Per-Sample Processing is a stage of the OpenGL rendering pipeline, where fragment data output from a Fragment Shader is processed and written to various buffers. There are a number of operations that are performed in the order below.
Pixel ownership test
Occlusion query updating
Fragments at this point are considered to have passed any conditional tests. Thus, their outputs will be written to one or more buffers (Write Masks not withstanding). As such, this is the point where the fragment is counted for Occlusion Query counters. If an occlusion query of type GL_SAMPLES_PASSED is active, then the query will be incremented every time a fragment hits this stage.
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