Contains the index of the sample currently being processed.
gl_SampleIDis a fragment language input variable that contains the index of the sample currently being processed. This variable is in the range 0 to gl_NumSamples - 1, where gl_NumSamples is the total number of samples in each fragment for the current framebuffer (and thus 1 if rendering to a non-multisample buffer). Any static use of this variable in a fragment shader causes the entire shader to be evaluated per-sample rather than per-fragment.
When rendering to a non-multisample buffer, or if multisample rasterization is disabled,gl_SampleIDwill always be zero. gl_NumSamples is the sample count of the framebuffer regardless of whether multisample rasterization is enabled or not.
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