gl_Layer

In the geometry language, is used to select a specific layer (or face and layer of cube map) in a multi-layer framebuffer attachment. The actual layer used will come from one of the vertices in the primitive being shaded. Which vertex the layer comes from is undefined and as such it is recommended to write the same value to for all vertices in the primitive. If a shader statically assigns a value to , layered rendering mode is enabled. If a shader statically assigns a value to , and there is an execution path through the shader that does not set , then the value of is undefined for invocations of the shader that take that path.

When used with an array of cube map textures, the output variable takes on a special value. Instead of referring to only the layer, it is used to select a cube map face and a layer. Setting to the value $\mathrm{layer}*6+\mathrm{face}$ will render to $\mathrm{face}$ of the cube defined in $\mathrm{layer}$ . The face values are as follows:

Face Value	Resulting Target
0	GL_TEXTURE_CUBEMAP_POSITIVE_X
1	GL_TEXTURE_CUBEMAP_NEGATIVE_X
2	GL_TEXTURE_CUBEMAP_POSITIVE_Y
3	GL_TEXTURE_CUBEMAP_NEGATIVE_Y
4	GL_TEXTURE_CUBEMAP_POSITIVE_Z
5	GL_TEXTURE_CUBEMAP_NEGATIVE_Z

For example, to render to the positive y cube map face located in the 5 layer of the cube map array, should be set to $5*6+2$ .

The input variable in the fragment language will have the same value that was written to the output variable in the geometry language. If the geometry stage does not dynamically assign a value to , the value of in the fragment stage will be undefined. If the geometry stage makes no static assignment to , the input in the fragment stage will be zero. Otherwise, the fragment stage will read the same value written by the geometry stage, even if that value is out of range. If a fragment shader contains a static access to , it will count against the implementation defined limit for the maximum number of inputs to the fragment stage.

gl_ViewportIndex

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