ViewportMaskNV(3) Manual Page

In a vertex, tessellation evaluation, or geometry shader, the variable decorated with ViewportMaskNV can be written to with the mask of which viewports the primitive produced by that shader will directed.

The ViewportMaskNV variable must be an array that has ⌈(VkPhysicalDeviceLimits::maxViewports / 32)⌉ elements. When a shader writes to this variable, bit B of element M controls whether a primitive is emitted to viewport 32 × M + B. The viewports indicated by the mask are used to select the viewport transform and scissor rectangle that a primitive will be transformed by.

The last active pre-rasterization shader stage (in pipeline order) controls the ViewportMaskNV that is used. Outputs in previous shader stages are not used, even if the last stage fails to write the ViewportMaskNV. When ViewportMaskNV is written by the final pre-rasterization shader stage, any variable decorated with ViewportIndex in the fragment shader will have the index of the viewport that was used in generating that fragment.

If a pre-rasterization shader stage shader entry point’s interface includes a variable decorated with ViewportMaskNV, it must write the same value to ViewportMaskNV for all output vertices of a given primitive.