gl_SamplePosition

is a fragment language input variable that contains the location within a fragment of the sample currently being processed. The x and y components of contain the sub-pixel coordinate of the current sample and will have values in the range 0.0 to 1.0. The sub-pixel coordinates of the center of the pixel are always (0.5, 0.5). Any static use of causes the entire fragment shader to be evaluated per-sample rather than per-fragment. When rendering to a non-multisample buffer, or if multisample rasterization is disabled, will be (0.5, 0.5).

gl_SampleID , gl_SampleMask , gl_FragCoord

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