp png_ptr, png_voidp ptr);

      Your malloc_fn() must return NULL in case of failure.  The
      png_malloc() function will normally call png_error() if it receives a
      NULL from the system memory allocator or from your replacement
      malloc_fn().



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      Your free_fn() will never be called with a NULL ptr, since libpng's
      png_free() checks for NULL before calling free_fn().

      Input/Output in libpng is done through png_read() and png_write(),
      which currently just call fread() and fwrite().  The FILE * is stored
      in png_struct and is initialized via png_init_io().  If you wish to
      change the method of I/O, the library supplies callbacks that you can
      set through the function png_set_read_fn() and png_set_write_fn() at
      run time, instead of calling the png_init_io() function.  These
      functions also provide a void pointer that can be retrieved via the
      function png_get_io_ptr().  For example:

          png_set_read_fn(png_structp read_ptr,
              voidp read_io_ptr, png_rw_ptr read_data_fn)

          png_set_write_fn(png_structp write_ptr,
              voidp write_io_ptr, png_rw_ptr write_data_fn,
              png_flush_ptr output_flush_fn);

          voidp read_io_ptr = png_get_io_ptr(read_ptr);
          voidp write_io_ptr = png_get_io_ptr(write_ptr);

      The replacement I/O functions must have prototypes as follows:

          void user_read_data(png_structp png_ptr,
              png_bytep data, png_size_t length);

          void user_write_data(png_structp png_ptr,
              png_bytep data, png_size_t length);

          void user_flush_data(png_structp png_ptr);

      The user_read_data() function is responsible for detecting and
      handling end-of-data errors.

      Supplying NULL for the read, write, or flush functions sets them back
      to using the default C stream functions, which expect the io_ptr to
      point to a standard *FILE structure.  It is probably a mistake to use
      NULL for one of write_data_fn and output_flush_fn but not both of
      them, unless you have built libpng with PNG_NO_WRITE_FLUSH defined.
      It is an error to read from a write stream, and vice versa.

      Error handling in libpng is done through png_error() and
      png_warning().  Errors handled through png_error() are fatal, meaning
      that png_error() should never return to its caller.  Currently, this
      is handled via setjmp() and longjmp() (unless you have compiled libpng
      with PNG_NO_SETJMP, in which case it is handled via PNG_ABORT()), but
      you could change this to do things like exit() if you should wish, as
      long as your function does not return.

      On non-fatal errors, png_warning() is called to print a warning



                                   - 99 -           Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      message, and then control returns to the calling code.  By default
      png_error() and png_warning() print a message on stderr via fprintf()
      unless the library is compiled with PNG_NO_CONSOLE_IO defined (because
      you don't want the messages) or PNG_NO_STDIO defined (because
      fprintf() isn't available).  If you wish to change the behavior of the
      error functions, you will need to set up your own message callbacks.
      These functions are normally supplied at the time that the png_struct
      is created.  It is also possible to redirect errors and warnings to
      your own replacement functions after png_create_*_struct() has been
      called by calling:

          png_set_error_fn(png_structp png_ptr,
              png_voidp error_ptr, png_error_ptr error_fn,
              png_error_ptr warning_fn);

          png_voidp error_ptr = png_get_error_ptr(png_ptr);

      If NULL is supplied for either error_fn or warning_fn, then the libpng
      default function will be used, calling fprintf() and/or longjmp() if a
      problem is encountered.  The replacement error functions should have
      parameters as follows:

          void user_error_fn(png_structp png_ptr,
              png_const_charp error_msg);

          void user_warning_fn(png_structp png_ptr,
              png_const_charp warning_msg);

      The motivation behind using setjmp() and longjmp() is the C++ throw
      and catch exception handling methods.  This makes the code much easier
      to write, as there is no need to check every return code of every
      function call.  However, there are some uncertainties about the status
      of local variables after a longjmp, so the user may want to be careful
      about doing anything after setjmp returns non-zero besides returning
      itself.  Consult your compiler documentation for more details.  For an
      alternative approach, you may wish to use the "cexcept" facility (see
      http://cexcept.sourceforge.net), which is illustrated in pngvalid.c
      and in contrib/visupng.

      Beginning in libpng-1.4.0, the png_set_benign_errors() API became
      available.  You can use this to handle certain errors (normally
      handled as errors) as warnings.

          png_set_benign_errors (png_ptr, int allowed);

          allowed: 0: treat png_benign_error() as an error.
                   1: treat png_benign_error() as a warning.

      As of libpng-1.6.0, the default condition is to treat benign errors as
      warnings while reading and as errors while writing.




                                   - 100 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



    Custom chunks
      If you need to read or write custom chunks, you may need to get deeper
      into the libpng code.  The library now has mechanisms for storing and
      writing chunks of unknown type; you can even declare callbacks for
      custom chunks.  However, this may not be good enough if the library
      code itself needs to know about interactions between your chunk and
      existing `intrinsic' chunks.

      If you need to write a new intrinsic chunk, first read the PNG
      specification. Acquire a first level of understanding of how it works.
      Pay particular attention to the sections that describe chunk names,
      and look at how other chunks were designed, so you can do things
      similarly.  Second, check out the sections of libpng that read and
      write chunks.  Try to find a chunk that is similar to yours and use it
      as a template.  More details can be found in the comments inside the
      code.  It is best to handle private or unknown chunks in a generic
      method, via callback functions, instead of by modifying libpng
      functions. This is illustrated in pngtest.c, which uses a callback
      function to handle a private "vpAg" chunk and the new "sTER" chunk,
      which are both unknown to libpng.

      If you wish to write your own transformation for the data, look
      through the part of the code that does the transformations, and check
      out some of the simpler ones to get an idea of how they work.  Try to
      find a similar transformation to the one you want to add and copy off
      of it.  More details can be found in the comments inside the code
      itself.


    Configuring for gui/windowing platforms:
      You will need to write new error and warning functions that use the
      GUI interface, as described previously, and set them to be the error
      and warning functions at the time that png_create_*_struct() is
      called, in order to have them available during the structure
      initialization.  They can be changed later via png_set_error_fn().  On
      some compilers, you may also have to change the memory allocators
      (png_malloc, etc.).


    Configuring zlib:
      There are special functions to configure the compression.  Perhaps the
      most useful one changes the compression level, which currently uses
      input compression values in the range 0 - 9.  The library normally
      uses the default compression level (Z_DEFAULT_COMPRESSION = 6).  Tests
      have shown that for a large majority of images, compression values in
      the range 3-6 compress nearly as well as higher levels, and do so much
      faster.  For online applications it may be desirable to have maximum
      speed (Z_BEST_SPEED = 1).  With versions of zlib after v0.99, you can
      also specify no compression (Z_NO_COMPRESSION = 0), but this would
      create files larger than just storing the raw bitmap.  You can specify
      the compression level by calling:



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          #include zlib.h
          png_set_compression_level(png_ptr, level);

      Another useful one is to reduce the memory level used by the library.
      The memory level defaults to 8, but it can be lowered if you are short
      on memory (running DOS, for example, where you only have 640K).  Note
      that the memory level does have an effect on compression; among other
      things, lower levels will result in sections of incompressible data
      being emitted in smaller stored blocks, with a correspondingly larger
      relative overhead of up to 15% in the worst case.

          #include zlib.h
          png_set_compression_mem_level(png_ptr, level);

      The other functions are for configuring zlib.  They are not
      recommended for normal use and may result in writing an invalid PNG
      file.  See zlib.h for more information on what these mean.

