## Name

gl_Layer — contains the selected layer of a multi-layer framebuffer attachment

## Declaration

    // Geometry stage

`out int gl_Layer ;`

    // Fragment stage

`in int gl_Layer ;`

## Description

In the geometry language, `gl_Layer` 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 `gl_Layer` for all vertices in the primitive. If a shader statically assigns a value to `gl_Layer`, layered rendering mode is enabled. If a shader statically assigns a value to `gl_Layer`, and there is an execution path through the shader that does not set `gl_Layer`, then the value of `gl_Layer` is undefined for invocations of the shader that take that path.

When used with an array of cube map textures, the `gl_Layer` 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 `gl_Layer` to the value layer*6+face will render to face of the cube defined in 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 5th layer of the cube map array, `gl_Layer` should be set to 5*6+2.

The input variable `gl_Layer` in the fragment language will have the same value that was written to the output variable `gl_Layer` in the geometry language. If the geometry stage does not dynamically assign a value to `gl_Layer`, the value of `gl_Layer` in the fragment stage will be undefined. If the geometry stage makes no static assignment to `gl_Layer`, the input `gl_Layer` 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 `gl_Layer`, it will count against the implementation defined limit for the maximum number of inputs to the fragment stage.

## Version Support

| OpenGL Shading Language Version
--- | ---
Function Name | 1.10 | 1.20 | 1.30 | 1.40 | 1.50 | 3.30 | 4.00 | 4.10 | 4.20 | 4.30 | 4.40 | 4.50
gl_Layer (geometry stage) | - | - | - | - | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔ | ✔
gl_Layer (fragment stage) | - | - | - | - | - | - | - | - | - | ✔ | ✔ | ✔

## See Also

gl_ViewportIndex

## Copyright

Copyright © 2011-2014 Khronos Group. This material may be distributed subject to the terms and conditions set forth in the Open Publication License, v 1.0, 8 June 1999. http://opencontent.org/openpub/.