JPEGTRAN(1) JPEGTRAN(1)
28 August 2019
NAME
jpegtran - lossless transformation of JPEG files
SYNOPSIS
jpegtran [ options ] [ filename ]
DESCRIPTION
jpegtran performs various useful transformations of JPEG files. It
can translate the coded representation from one variant of JPEG to
another, for example from baseline JPEG to progressive JPEG or vice
versa. It can also perform some rearrangements of the image data, for
example turning an image from landscape to portrait format by
rotation.
For EXIF files and JPEG files containing Exif data, you may prefer to
use exiftran instead.
jpegtran works by rearranging the compressed data (DCT coefficients),
without ever fully decoding the image. Therefore, its transformations
are lossless: there is no image degradation at all, which would not be
true if you used djpeg followed by cjpeg to accomplish the same
conversion. But by the same token, jpegtran cannot perform lossy
operations such as changing the image quality. However, while the
image data is losslessly transformed, metadata can be removed. See
the -copy option for specifics.
jpegtran reads the named JPEG/JFIF file, or the standard input if no
file is named, and produces a JPEG/JFIF file on the standard output.
OPTIONS
All switch names may be abbreviated; for example, -optimize may be
written -opt or -o. Upper and lower case are equivalent. British
spellings are also accepted (e.g., -optimise), though for brevity
these are not mentioned below.
To specify the coded JPEG representation used in the output file,
jpegtran accepts a subset of the switches recognized by cjpeg:
-optimize
Perform optimization of entropy encoding parameters.
-progressive
Create progressive JPEG file.
-restart N
Emit a JPEG restart marker every N MCU rows, or every N MCU
blocks if "B" is attached to the number.
-arithmetic
Use arithmetic coding.
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-scans file
Use the scan script given in the specified text file.
See cjpeg(1) for more details about these switches. If you specify
none of these switches, you get a plain baseline-JPEG output file.
The quality setting and so forth are determined by the input file.
The image can be losslessly transformed by giving one of these
switches:
-flip horizontal
Mirror image horizontally (left-right).
-flip vertical
Mirror image vertically (top-bottom).
-rotate 90
Rotate image 90 degrees clockwise.
-rotate 180
Rotate image 180 degrees.
-rotate 270
Rotate image 270 degrees clockwise (or 90 ccw).
-transpose
Transpose image (across UL-to-LR axis).
-transverse
Transverse transpose (across UR-to-LL axis).
The transpose transformation has no restrictions regarding image
dimensions. The other transformations operate rather oddly if
the image dimensions are not a multiple of the iMCU size (usually
8 or 16 pixels), because they can only transform complete blocks
of DCT coefficient data in the desired way.
jpegtran's default behavior when transforming an odd-size image
is designed to preserve exact reversibility and mathematical
consistency of the transformation set. As stated, transpose is
able to flip the entire image area. Horizontal mirroring leaves
any partial iMCU column at the right edge untouched, but is able
to flip all rows of the image. Similarly, vertical mirroring
leaves any partial iMCU row at the bottom edge untouched, but is
able to flip all columns. The other transforms can be built up
as sequences of transpose and flip operations; for consistency,
their actions on edge pixels are defined to be the same as the
end result of the corresponding transpose-and-flip sequence.
For practical use, you may prefer to discard any untransformable
edge pixels rather than having a strange-looking strip along the
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right and/or bottom edges of a transformed image. To do this,
add the -trim switch:
-trim
Drop non-transformable edge blocks.
Obviously, a transformation with -trim is not reversible, so
strictly speaking jpegtran with this switch is not lossless.
Also, the expected mathematical equivalences between the
transformations no longer hold. For example, -rot 270 -trim
trims only the bottom edge, but -rot 90 -trim followed by -rot
180 -trim trims both edges.
If you are only interested in perfect transformation, add the
-perfect switch:
-perfect
Fails with an error if the transformation is not perfect.
For example you may want to do
(jpegtran -rot 90 -perfect foo.jpg || djpeg foo.jpg | pnmflip
-r90 | cjpeg)
to do a perfect rotation if available or an approximated one if
not.
We also offer a lossless-crop option, which discards data outside a
given image region but losslessly preserves what is inside. Like the
rotate and flip transforms, lossless crop is restricted by the current
JPEG format: the upper left corner of the selected region must fall on
an iMCU boundary. If this does not hold for the given crop
parameters, we silently move the upper left corner up and/or left to
make it so, simultaneously increasing the region dimensions to keep
the lower right crop corner unchanged. (Thus, the output image covers
at least the requested region, but may cover more.) The adjustment of
the region dimensions may be optionally disabled by attaching an 'f'
character ("force") to the width or height number.
