Each release of OpenGL represents a core of graphics functionality and API calls which must be supported by any vendor claiming to support OpenGL. However this core of API functionality does not prevent individual implementors or groups of implementors from adding new features and API calls.
In fact, even at the time of a core release some official optional graphics capabilities may also be specified. All these categories enhanced functionality and the associated API calls and tokens are referred to as extensions. Each OpenGL extension is carefully specified in the context of the broader OpenGL specification and there are runtime checks that can be used to query the existence of any extension and generate the appropriate function call.
The OpenGL Extension Registry is maintained by SGI and contains the technical specifications of all known OpenGL extensions. The registry also contains guidelines for creating new extensions and has example C/C++ header files containing function entry points and token enums.
The extension registry can be intimidating with over 350 extensions plotting the development of the OpenGL API for more than a decade. However, most of the extensions have either been folded into the OpenGL core API or have been rendered obsolete by newer core functionality.
Some extensions start with GL_EXT, others with GL_NV, GL_ATI, GL_ARB, GL_MTX, GL_APPLE, GL_SGI. EXT means it is multi-vendor. It is developed by multiple vendors and other companies can include it in there drivers as well. Below follows a table with frequently found extension prefixes.
Regardless of the vendor prefix, vendor-specific OpenGL extensions may be implemented an/or licensed by multiple hardware vendors to ensure compatibility.
|Extension Prefix||Extension Vendor|
|ARB||OpenGL® ARB approved extensions.|
|NVX||NVIDIA Corporation. Experimental extension.1|
|ATI||ATI Technologies, Inc.|
|SUN||Sun Microsystems, Inc.|
|SGI||Silicon Graphics, Inc.|
|SGIX||Silicon Graphics, Inc. Experimental extension.1|
|SGIS||Silicon Graphics, Inc. Experimental extension.1|
|IBM||International Business Machines Corporation, or simply IBM.|
|MESA||The Mesa 3D Graphics Library.|
|GREMEDY||Graphic Remedy, Ltd.|
|OML||Khronos Group, Inc. API: OpenML®|
|OES||Khronos Group, Inc. API: OpenGL® ES|
|PGI||Portland Group Inc.|
|I3D||Intense3D, now 3DLABS Inc.|
|MTX||Matrox Electronic Systems Ltd.|
|Extension Prefix||Extension Platform|
|WGL_EXT, WGL_ARB, WGL_ATI, WGL_NV||Microsoft Corporation. WGL = Windows OpenGL|
|GLX_EXT, GLX_ARB, GLX_ATI, GLX_NV||Linux platforms.|
|AGL||Apple Inc. AGL = Apple OpenGL|
1, Experimental extensions should not be used in a production environment as they are subject to removal from implementation.
Proprietary extensions which prove their worth can become ARB approved (such as GL_ARB_multitexture) and shortly after become core. Most likely, an extension is already of type EXT, then becomes ARB, then core. In some cases, but not many, a proprietary extension becomes ARB, then core. Often, the original extension's developer allows other companies to also code it into their drivers without royalty requirements.
When a proprietary extension becomes an ARB approved core extension, the vendor will rarely – if ever – remove it from their extension implementation. This is done to ensure backwards compatibility with software which implements older extensions.
In some rare occasions, there is a difference between extension and the core function, for example the case of GLSL in OpenGL 2.0 and the older extensions, and the case for environment textures. Many of the extensions in the OpenGL Extension Registry (OpenGL Extension Registry) are deprecated, make sure deprecated extensions are not used.
Anyone may develop extensions for the OpenGL® API; the community gives feedback. An example case would be the GL_GREMEDY_string_marker extension, which was initially a private extension, is now included in the OpenGL Extension Registry.
Specific Extension Usage
Vertex submission extensions
- GL_ARB_vertex_buffer_object Core in GL 1.5, also known as VBO
- GL_NV_vertex_array_range VBO replaced this
- GL_EXT_compiled_vertex_array VBO replaced this
- GL_ARB_multitexture Core in GL 1.2.1
- GL_ARB_texture_env_combine Core in GL 1.3
- GL_ARB_texture_compression Core in GL 1.3
- Floating point and mipmapping and filtering