If a fragment shader is active and its entry point’s interface includes a built-in output variable decorated with SampleMask, the coverage mask is ANDed with the bits of the SampleMask built-in to generate a new coverage mask. If sample shading is enabled, bits written to SampleMask corresponding to samples that are not being shaded by the fragment shader invocation are ignored.
If no fragment shader is active, or if the active fragment shader does not include SampleMask in its interface, the coverage mask is not modified.
Next, the fragment alpha value and coverage mask are modified based on the alphaToCoverageEnabled and alphaToOneEnabled members of the afxPipelineMultisamplingConfig structure.
All alpha values in this section refer only to the alpha component of the fragment shader output that has a Location and Index decoration of zero (see the Fragment Output Interface section). If that shader output has an integer or unsigned integer type, then these operations are skipped.
If alphaToCoverageEnabled is enabled, a temporary coverage mask is generated where each bit is determined by the fragment’s alpha value, which is ANDed with the fragment coverage mask.
No specific algorithm is specified for converting the alpha value to a temporary coverage mask. It is intended that the number of 1’s in this value be proportional to the alpha value (clamped to [0,1]), with all 1’s corresponding to a value of 1.0 and all 0’s corresponding to 0.0. The algorithm may be different at different framebuffer coordinates.
Note: Using different algorithms at different framebuffer coordinates may help to avoid artifacts caused by regular coverage sample locations.
Finally, if alphaToOneEnabled is enabled, each alpha value is replaced by the maximum representable alpha value for fixed-point color attachments, or by 1.0 for floating-point attachments. Otherwise, the alpha values are not changed.