glClientActiveTexture

glClientActiveTexture is a function in the OpenGL API. It is used to specify which texture unit subsequent texture image specification commands will affect. Texture units are individual units within the graphics hardware that can hold and process a texture. By activating a specific texture unit, developers can direct operations like glClientActiveTexture to apply to that unit. This is crucial for applications that use multiple textures, such as applying different textures to different parts of a model or combining textures for effects like blending.

Before a texture can be applied to geometry, it needs to be loaded into a texture unit. The glClientActiveTexture function allows the application to select the target texture unit for this loading process. For example, if an application wants to load a diffuse texture into texture unit 0 and a normal map into texture unit 1, it would first call glClientActiveTexture(GL_TEXTURE0) and then use commands like glTexImage2D to load the diffuse texture. Subsequently, it would call glClientActiveTexture(GL_TEXTURE1) before loading the normal map.

The texture units themselves are typically numbered starting from GL_TEXTURE0. The OpenGL specification defines a minimum number of texture units supported, but hardware can provide many more. The maximum number of texture units available can be queried using glGetIntegerv with the GL_MAX_TEXTURE_UNITS parameter. Proper management of texture units is essential for efficient rendering, especially in complex scenes with numerous textured objects.