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NAME
glut - an introduction to the OpenGL Utility Toolkit
SYNOPSIS
#include <GL/glut.h>
DESCRIPTION
The OpenGL Utility Toolkit (GLUT) is a programming interface with ANSI
C and FORTRAN bindings for writing window system independent OpenGL
programs. The toolkit supports the following functionality:
Multiple windows for OpenGL rendering.
Callback driven event processing.
Sophisticated input devices.
An ``idle'' routine and timers.
A simple, cascading pop-up menu facility.
Utility routines to generate various solid and wire frame
objects.
Support for bitmap and stroke fonts.
Miscellaneous window management functions, including managing
overlays. An ANSI C implementation of GLUT for the X Window
System has been implemented by the author. Windows NT and OS/2
versions of GLUT are also available.
BACKGROUND
One of the major accomplishments in the specification of OpenGL was
the isolation of window system dependencies from OpenGL's rendering
model. The result is that OpenGL is window system independent.
Window system operations such as the creation of a rendering window
and the handling of window system events are left to the native window
system to define. Necessary interactions between OpenGL and the window
system such as creating and binding an OpenGL context to a window are
described separately from the OpenGL specification in a window system
dependent specification. For example, the GLX specification describes
the standard by which OpenGL interacts with the X Window System.
The predecessor to OpenGL is IRIS GL. Unlike OpenGL, IRIS GL does
specify how rendering windows are created and manipulated. IRIS GL's
windowing interface is reasonably popular largely because it is simple
to use. IRIS GL programmers can worry about graphics programming
without needing to be an expert in programming the native window
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system. Experience also demonstrated that IRIS GL's windowing
interface was high-level enough that it could be retargeted to
different window systems. Silicon Graphics migrated from NeWS to the
X Window System without any major changes to IRIS GL's basic windowing
interface.
Removing window system operations from OpenGL is a sound decision
because it allows the OpenGL graphics system to be retargeted to
various systems including powerful but expensive graphics workstations
as well as mass-production graphics systems like video games, set-top
boxes for interactive television, and PCs.
Unfortunately, the lack of a window system interface for OpenGL is a
gap in OpenGL's utility. Learning native window system APIs such as
the X Window System's Xlib or Motif can be daunting. Even those
familiar with native window system APIs need to understand the
interface that binds OpenGL to the native window system. And when an
OpenGL program is written using the native window system interface,
despite the portability of the program's OpenGL rendering code, the
program itself will be window system dependent.
Testing and documenting OpenGL's functionality lead to the development
of the tk and aux toolkits. The aux toolkit is used in the examples
found in the OpenGL Programming Guide. Unfortunately, aux has numerous
limitations and its utility is largely limited to toy programs. The tk
library has more functionality than aux but was developed in an ad hoc
fashion and still lacks much important functionality that IRIS GL
programmers expect, like pop-up menus and overlays.
GLUT is designed to fill the need for a window system independent
programming interface for OpenGL programs. The interface is designed
to be simple yet still meet the needs of useful OpenGL programs.
Features from the IRIS GL, aux, and tk interfaces are included to make
it easy for programmers used to these interfaces to develop programs
for GLUT.
PHILOSPHY
GLUT simplifies the implementation of programs using OpenGL rendering.
The GLUT application programming interface (API) requires very few
routines to display a graphics scene rendered using OpenGL. The GLUT
API (like the OpenGL API) is stateful. Most initial GLUT state is
defined and the initial state is reasonable for simple programs.
The GLUT routines also take relatively few parameters. No pointers are
returned. The only pointers passed into GLUT are pointers to character
strings (all strings passed to GLUT are copied, not referenced) and
opaque font handles.
The GLUT API is (as much as reasonable) window system independent. For
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this reason, GLUT does not return any native window system handles,
pointers, or other data structures. More subtle window system
dependencies such as reliance on window system dependent fonts are
avoided by GLUT; instead, GLUT supplies its own (limited) set of
fonts.
For programming ease, GLUT provides a simple menu sub-API. While the
menuing support is designed to be implemented as pop-up menus, GLUT
gives window system leeway to support the menu functionality in
another manner (pull-down menus for example).
Two of the most important pieces of GLUT state are the current window
and current menu. Most window and menu routines affect the current
window or menu respectively. Most callbacks implicitly set the current
window and menu to the appropriate window or menu responsible for the
callback. GLUT is designed so that a program with only a single window
and/or menu will not need to keep track of any window or menu
identifiers. This greatly simplifies very simple GLUT programs.
GLUT is designed for simple to moderately complex programs focused on
OpenGL rendering. GLUT implements its own event loop. For this reason,
mixing GLUT with other APIs that demand their own event handling
structure may be difficult. The advantage of a builtin event dispatch
loop is simplicity.
GLUT contains routines for rendering fonts and geometric objects,
however GLUT makes no claims on the OpenGL display list name space.
For this reason, none of the GLUT rendering routines use OpenGL
display lists. It is up to the GLUT programmer to compile the output
from GLUT rendering routines into display lists if this is desired.
GLUT routines are logically organized into several sub-APIs according
to their functionality. The sub-APIs are:
Initialization.
Command line processing, window system initialization, and
initial window creation state are controlled by these routines.
Beginning Event Processing.
