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is a process by which pixels on screen are tested against depth buffer.
Depth testing function is set by
glEnable(GL_DEPTH_TEST); // this enables depth testing
and then by
glDepthTest(GL_LESS); // this sets function for depth testing
calls. Most widely used is GL_LESS argument, which specifies that (after rasterization
) pixels that have z coordinate in screen space
LESS than z coordinate for that pixel in depth buffer ARE drawn, others are discarded.