          #include zlib.h
          png_set_compression_strategy(png_ptr,
              strategy);

          png_set_compression_window_bits(png_ptr,
              window_bits);

          png_set_compression_method(png_ptr, method);

      This controls the size of the IDAT chunks (default 8192):

          png_set_compression_buffer_size(png_ptr, size);

      As of libpng version 1.5.4, additional APIs became available to set
      these separately for non-IDAT compressed chunks such as zTXt, iTXt,
      and iCCP:

          #include zlib.h
          #if PNG_LIBPNG_VER >= 10504
          png_set_text_compression_level(png_ptr, level);

          png_set_text_compression_mem_level(png_ptr, level);

          png_set_text_compression_strategy(png_ptr,
              strategy);

          png_set_text_compression_window_bits(png_ptr,
              window_bits);

          png_set_text_compression_method(png_ptr, method);
          #endif





                                   - 102 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



    Controlling row filtering
      If you want to control whether libpng uses filtering or not, which
      filters are used, and how it goes about picking row filters, you can
      call one of these functions.  The selection and configuration of row
      filters can have a significant impact on the size and encoding speed
      and a somewhat lesser impact on the decoding speed of an image.
      Filtering is enabled by default for RGB and grayscale images (with and
      without alpha), but not for paletted images nor for any images with
      bit depths less than 8 bits/pixel.

      The 'method' parameter sets the main filtering method, which is
      currently only '0' in the PNG 1.2 specification.  The 'filters'
      parameter sets which filter(s), if any, should be used for each
      scanline.  Possible values are PNG_ALL_FILTERS, PNG_NO_FILTERS, or
      PNG_FAST_FILTERS to turn filtering on and off, or to turn on just the
      fast-decoding subset of filters, respectively.

      Individual filter types are PNG_FILTER_NONE, PNG_FILTER_SUB,
      PNG_FILTER_UP, PNG_FILTER_AVG, PNG_FILTER_PAETH, which can be bitwise
      ORed together with '|' to specify one or more filters to use.  These
      filters are described in more detail in the PNG specification.  If you
      intend to change the filter type during the course of writing the
      image, you should start with flags set for all of the filters you
      intend to use so that libpng can initialize its internal structures
      appropriately for all of the filter types.  (Note that this means the
      first row must always be adaptively filtered, because libpng currently
      does not allocate the filter buffers until png_write_row() is called
      for the first time.)

          filters = PNG_NO_FILTERS;
          filters = PNG_ALL_FILTERS;
          filters = PNG_FAST_FILTERS;

          or

          filters = PNG_FILTER_NONE | PNG_FILTER_SUB |
                    PNG_FILTER_UP | PNG_FILTER_AVG |
                    PNG_FILTER_PAETH;

          png_set_filter(png_ptr, PNG_FILTER_TYPE_BASE,
             filters);

                    The second parameter can also be
                    PNG_INTRAPIXEL_DIFFERENCING if you are
                    writing a PNG to be embedded in a MNG
                    datastream.  This parameter must be the
                    same as the value of filter_method used
                    in png_set_IHDR().






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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



    Requesting debug printout
      The macro definition PNG_DEBUG can be used to request debugging
      printout.  Set it to an integer value in the range 0 to 3.  Higher
      numbers result in increasing amounts of debugging information.  The
      information is printed to the "stderr" file, unless another file name
      is specified in the PNG_DEBUG_FILE macro definition.

      When PNG_DEBUG > 0, the following functions (macros) become available:

         png_debug(level, message)
         png_debug1(level, message, p1)
         png_debug2(level, message, p1, p2)

      in which "level" is compared to PNG_DEBUG to decide whether to print
      the message, "message" is the formatted string to be printed, and p1
      and p2 are parameters that are to be embedded in the string according
      to printf-style formatting directives.  For example,

         png_debug1(2, "foo=%d", foo);

      is expanded to

         if (PNG_DEBUG > 2)
            fprintf(PNG_DEBUG_FILE, "foo=%d\n", foo);

      When PNG_DEBUG is defined but is zero, the macros aren't defined, but
      you can still use PNG_DEBUG to control your own debugging:

         #ifdef PNG_DEBUG
             fprintf(stderr, ...
         #endif

      When PNG_DEBUG = 1, the macros are defined, but only png_debug
      statements having level = 0 will be printed.  There aren't any such
      statements in this version of libpng, but if you insert some they will
      be printed.


 VII. MNG support
      The MNG specification (available at http://www.libpng.org/pub/mng)
      allows certain extensions to PNG for PNG images that are embedded in
      MNG datastreams.  Libpng can support some of these extensions.  To
      enable them, use the png_permit_mng_features() function:

         feature_set = png_permit_mng_features(png_ptr, mask)

         mask is a png_uint_32 containing the bitwise OR of the
              features you want to enable.  These include
              PNG_FLAG_MNG_EMPTY_PLTE
              PNG_FLAG_MNG_FILTER_64
              PNG_ALL_MNG_FEATURES



                                   - 104 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



         feature_set is a png_uint_32 that is the bitwise AND of
            your mask with the set of MNG features that is
            supported by the version of libpng that you are using.

      It is an error to use this function when reading or writing a
      standalone PNG file with the PNG 8-byte signature.  The PNG datastream
      must be wrapped in a MNG datastream.  As a minimum, it must have the
      MNG 8-byte signature and the MHDR and MEND chunks.  Libpng does not
      provide support for these or any other MNG chunks; your application
      must provide its own support for them.  You may wish to consider using
      libmng (available at http://www.libmng.com) instead.


 VIII. Changes to Libpng from version
      It should be noted that versions of libpng later than 0.96 are not
      distributed by the original libpng author, Guy Schalnat, nor by
      Andreas Dilger, who had taken over from Guy during 1996 and 1997, and
      distributed versions 0.89 through 0.96, but rather by another member
      of the original PNG Group, Glenn Randers-Pehrson.  Guy and Andreas are
      still alive and well, but they have moved on to other things.

      The old libpng functions png_read_init(), png_write_init(),
      png_info_init(), png_read_destroy(), and png_write_destroy() have been
      moved to PNG_INTERNAL in version 0.95 to discourage their use.  These
      functions will be removed from libpng version 1.4.0.

      The preferred method of creating and initializing the libpng
      structures is via the png_create_read_struct(),
      png_create_write_struct(), and png_create_info_struct() because they
      isolate the size of the structures from the application, allow version
      error checking, and also allow the use of custom error handling
      routines during the initialization, which the old functions do not.
      The functions png_read_destroy() and png_write_destroy() do not
      actually free the memory that libpng allocated for these structs, but
      just reset the data structures, so they can be used instead of
      png_destroy_read_struct() and png_destroy_write_struct() if you feel
      there is too much system overhead allocating and freeing the
      png_struct for each image read.

      Setting the error callbacks via png_set_message_fn() before
      png_read_init() as was suggested in libpng-0.88 is no longer supported
      because this caused applications that do not use custom error
      functions to fail if the png_ptr was not initialized to zero.  It is
      still possible to set the error callbacks AFTER png_read_init(), or to
      change them with png_set_error_fn(), which is essentially the same
      function, but with a new name to force compilation errors with
      applications that try to use the old method.

      Support for the sCAL, iCCP, iTXt, and sPLT chunks was added at
      libpng-1.0.6; however, iTXt support was not enabled by default.




                                   - 105 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      Starting with version 1.0.7, you can find out which version of the
      library you are using at run-time:

         png_uint_32 libpng_vn = png_access_version_number();

      The number libpng_vn is constructed from the major version, minor
      version with leading zero, and release number with leading zero,
      (e.g., libpng_vn for version 1.0.7 is 10007).

      Note that this function does not take a png_ptr, so you can call it
      before you've created one.