This routine enters GLUT's event processing loop. This routine
never returns, and it continuously calls GLUT callbacks as
necessary.
Window Management.
These routines create and control windows.
Overlay Management.
These routines establish and manage overlays for windows.
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Menu Management.
These routines create and control pop-up menus.
Callback Registration.
These routines register callbacks to be called by the GLUT event
processing loop.
Color Index Colormap Management.
These routines allow the manipulation of color index colormaps
for windows.
State Retrieval.
These routines allows programs to retrieve state from GLUT.
Font Rendering.
These routines allow rendering of stroke and bitmap fonts.
Geometric Shape Rendering.
These routines allow the rendering of 3D geometric objects
including spheres, cones, icosahedrons, and teapots.
CONVENTIONS
GLUT window and screen coordinates are expressed in pixels. The upper
left hand corner of the screen or a window is (0,0). X coordinates
increase in a rightward direction; Y coordinates increase in a
downward direction. Note: This is inconsistent with OpenGL's
coordinate scheme that generally considers the lower left hand
coordinate of a window to be at (0,0) but is consistent with most
popular window systems.
Integer identifiers in GLUT begin with one, not zero. So window
identifiers, menu identifiers, and menu item indexes are based from
one, not zero.
In GLUT's ANSI C binding, for most routines, basic types (int, char*)
are used as parameters. In routines where the parameters are directly
passed to OpenGL routines, OpenGL types (GLfloat) are used.
The header files for GLUT should be included in GLUT programs with the
following include directive:
#include <GL/glut.h>
Because a very large window system software vendor (who will remain
nameless) has an apparent inability to appreciate that OpenGL's API is
independent of their window system API, portable ANSI C GLUT programs
should not directly include <GL/gl.h> or <GL/glu.h>. Instead, ANSI C
GLUT programs should rely on <GL/glut.h> to include the necessary
OpenGL and GLU related header files.
The ANSI C GLUT library archive is typically named libglut.a on Unix
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systems. GLUT programs need to link with the system's OpenGL and GLUT
libraries (and any libraries these libraries potentially depend on). A
set of window system dependent libraries may also be necessary for
linking GLUT programs. For example, programs using the X11 GLUT
implementation typically need to link with Xlib, the X extension
library, possibly the X Input extension library, the X miscellaneous
utilities library, and the math library. An example X11/Unix compile
line would look like:
cc -o foo foo.c -lglut -lGLU -lGL -lXmu -lXi -lXext -lX11 -lm
SEE ALSO
glutAddMenuEntry, glutAddSubMenu, glutAttachMenu, glutBitmapCharacter,
glutBitmapWidth, glutButtonBoxFunc, glutChangeToMenuEntry,
glutChangeToSubMenu, glutCopyColormap, glutCreateMenu,
glutCreateSubWindow, glutCreateWindow, glutDestroyMenu,
glutDestroyWindow, glutDeviceGet, glutDialsFunc, glutDisplayFunc,
glutEntryFunc, glutEstablishOverlay, glutExtensionSupported,
glutFullScreen, glutGet, glutGetColor, glutGetModifiers, glutIdleFunc,
glutInit, glutInitDisplayMode, glutInitWindowPosition,
glutKeyboardFunc, glutLayerGet, glutMainLoop, glutMenuStatusFunc,
glutMotionFunc, glutMouseFunc, glutOverlayDisplayFunc, glutPopWindow,
glutPositionWindow, glutPostOverlayRedisplay, glutPostRedisplay,
glutRemoveMenuItem, glutRemoveOverlay, glutReshapeFunc,
glutReshapeWindow, glutSetColor, glutSetCursor, glutSetMenu,
glutSetWindow, glutSetWindowTitle, glutShowOverlay, glutShowWindow,
glutSolidCone, glutSolidCube, glutSolidDodecahedron,
glutSolidIcosahedron, glutSolidOctahedron, glutSolidSphere,
glutSolidTeapot, glutSolidTetrahedron, glutSolidTorus,
glutSpaceballButtonFunc, glutSpaceballMotionFunc,
glutSpaceballRotateFunc, glutSpecialFunc, glutStrokeCharacter,
glutStrokeWidth, glutSwapBuffers, glutTabletButtonFunc,
glutTabletMotionFunc, glutTimerFunc, glutUseLayer, glutVisibilityFunc,
REFERENCES
Mark Kilgard, Programming OpenGL for the X Window System, Addison-
Wesley, ISBN 0-201-48359-9, 1996.
Mark Kilgard, The OpenGL Utility Toolkit (GLUT) Programming Interface
API Version 3 (the official GLUT specification).
WEB REFERENCES
Main GLUT page
http://reality.sgi.com/mjk/glut3/glut3.html
GLUT Frequently Asked Question list
http://reality.sgi.com/mjk/glut3/glut-faq.html
The OpenGL Utility Toolkit (GLUT) Programming Interface API Version 3
http://reality.sgi.com/mjk/spec3/spec3.html
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http://reality.sgi.com/mjk/glut3/glut-3.spec.ps.gz
OpenGL and X: An OpenGL Toolkit article (PostScript)
http://reality.sgi.com/mjk/glut3/glut.column1.ps.gz
AUTHOR
Mark J. Kilgard (mjk@nvidia.com)
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