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

Per-Fragment Operations

Additional fragment processing, outside of what you can currently do in a fragment shader is the final processing of individual fragments. During this stage a fragment’s visibility is determined using depth testing (also commonly known as z-buffering) and stencil testing.

If a fragment successfully makes it through all of the enabled tests, it may be written directly to the framebuffer, updating the color (and possibly depth value) of its pixel, or if blending is enabled, the fragment’s color will be combined with the pixel’s current color to generate a new color that is written into the framebuffer.

As you saw in Figure 1.2, there’s also a path for pixel data. Generally, pixel data comes from an image file, although it ...