      You can also check which version of png.h you used when compiling your
      application:

         png_uint_32 application_vn = PNG_LIBPNG_VER;


 IX. Changes to Libpng from version
      Support for user memory management was enabled by default.  To
      accomplish this, the functions png_create_read_struct_2(),
      png_create_write_struct_2(), png_set_mem_fn(), png_get_mem_ptr(),
      png_malloc_default(), and png_free_default() were added.

      Support for the iTXt chunk has been enabled by default as of version
      1.2.41.

      Support for certain MNG features was enabled.

      Support for numbered error messages was added.  However, we never got
      around to actually numbering the error messages.  The function
      png_set_strip_error_numbers() was added (Note: the prototype for this
      function was inadvertently removed from png.h in PNG_NO_ASSEMBLER_CODE
      builds of libpng-1.2.15.  It was restored in libpng-1.2.36).

      The png_malloc_warn() function was added at libpng-1.2.3.  This issues
      a png_warning and returns NULL instead of aborting when it fails to
      acquire the requested memory allocation.

      Support for setting user limits on image width and height was enabled
      by default.  The functions png_set_user_limits(),
      png_get_user_width_max(), and png_get_user_height_max() were added at
      libpng-1.2.6.

      The png_set_add_alpha() function was added at libpng-1.2.7.

      The function png_set_expand_gray_1_2_4_to_8() was added at libpng-
      1.2.9.  Unlike png_set_gray_1_2_4_to_8(), the new function does not
      expand the tRNS chunk to alpha. The png_set_gray_1_2_4_to_8() function
      is deprecated.




                                   - 106 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      A number of macro definitions in support of runtime selection of
      assembler code features (especially Intel MMX code support) were added
      at libpng-1.2.0:

          PNG_ASM_FLAG_MMX_SUPPORT_COMPILED
          PNG_ASM_FLAG_MMX_SUPPORT_IN_CPU
          PNG_ASM_FLAG_MMX_READ_COMBINE_ROW
          PNG_ASM_FLAG_MMX_READ_INTERLACE
          PNG_ASM_FLAG_MMX_READ_FILTER_SUB
          PNG_ASM_FLAG_MMX_READ_FILTER_UP
          PNG_ASM_FLAG_MMX_READ_FILTER_AVG
          PNG_ASM_FLAG_MMX_READ_FILTER_PAETH
          PNG_ASM_FLAGS_INITIALIZED
          PNG_MMX_READ_FLAGS
          PNG_MMX_FLAGS
          PNG_MMX_WRITE_FLAGS
          PNG_MMX_FLAGS

      We added the following functions in support of runtime selection of
      assembler code features:

          png_get_mmx_flagmask()
          png_set_mmx_thresholds()
          png_get_asm_flags()
          png_get_mmx_bitdepth_threshold()
          png_get_mmx_rowbytes_threshold()
          png_set_asm_flags()

      We replaced all of these functions with simple stubs in libpng-1.2.20,
      when the Intel assembler code was removed due to a licensing issue.

      These macros are deprecated:

          PNG_READ_TRANSFORMS_NOT_SUPPORTED
          PNG_PROGRESSIVE_READ_NOT_SUPPORTED
          PNG_NO_SEQUENTIAL_READ_SUPPORTED
          PNG_WRITE_TRANSFORMS_NOT_SUPPORTED
          PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED
          PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED

      They have been replaced, respectively, by:

          PNG_NO_READ_TRANSFORMS
          PNG_NO_PROGRESSIVE_READ
          PNG_NO_SEQUENTIAL_READ
          PNG_NO_WRITE_TRANSFORMS
          PNG_NO_READ_ANCILLARY_CHUNKS
          PNG_NO_WRITE_ANCILLARY_CHUNKS

      PNG_MAX_UINT was replaced with PNG_UINT_31_MAX.  It has been
      deprecated since libpng-1.0.16 and libpng-1.2.6.



                                   - 107 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      The function
          png_check_sig(sig, num) was replaced with
          !png_sig_cmp(sig, 0, num) It has been deprecated since libpng-
      0.90.

      The function
          png_set_gray_1_2_4_to_8() which also expands tRNS to alpha was
      replaced with
          png_set_expand_gray_1_2_4_to_8() which does not. It has been
      deprecated since libpng-1.0.18 and 1.2.9.


 X. Changes to Libpng from version
      Private libpng prototypes and macro definitions were moved from png.h
      and pngconf.h into a new pngpriv.h header file.

      Functions png_set_benign_errors(), png_benign_error(), and
      png_chunk_benign_error() were added.

      Support for setting the maximum amount of memory that the application
      will allocate for reading chunks was added, as a security measure.
      The functions png_set_chunk_cache_max() and png_get_chunk_cache_max()
      were added to the library.

      We implemented support for I/O states by adding png_ptr member
      io_state and functions png_get_io_chunk_name() and png_get_io_state()
      in pngget.c

      We added PNG_TRANSFORM_GRAY_TO_RGB to the available high-level input
      transforms.

      Checking for and reporting of errors in the IHDR chunk is more
      thorough.

      Support for global arrays was removed, to improve thread safety.

      Some obsolete/deprecated macros and functions have been removed.

      Typecasted NULL definitions such as
         #define png_voidp_NULL            (png_voidp)NULL were eliminated.
      If you used these in your application, just use NULL instead.

      The png_struct and info_struct members "trans" and "trans_values" were
      changed to "trans_alpha" and "trans_color", respectively.

      The obsolete, unused pnggccrd.c and pngvcrd.c files and related
      makefiles were removed.

      The PNG_1_0_X and PNG_1_2_X macros were eliminated.

      The PNG_LEGACY_SUPPORTED macro was eliminated.



                                   - 108 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      Many WIN32_WCE #ifdefs were removed.

      The functions png_read_init(info_ptr), png_write_init(info_ptr),
      png_info_init(info_ptr), png_read_destroy(), and png_write_destroy()
      have been removed.  They have been deprecated since libpng-0.95.

      The png_permit_empty_plte() was removed. It has been deprecated since
      libpng-1.0.9.  Use png_permit_mng_features() instead.

      We removed the obsolete stub functions png_get_mmx_flagmask(),
      png_set_mmx_thresholds(), png_get_asm_flags(),
      png_get_mmx_bitdepth_threshold(), png_get_mmx_rowbytes_threshold(),
      png_set_asm_flags(), and png_mmx_supported()

      We removed the obsolete png_check_sig(), png_memcpy_check(), and
      png_memset_check() functions.  Instead use !png_sig_cmp(), memcpy(),
      and memset(), respectively.

      The function png_set_gray_1_2_4_to_8() was removed. It has been
      deprecated since libpng-1.0.18 and 1.2.9, when it was replaced with
      png_set_expand_gray_1_2_4_to_8() because the former function also
      expanded any tRNS chunk to an alpha channel.

      Macros for png_get_uint_16, png_get_uint_32, and png_get_int_32 were
      added and are used by default instead of the corresponding functions.
      Unfortunately, from libpng-1.4.0 until 1.4.4, the png_get_uint_16
      macro (but not the function) incorrectly returned a value of type
      png_uint_32.

      We changed the prototype for png_malloc() from
          png_malloc(png_structp png_ptr, png_uint_32 size) to
          png_malloc(png_structp png_ptr, png_alloc_size_t size)

      This also applies to the prototype for the user replacement
      malloc_fn().

      The png_calloc() function was added and is used in place of of
      "png_malloc(); memset();" except in the case in png_read_png() where
      the array consists of pointers; in this case a "for" loop is used
      after the png_malloc() to set the pointers to NULL, to give robust.
      behavior in case the application runs out of memory part-way through
      the process.

