OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Testing and Operating on Fragments

When you draw geometry on the screen, OpenGL starts processing it by executing the currently bound vertex shader; then the tessellation, and geometry shaders, if they’re bound; and then assembles the final geometry into primitives that get sent to the rasterizer, which figures out which pixels in the window are affected. After OpenGL determines that an individual fragment should be generated, its fragment shader is executed, and then several processing stages, which control how and whether the fragment is drawn as a pixel into the framebuffer, remain. For example, if the fragment is outside a rectangular region or if it’s farther from the viewpoint than the pixel that’s already in the framebuffer, its processing ...