Layer(3) Manual Page

Decorating a variable with the Layer built-in decoration will make that variable contain the select layer of a multi-layer framebuffer attachment.

In a mesh, vertex, tessellation evaluation, or geometry shader, any variable decorated with Layer can be written with the framebuffer layer index to which the primitive produced by that shader will be directed.

The last active pre-rasterization shader stage (in pipeline order) controls the Layer that is used. Outputs in previous shader stages are not used, even if the last stage fails to write the Layer.

If the last active pre-rasterization shader stage shader entry point’s interface does not include a variable decorated with Layer, then the first layer is used. If a pre-rasterization shader stage shader entry point’s interface includes a variable decorated with Layer, it must write the same value to Layer for all output vertices of a given primitive. If the Layer value is less than 0 or greater than or equal to the number of layers in the framebuffer, then primitives may still be rasterized, fragment shaders may be executed, and the framebuffer values for all layers are undefined. In a mesh shader this also applies when the Layer value is greater than or equal to the maxMeshOutputLayers limit.

If a variable with the Layer decoration is also decorated with ViewportRelativeNV, then the ViewportIndex is added to the layer that is used for rendering and that is made available in the fragment shader.

If the shader writes to a variable decorated ViewportMaskNV, then the layer selected has a different value for each viewport a primitive is rendered to.

In a fragment shader, a variable decorated with Layer contains the layer index of the primitive that the fragment invocation belongs to.