Primitive Assembly

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Primitive Assembly is the stage in the OpenGL rendering pipeline where Primitives are divided into a sequence of individual base primitives. After some minor processing, as described below, they are passed along to the rasterizer to be rendered.

Early primitive assembly

The purpose of the primitive assembly step is to convert a vertex stream into a sequence of base primitives. For example, a primitive which is a line list of 12 vertices needs to generate 11 line base primitives.

The full primitive assembly step (including the processing below) will always happen after Vertex Post-Processing. However, some Vertex Processing steps require that a primitive be decomposed into a sequence of base primitives. For example, a Geometry Shader operates on each input base primitive in the primitive sequence. Therefore, a form of primitive assembly must happen before the GS can execute.

This early primitive assembly only performs the conversion to base primitives. It does not perform any of the below processing steps.

Such early processing must happen if a Geometry Shader or Tessellation is active. The early assembly step for Tessellation is simplified, since Patch Primitives are always sequences of patches.

Rasterizer discard

All primitives can be discarded by enabling GL_RASTERIZER_DISCARD. This prevents any primitives from being rasterized.

This is useful for testing the performance of prior rendering stages, but it is also useful to prevent rendering of primitives generated during Transform Feedback operations.

Warning: Framebuffer Clearing functions will do nothing if rasterizer discard is enabled.

Face culling

The culling of triangle primitives happens during this point in the pipeline.