Chapter 6 - Rasterization and fragment processing

Abstract

This chapter reviews the rasterization and fragment processing parts of the OpenGL pipeline, emphasizing details and potential problems that affect multipass rendering techniques. The details of this section of the OpenGL pipeline have a large influence on the design of a multipass algorithm. The rasterization and fragment processing operations provide many mechanisms for modifying a fragment value or discarding it outright. OpenGL's rasterization phase is divided into several stages: scan conversion, texture lookup and environment, color sum and fog. This part of the pipeline can be broken into two major groups—rasterization and fragment processing. Rasterization, a primitive, described as a set of vortex coordinates, associated colors, and texture coordinates, is converted into a series of fragments. Here, the pixel locations covered by the primitives are determined and the associated color and depth values at these locations are computed. The current texturing and fog state are used by the rasterizer to modify the fragments appropriately. After rasterization, the fragments undergo a series of processing steps, called fragment operations including scissoring, alpha test, depth, and stencil test and blending.