SampleMask(3) Manual Page

A variable in the Input storage class decorated with SampleMask will contain a bitmask of the set of samples covered by the primitive generating the fragment during rasterization. It has a sample bit set if and only if the sample is considered covered for this fragment shader invocation. SampleMask[] is an array of integers. Bits are mapped to samples in a manner where bit B of mask M (SampleMask[M]) corresponds to sample 32 × M + B.

A variable in the Output storage class decorated with SampleMask is an array of integers forming a bit array in a manner similar to an input variable decorated with SampleMask, but where each bit represents coverage as computed by the shader. This computed SampleMask is combined with the generated coverage mask in the multisample coverage operation.

Variables decorated with SampleMask must be either an unsized array, or explicitly sized to be no larger than the implementation-dependent maximum sample-mask (as an array of 32-bit elements), determined by the maximum number of samples.

If a fragment shader entry point’s interface includes an output variable decorated with SampleMask, the sample mask will be undefined for any array elements of any fragment shader invocations that fail to assign a value. If a fragment shader entry point’s interface does not include an output variable decorated with SampleMask, the sample mask has no effect on the processing of a fragment.