      We changed the prototypes of png_get_compression_buffer_size() and
      png_set_compression_buffer_size() to work with png_size_t instead of
      png_uint_32.

      Support for numbered error messages was removed by default, since we
      never got around to actually numbering the error messages. The
      function png_set_strip_error_numbers() was removed from the library by
      default.



                                   - 109 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      The png_zalloc() and png_zfree() functions are no longer exported.
      The png_zalloc() function no longer zeroes out the memory that it
      allocates.  Applications that called png_zalloc(png_ptr, number, size)
      can call png_calloc(png_ptr, number*size) instead, and can call
      png_free() instead of png_zfree().

      Support for dithering was disabled by default in libpng-1.4.0, because
      it has not been well tested and doesn't actually "dither".  The code
      was not removed, however, and could be enabled by building libpng with
      PNG_READ_DITHER_SUPPORTED defined.  In libpng-1.4.2, this support was
      re-enabled, but the function was renamed png_set_quantize() to reflect
      more accurately what it actually does.  At the same time, the
      PNG_DITHER_[RED,GREEN_BLUE]_BITS macros were also renamed to
      PNG_QUANTIZE_[RED,GREEN,BLUE]_BITS, and PNG_READ_DITHER_SUPPORTED was
      renamed to PNG_READ_QUANTIZE_SUPPORTED.

      We removed the trailing '.' from the warning and error messages.


 XI. Changes to Libpng from version
      From libpng-1.4.0 until 1.4.4, the png_get_uint_16 macro (but not the
      function) incorrectly returned a value of type png_uint_32.  The
      incorrect macro was removed from libpng-1.4.5.

      Checking for invalid palette index on write was added at libpng
      1.5.10.  If a pixel contains an invalid (out-of-range) index libpng
      issues a benign error.  This is enabled by default because this
      condition is an error according to the PNG specification, Clause
      11.3.2, but the error can be ignored in each png_ptr with

         png_set_check_for_invalid_index(png_ptr, allowed);

            allowed  - one of
                       0: disable benign error (accept the
                          invalid data without warning).
                       1: enable benign error (treat the
                          invalid data as an error or a
                          warning).

      If the error is ignored, or if png_benign_error() treats it as a
      warning, any invalid pixels are decoded as opaque black by the decoder
      and written as-is by the encoder.

      Retrieving the maximum palette index found was added at libpng-1.5.15.
      This statement must appear after png_read_png() or png_read_image()
      while reading, and after png_write_png() or png_write_image() while
      writing.

         int max_palette = png_get_palette_max(png_ptr, info_ptr);

      This will return the maximum palette index found in the image, or "-1"



                                   - 110 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      if the palette was not checked, or "0" if no palette was found.  Note
      that this does not account for any palette index used by ancillary
      chunks such as the bKGD chunk; you must check those separately to
      determine the maximum palette index actually used.

      There are no substantial API changes between the non-deprecated parts
      of the 1.4.5 API and the 1.5.0 API; however, the ability to directly
      access members of the main libpng control structures, png_struct and
      png_info, deprecated in earlier versions of libpng, has been
      completely removed from libpng 1.5, and new private "pngstruct.h",
      "pnginfo.h", and "pngdebug.h" header files were created.

      We no longer include zlib.h in png.h.  The include statement has been
      moved to pngstruct.h, where it is not accessible by applications.
      Applications that need access to information in zlib.h will need to
      add the '#include "zlib.h"' directive.  It does not matter whether
      this is placed prior to or after the '"#include png.h"' directive.

      The png_sprintf(), png_strcpy(), and png_strncpy() macros are no
      longer used and were removed.

      We moved the png_strlen(), png_memcpy(), png_memset(), and
      png_memcmp() macros into a private header file (pngpriv.h) that is not
      accessible to applications.

      In png_get_iCCP, the type of "profile" was changed from png_charpp to
      png_bytepp, and in png_set_iCCP, from png_charp to png_const_bytep.

      There are changes of form in png.h, including new and changed macros
      to declare parts of the API.  Some API functions with arguments that
      are pointers to data not modified within the function have been
      corrected to declare these arguments with PNG_CONST.

      Much of the internal use of C macros to control the library build has
      also changed and some of this is visible in the exported header files,
      in particular the use of macros to control data and API elements
      visible during application compilation may require significant
      revision to application code.  (It is extremely rare for an
      application to do this.)

      Any program that compiled against libpng 1.4 and did not use
      deprecated features or access internal library structures should
      compile and work against libpng 1.5, except for the change in the
      prototype for png_get_iCCP() and png_set_iCCP() API functions
      mentioned above.

      libpng 1.5.0 adds PNG_ PASS macros to help in the reading and writing
      of interlaced images.  The macros return the number of rows and
      columns in each pass and information that can be used to de-interlace
      and (if absolutely necessary) interlace an image.




                                   - 111 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      libpng 1.5.0 adds an API png_longjmp(png_ptr, value).  This API calls
      the application-provided png_longjmp_ptr on the internal, but
      application initialized, longjmp buffer.  It is provided as a
      convenience to avoid the need to use the png_jmpbuf macro, which had
      the unnecessary side effect of resetting the internal png_longjmp_ptr
      value.

      libpng 1.5.0 includes a complete fixed point API.  By default this is
      present along with the corresponding floating point API.  In general
      the fixed point API is faster and smaller than the floating point one
      because the PNG file format used fixed point, not floating point.
      This applies even if the library uses floating point in internal
      calculations.  A new macro, PNG_FLOATING_ARITHMETIC_SUPPORTED, reveals
      whether the library uses floating point arithmetic (the default) or
      fixed point arithmetic internally for performance critical
      calculations such as gamma correction.  In some cases, the gamma
      calculations may produce slightly different results.  This has changed
      the results in png_rgb_to_gray and in alpha composition
      (png_set_background for example). This applies even if the original
      image was already linear (gamma == 1.0) and, therefore, it is not
      necessary to linearize the image.  This is because libpng has *not*
      been changed to optimize that case correctly, yet.

      Fixed point support for the sCAL chunk comes with an important caveat;
      the sCAL specification uses a decimal encoding of floating point
      values and the accuracy of PNG fixed point values is insufficient for
      representation of these values. Consequently a "string" API
      (png_get_sCAL_s and png_set_sCAL_s) is the only reliable way of
      reading arbitrary sCAL chunks in the absence of either the floating
      point API or internal floating point calculations.  Starting with
      libpng-1.5.0, both of these functions are present when
      PNG_sCAL_SUPPORTED is defined.  Prior to libpng-1.5.0, their presence
      also depended upon PNG_FIXED_POINT_SUPPORTED being defined and
      PNG_FLOATING_POINT_SUPPORTED not being defined.

      Applications no longer need to include the optional distribution
      header file pngusr.h or define the corresponding macros during
      application build in order to see the correct variant of the libpng
      API.  From 1.5.0 application code can check for the corresponding
      _SUPPORTED macro:

      #ifdef PNG_INCH_CONVERSIONS_SUPPORTED
         /* code that uses the inch conversion APIs. */ #endif

      This macro will only be defined if the inch conversion functions have
      been compiled into libpng.  The full set of macros, and whether or not
      support has been compiled in, are available in the header file
      pnglibconf.h.  This header file is specific to the libpng build.
      Notice that prior to 1.5.0 the _SUPPORTED macros would always have the
      default definition unless reset by pngusr.h or by explicit settings on
      the compiler command line.  These settings may produce compiler



                                   - 112 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      warnings or errors in 1.5.0 because of macro redefinition.

