Multitexture refers to the ability to apply two or more distinct textures to a single fragment. Each texture has the ability to supply its own texture color to rasterized fragments. Without multitexture, there is only a single supported texture unit. OpenGL's multitexture support requires that every texture unit be fully functional and maintain state that is independent of any other texture units. Each texture unit has its own texture coordinate generation state, texture matrix state, texture enable state, and texture environment state. However, each texture unit within an OpenGL context shares the same set of texture objects.
Rendering algorithms that require multiple rendering passes can often be reimplemented to use multitexture in operate in less rendering passes. Some effects are only viable with multitexture.
Many OpenGL games such as Quake and Unreal use light maps to improve the lighting quality within their scenes. Without multitexture, light map textures must be modulated into the scene with a second blended rendering pass in addition to a first pass to render the base surface texture. With multitexture, the light maps and base surface texture can be rendered in a single rendering pass. This can cut the transformation overhead almost in half when rendering light maps because a single multitexture rendering pass means that polygons need to only be transformed once. The framebuffer update overhead is also lower when using multitexture to render light maps. When multitexture is used, the overhead of blending in the second rendering pass is completely eliminated. A single multitextured rendering pass can render both the surface texture and the light map texture without any framebuffer blending because the modulation of the surface texture with the light map texture occurs as part of the multitexture texture environment. Light maps are described in more detail in Section 10.2.
The OpenGL 1.2.1 revision of the OpenGL specification [88] includes an Appendix F that introduces the concept of OpenGL Architecture Review Board (ARB) approved extensions and specifies the ARB_multitexture extension, the first distinct ARB extension. The original OpenGL 1.2 specification includes an ARB extension called the ARB_imaging extension, but the ARB_imaging description is intermingled with the core OpenGL 1.2 specification. The ARB_multitexture extension is the first ARB extension that is specified in an Appendix distinct from the core specification. The purpose of ARB extensions is to add important new functionality to OpenGL in a modular way that makes it easier and quicker for OpenGL implementors to make available standard OpenGL feature subsets because an ARB extension does not require a complete update of the core OpenGL specification.