OpenGL Programming Guide

Chapter 10The Framebuffer

Testing and Operating on Fragments

When you draw geometry, text, or images on the screen, OpenGL performs several calculations to rotate, translate, scale, determine the lighting, project the object(s) into perspective, figure out which pixels in the window are affected, and determine what colors those pixels should be drawn. Many of the earlier chapters in this book give some information about how to control these operations. After OpenGL determines that an individual fragment should be generated and what its color should be, several processing stages remain that control how and whether the fragment is drawn as a pixel into the framebuffer. For example, if it's outside the window or if it's farther from the viewpoint than the pixel that's already in the framebuffer, it isn't drawn. In another stage, the fragment's color is blended with the color of the pixel already in the framebuffer.

This section describes both the complete set of tests that a fragment must pass before it goes into the framebuffer and the possible final operations that can be performed on the fragment as it's written. The tests and operations occur in the following order; if a fragment is eliminated in an early test, none of the later tests or operations take place.

1. Scissor test
   
   

2. Alpha test
   
   

3. Stencil test
   
   

4. Depth test
   
   

5. Blending
   
   

6. Dithering
   
   

7. Logical operation

Each of these tests and operations is described in detail in the following sections.

Scissor Test
Alpha Test
Stencil Test
Depth Test
Blending, Dithering, and Logical Operations

OpenGL Programming Guide