ViewportIndex

ViewportIndex

Decorating a variable with the ViewportIndex built-in decoration will make that variable contain the index of the viewport.

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

The selected viewport index is used to select the viewport transform, scissor rectangle, and exclusive scissor rectangle.

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

If the last active pre-rasterization shader stage shader entry point’s interface does not include a variable decorated with ViewportIndex , and if the multiviewPerViewViewports feature is not enabled, then the first viewport is used. If a pre-rasterization shader stage shader entry point’s interface includes a variable decorated with ViewportIndex, it must write the same value to ViewportIndex for all output vertices of a given primitive.

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

If the multiviewPerViewViewports feature is enabled, and if the last active pre-rasterization shader stage shader entry point’s interface does not include a variable decorated with ViewportIndex, then the value of ViewIndex is used as an index to select the viewport transform and scissor rectangle, and the value of ViewportIndex in the fragment shader is undefined::.