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Last Update: 07-31-17

# The OpenGL Extension Wrangler Library

## Initializing GLEW

First you need to create a valid OpenGL rendering context and call `glewInit()` to initialize the extension entry points. If `glewInit()` returns `GLEW_OK`, the initialization succeeded and you can use the available extensions as well as core OpenGL functionality. For example:

#include <GL/glew.h>
#include <GL/glut.h>
...
glutInit(&argc, argv);
glutCreateWindow("GLEW Test");
GLenum err = glewInit();
if (GLEW_OK != err)
{
/* Problem: glewInit failed, something is seriously wrong. */
fprintf(stderr, "Error: %s\n", glewGetErrorString(err));
...
}
fprintf(stdout, "Status: Using GLEW %s\n", glewGetString(GLEW_VERSION));

## Checking for Extensions

Starting from GLEW 1.1.0, you can find out if a particular extension is available on your platform by querying globally defined variables of the form `GLEW_{extension_name}`:

if (GLEW_ARB_vertex_program)
{
/* It is safe to use the ARB_vertex_program extension here. */
glGenProgramsARB(...);
}

In GLEW 1.0.x, a global structure was used for this task. To ensure binary compatibility between releases, the struct was replaced with a set of variables.

You can also check for core OpenGL functionality. For example, to see if OpenGL 1.3 is supported, do the following:

if (GLEW_VERSION_1_3)
{
/* Yay! OpenGL 1.3 is supported! */
}

In general, you can check if `GLEW_{extension_name}` or `GLEW_VERSION_{version}` is true or false.

It is also possible to perform extension checks from string input. Starting from the 1.3.0 release, use `glewIsSupported` to check if the required core or extension functionality is available:

if (glewIsSupported("GL_VERSION_1_4 GL_ARB_point_sprite"))
{
/* Great, we have OpenGL 1.4 + point sprites. */
}

For extensions only, `glewGetExtension` provides a slower alternative (GLEW 1.0.x-1.2.x). Note that in the 1.3.0 release `glewGetExtension` was replaced with `glewIsSupported`.

if (glewGetExtension("GL_ARB_fragment_program"))
{
/* Looks like ARB_fragment_program is supported. */
}

## Experimental Drivers

GLEW obtains information on the supported extensions from the graphics driver. Experimental or pre-release drivers, however, might not report every available extension through the standard mechanism, in which case GLEW will report it unsupported. To circumvent this situation, the `glewExperimental` global switch can be turned on by setting it to `GL_TRUE` before calling `glewInit()`, which ensures that all extensions with valid entry points will be exposed.

## Platform Specific Extensions

Platform specific extensions are separated into two header files: `wglew.h` and `glxew.h`, which define the available `WGL` and `GLX` extensions. To determine if a certain extension is supported, query `WGLEW_{extension name}` or `GLXEW_{extension_name}`. For example:

#include <GL/wglew.h>
if (WGLEW_ARB_pbuffer)
{
/* OK, we can use pbuffers. */
}
else
{
/* Sorry, pbuffers will not work on this platform. */
}

Alternatively, use `wglewIsSupported` or `glxewIsSupported` to check for extensions from a string:

if (wglewIsSupported("WGL_ARB_pbuffer"))
{
/* OK, we can use pbuffers. */
}

## Utilities

GLEW provides two command-line utilities: one for creating a list of available extensions and visuals; and another for verifying extension entry points.

### visualinfo: extensions and visuals

`visualinfo` is an extended version of `glxinfo`. The Windows version creates a file called `visualinfo.txt`, which contains a list of available OpenGL, WGL, and GLU extensions as well as a table of visuals aka. pixel formats. Pbuffer and MRT capable visuals are also included. For additional usage information, type `visualinfo -h`.

### glewinfo: extension verification utility

`glewinfo` allows you to verify the entry points for the extensions supported on your platform. The Windows version reports the results to a text file called `glewinfo.txt`. The Unix version prints the results to `stdout`.

Windows usage:

>     glewinfo [-pf <id>]

where `<id>` is the pixel format id for which the capabilities are displayed.

Unix usage:

>     glewinfo [-display <dpy>] [-visual <id>]

where `<dpy>` is the X11 display and `<id>` is the visual id for which the capabilities are displayed.