      Applications can now choose whether to use these macros or to call the
      corresponding function by defining PNG_USE_READ_MACROS or
      PNG_NO_USE_READ_MACROS before including png.h.  Notice that this is
      only supported from 1.5.0; defining PNG_NO_USE_READ_MACROS prior to
      1.5.0 will lead to a link failure.

      Prior to libpng-1.5.4, the zlib compressor used the same set of
      parameters when compressing the IDAT data and textual data such as
      zTXt and iCCP.  In libpng-1.5.4 we reinitialized the zlib stream for
      each type of data.  We added five png_set_text_*() functions for
      setting the parameters to use with textual data.

      Prior to libpng-1.5.4, the PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED
      option was off by default, and slightly inaccurate scaling occurred.
      This option can no longer be turned off, and the choice of accurate or
      inaccurate 16-to-8 scaling is by using the new png_set_scale_16_to_8()
      API for accurate scaling or the old png_set_strip_16_to_8() API for
      simple chopping.  In libpng-1.5.4, the
      PNG_READ_16_TO_8_ACCURATE_SCALE_SUPPORTED macro became
      PNG_READ_SCALE_16_TO_8_SUPPORTED, and the PNG_READ_16_TO_8 macro
      became PNG_READ_STRIP_16_TO_8_SUPPORTED, to enable the two
      png_set_*_16_to_8() functions separately.

      Prior to libpng-1.5.4, the png_set_user_limits() function could only
      be used to reduce the width and height limits from the value of
      PNG_USER_WIDTH_MAX and PNG_USER_HEIGHT_MAX, although this document
      said that it could be used to override them.  Now this function will
      reduce or increase the limits.

      Starting in libpng-1.5.10, the user limits can be set en masse with
      the configuration option PNG_SAFE_LIMITS_SUPPORTED.  If this option is
      enabled, a set of "safe" limits is applied in pngpriv.h.  These can be
      overridden by application calls to png_set_user_limits(),
      png_set_user_chunk_cache_max(), and/or png_set_user_malloc_max() that
      increase or decrease the limits.  Also, in libpng-1.5.10 the default
      width and height limits were increased from 1,000,000 to 0x7fffffff
      (i.e., made unlimited).  Therefore, the limits are now
                                     default      safe
         png_user_width_max        0x7fffffff    1,000,000
         png_user_height_max       0x7fffffff    1,000,000
         png_user_chunk_cache_max  0 (unlimited)   128
         png_user_chunk_malloc_max 0 (unlimited) 8,000,000

      The png_set_option() function (and the "options" member of the png
      struct) was added to libpng-1.5.15, with option PNG_ARM_NEON.

      The library now supports a complete fixed point implementation and can
      thus be used on systems that have no floating point support or very
      limited or slow support.  Previously gamma correction, an essential



                                   - 113 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      part of complete PNG support, required reasonably fast floating point.

      As part of this the choice of internal implementation has been made
      independent of the choice of fixed versus floating point APIs and all
      the missing fixed point APIs have been implemented.

      The exact mechanism used to control attributes of API functions has
      changed, as described in the INSTALL file.

      A new test program, pngvalid, is provided in addition to pngtest.
      pngvalid validates the arithmetic accuracy of the gamma correction
      calculations and includes a number of validations of the file format.
      A subset of the full range of tests is run when "make check" is done
      (in the 'configure' build.)  pngvalid also allows total allocated
      memory usage to be evaluated and performs additional memory overwrite
      validation.

      Many changes to individual feature macros have been made. The
      following are the changes most likely to be noticed by library
      builders who configure libpng:

      1) All feature macros now have consistent naming:

      #define PNG_NO_feature turns the feature off #define
      PNG_feature_SUPPORTED turns the feature on

      pnglibconf.h contains one line for each feature macro which is either:

      #define PNG_feature_SUPPORTED

      if the feature is supported or:

      /*#undef PNG_feature_SUPPORTED*/

      if it is not.  Library code consistently checks for the 'SUPPORTED'
      macro.  It does not, and libpng applications should not, check for the
      'NO' macro which will not normally be defined even if the feature is
      not supported.  The 'NO' macros are only used internally for setting
      or not setting the corresponding 'SUPPORTED' macros.

      Compatibility with the old names is provided as follows:

      PNG_INCH_CONVERSIONS turns on PNG_INCH_CONVERSIONS_SUPPORTED

      And the following definitions disable the corresponding feature:

      PNG_SETJMP_NOT_SUPPORTED disables SETJMP
      PNG_READ_TRANSFORMS_NOT_SUPPORTED disables READ_TRANSFORMS
      PNG_NO_READ_COMPOSITED_NODIV disables READ_COMPOSITE_NODIV
      PNG_WRITE_TRANSFORMS_NOT_SUPPORTED disables WRITE_TRANSFORMS
      PNG_READ_ANCILLARY_CHUNKS_NOT_SUPPORTED disables READ_ANCILLARY_CHUNKS



                                   - 114 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      PNG_WRITE_ANCILLARY_CHUNKS_NOT_SUPPORTED disables
      WRITE_ANCILLARY_CHUNKS

      Library builders should remove use of the above, inconsistent, names.

      2) Warning and error message formatting was previously conditional on
      the STDIO feature. The library has been changed to use the CONSOLE_IO
      feature instead. This means that if CONSOLE_IO is disabled the library
      no longer uses the printf(3) functions, even though the default
      read/write implementations use (FILE) style stdio.h functions.

      3) Three feature macros now control the fixed/floating point
      decisions:

      PNG_FLOATING_POINT_SUPPORTED enables the floating point APIs

      PNG_FIXED_POINT_SUPPORTED enables the fixed point APIs; however, in
      practice these are normally required internally anyway (because the
      PNG file format is fixed point), therefore in most cases
      PNG_NO_FIXED_POINT merely stops the function from being exported.

      PNG_FLOATING_ARITHMETIC_SUPPORTED chooses between the internal
      floating point implementation or the fixed point one.  Typically the
      fixed point implementation is larger and slower than the floating
      point implementation on a system that supports floating point;
      however, it may be faster on a system which lacks floating point
      hardware and therefore uses a software emulation.

      4) Added PNG_{READ,WRITE}_INT_FUNCTIONS_SUPPORTED.  This allows the
      functions to read and write ints to be disabled independently of
      PNG_USE_READ_MACROS, which allows libpng to be built with the
      functions even though the default is to use the macros - this allows
      applications to choose at app buildtime whether or not to use macros
      (previously impossible because the functions weren't in the default
      build.)


 XII. Changes to Libpng from version
      A "simplified API" has been added (see documentation in png.h and a
      simple example in contrib/examples/pngtopng.c).  The new publicly
      visible API includes the following:

         macros:
           PNG_FORMAT_*
           PNG_IMAGE_*
         structures:
           png_control
           png_image
         read functions
           png_image_begin_read_from_file()
           png_image_begin_read_from_stdio()



                                   - 115 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



           png_image_begin_read_from_memory()
           png_image_finish_read()
           png_image_free()
         write functions
           png_image_write_to_file()
           png_image_write_to_memory()
           png_image_write_to_stdio()

      Starting with libpng-1.6.0, you can configure libpng to prefix all
      exported symbols, using the PNG_PREFIX macro.

      We no longer include string.h in png.h.  The include statement has
      been moved to pngpriv.h, where it is not accessible by applications.
      Applications that need access to information in string.h must add an
      '#include <string.h>' directive.  It does not matter whether this is
      placed prior to or after the '#include "png.h"' directive.

