Geometry Pipelines, also called Geometry Engines(GE) are the first stage in a classical Graphics Pipeline, such as the Reality Engine. They do the transformation from 3D coordinates used to specify the geometry to a unified coordinate system used by the Raster Manager (RM) to rasterize the geometry into framebuffer pixels. The Display Generator(DG) scans these pixels into a video signal understood by a monitor. In OpenGL, this transformation is defined by the Modelview Matrix and the Projection Matrix. Typically, the modelview matrix defines the transformation of the incoming vertices into world coordinates, a coordinate system used for all vertices. The projection matrix defines how this 3-dimensional coordinate space is projected to the Viewport. In addition to this transformation, the GEs compute the vertex colors based on the light settings, may perform texture coordinate generation as well as clipping of the geometry. The Geforce graphics cards from nVidia introduced these functionalities for the first time in the consumer market, labelled as hardware-based Transform and Lighting(T&L).
The elements of a pipeline are often executed in parallel or in time-sliced fashion; in that case, some amount of buffer storage is often inserted between elements.
Instruction pipelines, such as the classic RISC pipeline, which are used in processors to allow the parallel execution of two or more consecutive instructions from a nominally sequential stream; the processing elements are the logical circuits that implement the various stages of an instruction (address decoding and arithmetic, register fetching, cache lookup, etc.).
Software pipelines, consisting of multiple processes arranged so that the output stream of one process is automatically and promptly fed as the input stream of the next one.
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