The image can be losslessly cropped by giving the switch:
-crop WxH+X+Y
Crop to a rectangular subarea of width W, height H starting at
point X,Y.
Crop extension: The width or height parameters can be made larger than
the source image. In this case the extra area is filled in with zero
(neutral gray). A larger width parameter has two more options:
Attaching an 'f' character ("flatten") to the width number will fill
in the extra area with the DC of the adjacent block, instead of gray
out. Attaching an 'r' character ("reflect") to the width number will
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fill in the extra area with repeated reflections of the source region,
instead of gray out.
A complementary lossless-wipe option is provided to discard (gray out)
data inside a given image region while losslessly preserving what is
outside:
-wipe WxH+X+Y
Wipe (gray out) a rectangular subarea of width W, height H
starting at point X,Y.
Attaching an 'f' character ("flatten") to the width number will fill
the region with the average of adjacent blocks, instead of gray out.
In case the wipe region and outside area form two horizontally
adjacent rectangles, attaching an 'r' character ("reflect") to the
width number will fill the region with repeated reflections of the
outside area, instead of gray out.
Another option is lossless-drop, which replaces data at a given image
position by another image:
-drop +X+Y filename
Drop another image
Both source images must have the same subsampling values. It is best
if they also have the same quantization, otherwise quantization
adaption occurs. The trim option can be used with the drop option to
requantize the drop file to the source file.
Other not-strictly-lossless transformation switches are:
-grayscale
Force grayscale output.
This option discards the chrominance channels if the input image
is YCbCr (ie, a standard color JPEG), resulting in a grayscale
JPEG file. The luminance channel is preserved exactly, so this
is a better method of reducing to grayscale than decompression,
conversion, and recompression. This switch is particularly handy
for fixing a monochrome picture that was mistakenly encoded as a
color JPEG. (In such a case, the space savings from getting rid
of the near-empty chroma channels won't be large; but the
decoding time for a grayscale JPEG is substantially less than
that for a color JPEG.)
-scale M/N
Scale the output image by a factor M/N.
Currently supported scale factors are M/N with all M from 1 to
16, where N is the source DCT size, which is 8 for baseline JPEG.
If the /N part is omitted, then M specifies the DCT scaled size
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to be applied on the given input. For baseline JPEG this is
equivalent to M/8 scaling, since the source DCT size for baseline
JPEG is 8. Caution: An implementation of the JPEG SmartScale
extension is required for this feature. SmartScale enabled JPEG
is not yet widely implemented, so many decoders will be unable to
view a SmartScale extended JPEG file at all.
jpegtran also recognizes these switches that control what to do with
"extra" markers, such as comment blocks:
-copy none
Copy no extra markers from source file. This setting suppresses
all comments and other metadata in the source file.
-copy comments
Copy only comment markers. This setting copies comments from the
source file, but discards any other metadata.
-copy all
Copy all extra markers. This setting preserves metadata found in
the source file, such as JFIF thumbnails, Exif data, and
Photoshop settings. In some files these extra markers can be
sizable. Note that this option will copy thumbnails as-is; they
will not be transformed.
The default behavior is -copy comments. (Note: in IJG releases
v6 and v6a, jpegtran always did the equivalent of -copy none.)
Additional switches recognized by jpegtran are:
-maxmemory N
Set limit for amount of memory to use in processing large images.
Value is in thousands of bytes, or millions of bytes if "M" is
attached to the number. For example, -max 4m selects 4000000
bytes. If more space is needed, temporary files will be used.
-outfile name
Send output image to the named file, not to standard output.
-verbose
Enable debug printout. More -v's give more output. Also,
version information is printed at startup.
-debug
Same as -verbose.
EXAMPLES
This example converts a baseline JPEG file to progressive form:
jpegtran -progressive foo.jpg > fooprog.jpg
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This example rotates an image 90 degrees clockwise, discarding any
unrotatable edge pixels:
jpegtran -rot 90 -trim foo.jpg > foo90.jpg
ENVIRONMENT
JPEGMEM
If this environment variable is set, its value is the default
memory limit. The value is specified as described for the
-maxmemory switch. JPEGMEM overrides the default value specified
when the program was compiled, and itself is overridden by an
explicit -maxmemory.
SEE ALSO
cjpeg(1), djpeg(1), rdjpgcom(1), wrjpgcom(1)
Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
AUTHOR
Independent JPEG Group
BUGS
The transform options can't transform odd-size images perfectly. Use
-trim or -perfect if you don't like the results.
The entire image is read into memory and then written out again, even
in cases where this isn't really necessary. Expect swapping on large
images, especially when using the more complex transform options.
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