      The following API are now DEPRECATED:
         png_info_init_3()
         png_convert_to_rfc1123() which has been replaced
           with png_convert_to_rfc1123_buffer()
         png_malloc_default()
         png_free_default()
         png_reset_zstream()

      The following have been removed:
         png_get_io_chunk_name(), which has been replaced
           with png_get_io_chunk_type().  The new
           function returns a 32-bit integer instead of
           a string.
         The png_sizeof(), png_strlen(), png_memcpy(), png_memcmp(), and
           png_memset() macros are no longer used in the libpng sources and
           have been removed.  These had already been made invisible to
      applications
           (i.e., defined in the private pngpriv.h header file) since
      libpng-1.5.0.

      The signatures of many exported functions were changed, such that
         png_structp became png_structrp or png_const_structrp
         png_infop became png_inforp or png_const_inforp where "rp"
      indicates a "restricted pointer".

      Dropped support for 16-bit platforms. The support for FAR/far types
      has been eliminated and the definition of png_alloc_size_t is now
      controlled by a flag so that 'small size_t' systems can select it if
      necessary.

      Error detection in some chunks has improved; in particular the iCCP
      chunk reader now does pretty complete validation of the basic format.
      Some bad profiles that were previously accepted are now accepted with
      a warning or rejected, depending upon the png_set_benign_errors()



                                   - 116 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      setting, in particular the very old broken Microsoft/HP 3144-byte sRGB
      profile.  Starting with libpng-1.6.11, recognizing and checking sRGB
      profiles can be avoided by means of

          #if defined(PNG_SKIP_sRGB_CHECK_PROFILE) &&
      defined(PNG_SET_OPTION_SUPPORTED)
             png_set_option(png_ptr, PNG_SKIP_sRGB_CHECK_PROFILE,
                 PNG_OPTION_ON);
          #endif

      It's not a good idea to do this if you are using the "simplified API",
      which needs to be able to recognize sRGB profiles conveyed via the
      iCCP chunk.

      The PNG spec requirement that only grayscale profiles may appear in
      images with color type 0 or 4 and that even if the image only contains
      gray pixels, only RGB profiles may appear in images with color type 2,
      3, or 6, is now enforced.  The sRGB chunk is allowed to appear in
      images with any color type and is interpreted by libpng to convey a
      one-tracer-curve gray profile or a three-tracer-curve RGB profile as
      appropriate.

      Libpng 1.5.x erroneously used /MD for Debug DLL builds; if you used
      the debug builds in your app and you changed your app to use /MD you
      will need to change it back to /MDd for libpng 1.6.x.

      Prior to libpng-1.6.0 a warning would be issued if the iTXt chunk
      contained an empty language field or an empty translated keyword.
      Both of these are allowed by the PNG specification, so these warnings
      are no longer issued.

      The library now issues an error if the application attempts to set a
      transform after it calls png_read_update_info() or if it attempts to
      call both png_read_update_info() and png_start_read_image() or to call
      either of them more than once.

      The default condition for benign_errors is now to treat benign errors
      as warnings while reading and as errors while writing.

      The library now issues a warning if both background processing and RGB
      to gray are used when gamma correction happens. As with previous
      versions of the library the results are numerically very incorrect in
      this case.

      There are some minor arithmetic changes in some transforms such as
      png_set_background(), that might be detected by certain regression
      tests.

      Unknown chunk handling has been improved internally, without any API
      change.  This adds more correct option control of the unknown
      handling, corrects a pre-existing bug where the per-chunk 'keep'



                                   - 117 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      setting is ignored, and makes it possible to skip IDAT chunks in the
      sequential reader.

      The machine-generated configure files are no longer included in
      branches libpng16 and later of the GIT repository.  They continue to
      be included in the tarball releases, however.

      Libpng-1.6.0 through 1.6.2 used the CMF bytes at the beginning of the
      IDAT stream to set the size of the sliding window for reading instead
      of using the default 32-kbyte sliding window size.  It was discovered
      that there are hundreds of PNG files in the wild that have incorrect
      CMF bytes that caused zlib to issue the "invalid distance too far
      back" error and reject the file.  Libpng-1.6.3 and later calculate
      their own safe CMF from the image dimensions, provide a way to revert
      to the libpng-1.5.x behavior (ignoring the CMF bytes and using a 32-
      kbyte sliding window), by using

          png_set_option(png_ptr, PNG_MAXIMUM_INFLATE_WINDOW,
              PNG_OPTION_ON);

      and provide a tool (contrib/tools/pngfix) for rewriting a PNG file
      while optimizing the CMF bytes in its IDAT chunk correctly.

      Libpng-1.6.0 and libpng-1.6.1 wrote uncompressed iTXt chunks with the
      wrong length, which resulted in PNG files that cannot be read beyond
      the bad iTXt chunk.  This error was fixed in libpng-1.6.3, and a tool
      (called contrib/tools/png-fix-itxt) has been added to the libpng
      distribution.

      Starting with libpng-1.6.17, the PNG_SAFE_LIMITS macro was eliminated
      and safe limits are used by default (users who need larger limits can
      still override them at compile time or run time, as described above).

      The new limits are
                                      default   spec limit
         png_user_width_max         1,000,000  2,147,483,647
         png_user_height_max        1,000,000  2,147,483,647
         png_user_chunk_cache_max         128  unlimited
         png_user_chunk_malloc_max  8,000,000  unlimited

      Starting with libpng-1.6.18, a PNG_RELEASE_BUILD macro was added,
      which allows library builders to control compilation for an installed
      system (a release build).  It can be set for testing debug or beta
      builds to ensure that they will compile when the build type is
      switched to RC or STABLE. In essence this overrides the
      PNG_LIBPNG_BUILD_BASE_TYPE definition which is not directly user
      controllable.

      Starting with libpng-1.6.19, attempting to set an over-length PLTE
      chunk is an error. Previously this requirement of the PNG
      specification was not enforced, and the palette was always limited to



                                   - 118 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      256 entries. An over-length PLTE chunk found in an input PNG is
      silently truncated.


 XIII. Detecting libpng
      The png_get_io_ptr() function has been present since libpng-0.88, has
      never changed, and is unaffected by conditional compilation macros.
      It is the best choice for use in configure scripts for detecting the
      presence of any libpng version since 0.88.  In an autoconf
      "configure.in" you could use

          AC_CHECK_LIB(png, png_get_io_ptr, ...


 XV. Source code repository
      Since about February 2009, version 1.2.34, libpng has been under "git"
      source control.  The git repository was built from old libpng-
      x.y.z.tar.gz files going back to version 0.70.  You can access the git
      repository (read only) at

          git://git.code.sf.net/p/libpng/code

      or you can browse it with a web browser by selecting the "code" button
      at

          https://sourceforge.net/projects/libpng

      Patches can be sent to glennrp at users.sourceforge.net or to png-
      mng-implement at lists.sourceforge.net or you can upload them to the
      libpng bug tracker at

          http://libpng.sourceforge.net

      We also accept patches built from the tar or zip distributions, and
      simple verbal discriptions of bug fixes, reported either to the
      SourceForge bug tracker, to the png-mng-implement at lists.sf.net
      mailing list, or directly to glennrp.


 XV. Coding style
      Our coding style is similar to the "Allman" style (See
      http://en.wikipedia.org/wiki/Indent_style#Allman_style), with curly
      braces on separate lines:

          if (condition)
          {
             action;
          }

          else if (another condition)
          {



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



             another action;
          }

      The braces can be omitted from simple one-line actions:

          if (condition)
             return (0);

      We use 3-space indentation, except for continued statements which are
      usually indented the same as the first line of the statement plus four
      more spaces.

      For macro definitions we use 2-space indentation, always leaving the
      "#" in the first column.

