The Graphics Pipeline and Per-Fragment Operations

The model of command execution in OpenGL is that of a pipeline with a fixed topology (although stages may be switched in or out). The pipeline is meant to mimic the organization of graphics subsystems. The final stages of the pipeline, for example, consist of a series of tests on and modifications to fragments before they are eventually placed in the framebuffer. To draw a complex scene in a short amount of time, many fragments must pass through these final stages on their way to the framebuffer, leaving little time to process each fragment. Such high fill rates demand special purpose hardware that can only perform fixed operations with minimum access to external data.

Even though fragment operations are limited, many interesting and useful effects may be obtained by combining the operations appropriately. Per-fragment operations provided by OpenGL include

• alpha blending: blend a fragment's color with that of the corresponding pixel in the framebuffer based on an alpha value;
• depth test: compare a depth value associated with a fragment with the corresponding value already present in the framebuffer and discard or keep the fragment based on the outcome of the comparison;
• stencil test: compare a reference value with a corresponding value stored in the framebuffer and update the value or discard the fragment based on the outcome of the comparison.

The OpenGL graphics pipeline also induces a kind of orthogonality among primitives. Each vertex, whether it belongs to a point, line segment, or polygon primitive, is treated in the same way: its coordinates are transformed and lighting (if enabled) assigns it a color. The primitive defined by these vertices is then rasterized and converted to fragments, as is a bitmap or image rectangle primitive. All fragments, no matter what their origin, are treated identically. This homogeneity among operations removes unneeded special cases (for each primitive type) from the pipeline. It also makes natural the combination of diverse primitives in the same scene without having to set special modes for each primitive type.