          #ifndef PNG_NO_FEATURE
          #  ifndef PNG_FEATURE_SUPPORTED
          #    define PNG_FEATURE_SUPPORTED
          #  endif
          #endif

      Comments appear with the leading "/*" at the same indentation as the
      statement that follows the comment:

          /* Single-line comment */
          statement;

          /* This is a multiple-line
           * comment.
           */
          statement;

      Very short comments can be placed after the end of the statement to
      which they pertain:

          statement;    /* comment */

      We don't use C++ style ("//") comments. We have, however, used them in
      the past in some now-abandoned MMX assembler code.

      Functions and their curly braces are not indented, and exported
      functions are marked with PNGAPI:

       /* This is a public function that is visible to
        * application programmers. It does thus-and-so.
        */
       void PNGAPI
       png_exported_function(png_ptr, png_info, foo)
       {
          body;
       }



                                   - 120 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      The return type and decorations are placed on a separate line ahead of
      the function name, as illustrated above.

      The prototypes for all exported functions appear in png.h, above the
      comment that says

          /* Maintainer: Put new public prototypes here ... */

      We mark all non-exported functions with "/* PRIVATE */"":

       void /* PRIVATE */
       png_non_exported_function(png_ptr, png_info, foo)
       {
          body;
       }

      The prototypes for non-exported functions (except for those in
      pngtest) appear in pngpriv.h above the comment that says

        /* Maintainer: Put new private prototypes here ^ */

      To avoid polluting the global namespace, the names of all exported
      functions and variables begin with "png_", and all publicly visible C
      preprocessor macros begin with "PNG".  We request that applications
      that use libpng *not* begin any of their own symbols with either of
      these strings.

      We put a space after the "sizeof" operator and we omit the optional
      parentheses around its argument when the argument is an expression,
      not a type name, and we always enclose the sizeof operator, with its
      argument, in parentheses:

        (sizeof (png_uint_32))
        (sizeof array)

      Prior to libpng-1.6.0 we used a "png_sizeof()" macro, formatted as
      though it were a function.

      Control keywords if, for, while, and switch are always followed by a
      space to distinguish them from function calls, which have no trailing
      space.

      We put a space after each comma and after each semicolon in "for"
      statements, and we put spaces before and after each C binary operator
      and after "for" or "while", and before "?".  We don't put a space
      between a typecast and the expression being cast, nor do we put one
      between a function name and the left parenthesis that follows it:

          for (i = 2; i > 0; --i)
             y[i] = a(x) + (int)b;




                                   - 121 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      We prefer #ifdef and #ifndef to #if defined() and #if !defined() when
      there is only one macro being tested.  We always use parentheses with
      "defined".

      We express integer constants that are used as bit masks in hex format,
      with an even number of lower-case hex digits, and to make them
      unsigned (e.g., 0x00U, 0xffU, 0x0100U) and long if they are greater
      than 0x7fff (e.g., 0xffffUL).

      We prefer to use underscores rather than camelCase in names, except
      for a few type names that we inherit from zlib.h.

      We prefer "if (something != 0)" and "if (something == 0)" over "if
      (something)" and if "(!something)", respectively, and for pointers we
      prefer "if (some_pointer != NULL)" or "if (some_pointer == NULL)".

      We do not use the TAB character for indentation in the C sources.

      Lines do not exceed 80 characters.

      Other rules can be inferred by inspecting the libpng source.


 XVI. Y2K Compliance in libpng
      Since the PNG Development group is an ad-hoc body, we can't make an
      official declaration.

      This is your unofficial assurance that libpng from version 0.71 and
      upward through 1.6.29 are Y2K compliant.  It is my belief that earlier
      versions were also Y2K compliant.

      Libpng only has two year fields.  One is a 2-byte unsigned integer
      that will hold years up to 65535.  The other, which is deprecated,
      holds the date in text format, and will hold years up to 9999.

      The integer is
          "png_uint_16 year" in png_time_struct.

      The string is
          "char time_buffer[29]" in png_struct.  This is no longer used in
      libpng-1.6.x and will be removed from libpng-1.7.0.

      There are seven time-related functions:

          png_convert_to_rfc_1123_buffer() in png.c
            (formerly png_convert_to_rfc_1152() in error, and
            also formerly png_convert_to_rfc_1123())
          png_convert_from_struct_tm() in pngwrite.c, called
            in pngwrite.c
          png_convert_from_time_t() in pngwrite.c
          png_get_tIME() in pngget.c



                                   - 122 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



          png_handle_tIME() in pngrutil.c, called in pngread.c
          png_set_tIME() in pngset.c
          png_write_tIME() in pngwutil.c, called in pngwrite.c

      All appear to handle dates properly in a Y2K environment.  The
      png_convert_from_time_t() function calls gmtime() to convert from
      system clock time, which returns (year - 1900), which we properly
      convert to the full 4-digit year.  There is a possibility that
      applications using libpng are not passing 4-digit years into the
      png_convert_to_rfc_1123() function, or that they are incorrectly
      passing only a 2-digit year instead of "year - 1900" into the
      png_convert_from_struct_tm() function, but this is not under our
      control.  The libpng documentation has always stated that it works
      with 4-digit years, and the APIs have been documented as such.

      The tIME chunk itself is also Y2K compliant.  It uses a 2-byte
      unsigned integer to hold the year, and can hold years as large as
      65535.

      zlib, upon which libpng depends, is also Y2K compliant.  It contains
      no date-related code.


         Glenn Randers-Pehrson
         libpng maintainer
         PNG Development Group


 NOTE
      Note about libpng version numbers:

      Due to various miscommunications, unforeseen code incompatibilities
      and occasional factors outside the authors' control, version numbering
      on the library has not always been consistent and straightforward.
      The following table summarizes matters since version 0.89c, which was
      the first widely used release:

       source             png.h  png.h  shared-lib
       version            string   int  version
       -------            ------  ----- ----------
       0.89c "1.0 beta 3"     0.89      89  1.0.89
       0.90  "1.0 beta 4"     0.90      90  0.90  [should have been 2.0.90]
       0.95  "1.0 beta 5"     0.95      95  0.95  [should have been 2.0.95]
       0.96  "1.0 beta 6"     0.96      96  0.96  [should have been 2.0.96]
       0.97b "1.00.97 beta 7" 1.00.97   97  1.0.1 [should have been 2.0.97]
       0.97c                  0.97      97  2.0.97
       0.98                   0.98      98  2.0.98
       0.99                   0.99      98  2.0.99
       0.99a-m                0.99      99  2.0.99
       1.00                   1.00     100  2.1.0 [100 should be 10000]
       1.0.0      (from here on, the   100  2.1.0 [100 should be 10000]



                                   - 123 -          Formatted:  May 25, 2017






 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



       1.0.1       png.h string is   10001  2.1.0
       1.0.1a-e    identical to the  10002  from here on, the shared library
       1.0.2       source version)   10002  is 2.V where V is the source
      code
       1.0.2a-b                      10003  version, except as noted.
       1.0.3                         10003
       1.0.3a-d                      10004
       1.0.4                         10004
       1.0.4a-f                      10005
       1.0.5 (+ 2 patches)           10005
       1.0.5a-d                      10006
       1.0.5e-r                      10100 (not source compatible)
       1.0.5s-v                      10006 (not binary compatible)
       1.0.6 (+ 3 patches)           10006 (still binary incompatible)
       1.0.6d-f                      10007 (still binary incompatible)
       1.0.6g                        10007
       1.0.6h                        10007  10.6h (testing xy.z so-
      numbering)
       1.0.6i                        10007  10.6i
       1.0.6j                        10007  2.1.0.6j (incompatible with
      1.0.0)
       1.0.7beta11-14        DLLNUM  10007  2.1.0.7beta11-14 (binary
      compatible)
       1.0.7beta15-18           1    10007  2.1.0.7beta15-18 (binary
      compatible)
       1.0.7rc1-2               1    10007  2.1.0.7rc1-2 (binary compatible)
       1.0.7                    1    10007  (still compatible)
       ...
       1.0.19                  10    10019  10.so.0.19[.0]
       ...
       1.2.57                  13    10257  12.so.0.56[.0]
       ...
       1.5.28                  15    10528  15.so.15.28[.0]
       ...
       1.6.29                  16    10629  16.so.16.29[.0]

      Henceforth the source version will match the shared-library minor and
      patch numbers; the shared-library major version number will be used
      for changes in backward compatibility, as it is intended.  The
      PNG_PNGLIB_VER macro, which is not used within libpng but is available
      for applications, is an unsigned integer of the form xyyzz
      corresponding to the source version x.y.z (leading zeros in y and z).
      Beta versions were given the previous public release number plus a
      letter, until version 1.0.6j; from then on they were given the
      upcoming public release number plus "betaNN" or "rcNN".


 SEE ALSO
      libpngpf(3), png(5) libpng:





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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



           http://libpng.sourceforge.net (follow the [DOWNLOAD] link)
           http://www.libpng.org/pub/png

           zlib:

           (generally) at the same location as libpng or at
           ftp://ftp.info-zip.org/pub/infozip/zlib

           PNGspecification:RFC2083

           (generally) at the same location as libpng or at
           ftp://ftp.rfc-editor.org:/in-notes/rfc2083.txt
           or (as a W3C Recommendation) at
           http://www.w3.org/TR/REC-png.html

           In the case of any inconsistency between the PNG specification
           and this library, the specification takes precedence.


 AUTHORS
      This man page: Glenn Randers-Pehrson <glennrp at
      users.sourceforge.net>

      The contributing authors would like to thank all those who helped with
      testing, bug fixes, and patience.  This wouldn't have been possible
      without all of you.

      Thanks to Frank J. T. Wojcik for helping with the documentation.

      Libpng version 1.6.29 - March 16, 2017: Initially created in 1995 by
      Guy Eric Schalnat, then of Group 42, Inc.  Currently maintained by
      Glenn Randers-Pehrson (glennrp at users.sourceforge.net).

      Supported by the PNG development group
      png-mng-implement at lists.sf.net (subscription required; visit png-
      mng-implement at lists.sourceforge.net (subscription required; visit
      https://lists.sourceforge.net/lists/listinfo/png-mng-implement to
      subscribe).


 NOTICES:
      This copy of the libpng notices is provided for your convenience.  In
      case of any discrepancy between this copy and the notices in the file
      png.h that is included in the libpng distribution, the latter shall
      prevail.

      COPYRIGHT NOTICE, DISCLAIMER, and LICENSE:

      If you modify libpng you may insert additional notices immediately
      following this sentence.




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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      This code is released under the libpng license.

      libpng versions 1.0.7, July 1, 2000 through 1.6.29, March 16, 2017 are
      Copyright (c) 2000-2002, 2004, 2006-2017 Glenn Randers-Pehrson, are
      derived from libpng-1.0.6, and are distributed according to the same
      disclaimer and license as libpng-1.0.6 with the following individuals
      added to the list of Contributing Authors:

         Simon-Pierre Cadieux
         Eric S. Raymond
         Mans Rullgard
         Cosmin Truta
         Gilles Vollant
         James Yu
         Mandar Sahastrabuddhe
         Google Inc.
         Vadim Barkov

      and with the following additions to the disclaimer:

         There is no warranty against interference with your enjoyment of
      the
         library or against infringement.  There is no warranty that our
         efforts or the library will fulfill any of your particular purposes
         or needs.  This library is provided with all faults, and the entire
         risk of satisfactory quality, performance, accuracy, and effort is
      with
         the user.

      Some files in the "contrib" directory and some configure-generated
      files that are distributed with libpng have other copyright owners and
      are released under other open source licenses.

      libpng versions 0.97, January 1998, through 1.0.6, March 20, 2000, are
      Copyright (c) 1998-2000 Glenn Randers-Pehrson, are derived from
      libpng-0.96, and are distributed according to the same disclaimer and
      license as libpng-0.96, with the following individuals added to the
      list of Contributing Authors:

         Tom Lane
         Glenn Randers-Pehrson
         Willem van Schaik

      libpng versions 0.89, June 1996, through 0.96, May 1997, are Copyright
      (c) 1996-1997 Andreas Dilger, are derived from libpng-0.88, and are
      distributed according to the same disclaimer and license as libpng-
      0.88, with the following individuals added to the list of Contributing
      Authors:

         John Bowler
         Kevin Bracey



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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



         Sam Bushell
         Magnus Holmgren
         Greg Roelofs
         Tom Tanner

      Some files in the "scripts" directory have other copyright owners but
      are released under this license.

      libpng versions 0.5, May 1995, through 0.88, January 1996, are
      Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.

      For the purposes of this copyright and license, "Contributing Authors"
      is defined as the following set of individuals:

         Andreas Dilger
         Dave Martindale
         Guy Eric Schalnat
         Paul Schmidt
         Tim Wegner

      The PNG Reference Library is supplied "AS IS".  The Contributing
      Authors and Group 42, Inc. disclaim all warranties, expressed or
      implied, including, without limitation, the warranties of
      merchantability and of fitness for any purpose.  The Contributing
      Authors and Group 42, Inc.  assume no liability for direct, indirect,
      incidental, special, exemplary, or consequential damages, which may
      result from the use of the PNG Reference Library, even if advised of
      the possibility of such damage.

      Permission is hereby granted to use, copy, modify, and distribute this
      source code, or portions hereof, for any purpose, without fee, subject
      to the following restrictions:

        1. The origin of this source code must not be misrepresented.

        2. Altered versions must be plainly marked as such and must not
           be misrepresented as being the original source.

        3. This Copyright notice may not be removed or altered from any
           source or altered source distribution.

      The Contributing Authors and Group 42, Inc. specifically permit,
      without fee, and encourage the use of this source code as a component
      to supporting the PNG file format in commercial products.  If you use
      this source code in a product, acknowledgment is not required but
      would be appreciated.

      END OF COPYRIGHT NOTICE, DISCLAIMER, and LICENSE.

      TRADEMARK:




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 LIBPNG(3)                                                         LIBPNG(3)
                               March 16, 2017



      The name "libpng" has not been registered by the Copyright owner as a
      trademark in any jurisdiction.  However, because libpng has been
      distributed and maintained world-wide, continually since 1995, the
      Copyright owner claims "common-law trademark protection" in any
      jurisdiction where common-law trademark is recognized.

      OSI CERTIFICATION:

      Libpng is OSI Certified Open Source Software.  OSI Certified Open
      Source is a certification mark of the Open Source Initiative. OSI has
      not addressed the additional disclaimers inserted at version 1.0.7.

      EXPORT CONTROL:

      The Copyright owner believes that the Export Control Classification
      Number (ECCN) for libpng is EAR99, which means not subject to export
      controls or International Traffic in Arms Regulations (ITAR) because
      it is open source, publicly available software, that does not contain
      any encryption software.  See the EAR, paragraphs 734.3(b)(3) and
      734.7(b).

      A "png_get_copyright" function is available, for convenient use in
      "about" boxes and the like:

         printf("%s", png_get_copyright(NULL));

      Also, the PNG logo (in PNG format, of course) is supplied in the files
      "pngbar.png" and "pngbar.jpg (88x31) and "pngnow.png" (98x31).

      Glenn Randers-Pehrson glennrp at users.sourceforge.net March 16, 2017
























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