Graphics pipeline
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In 3D computer graphics, the terms graphics pipeline or rendering pipeline most commonly refers to the current state of the art method of rasterization-based rendering as supported by commodity graphics hardware[1]. The graphics pipeline typically accepts some representation of a three-dimensional primitive as input and results in a 2D raster image as output. OpenGL and Direct3D are two notable 3d graphic standards, both describing very similar graphic pipelines.
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Stages of the graphics pipeline [edit]
Per-vertex lighting and shading [edit]
Geometry in the complete 3D scene is lit according to the defined locations of light sources, reflectance, and other surface properties. Some (mostly older) hardware implementations of the graphics pipeline compute lighting only at the vertices of the polygons being rendered; the lighting values between vertices are then interpolated during rasterization. Per-fragment or per-pixel lighting, as well as other effects, can be done on modern graphics hardware as a post-rasterization process by means of a shader program. Modern graphics hardware also supports per-vertex shading through the use of vertex shaders.
Clipping [edit]
Geometric primitives that now fall completely outside of the viewing frustum will not be visible and are discarded at this stage.
Projection transformation [edit]
In the case of a Perspective projection, objects which are distant from the camera are made smaller. This is achieved by dividing the X and Y coordinates of each vertex of each primitive by its Z coordinate(which represents its distance from the camera). In an orthographic projection, objects retain their original size regardless of distance from the camera.
Viewport transformation [edit]
The post-clip vertices are transformed once again to be in window space. In practice, this transform is very simple: applying a scale (multiplying by the width of the window) and a bias (adding to the offset from the screen origin). At this point, the vertices have coordinates which directly relate to pixels in a raster.
Scan conversion or rasterization [edit]
Rasterization is the process by which the 2D image space representation of the scene is converted into raster format and the correct resulting pixel values are determined. From now on, operations will be carried out on each single pixel. This stage is rather complex, involving multiple steps often referred as a group under the name of pixel pipeline.
Texturing, fragment shading [edit]
At this stage of the pipeline individual fragments (or pre-pixels) are assigned a color based on values interpolated from the vertices during rasterization, from a texture in memory, or from a shader program.
Display [edit]
The final colored pixels can then be displayed on a computer monitor or other display.
The graphics pipeline in hardware [edit]
The rendering pipeline is mapped onto current graphics acceleration hardware such that the input to the graphics card (GPU) is in the form of vertices. These vertices then undergo transformation and per-vertex lighting. At this point in modern GPU pipelines a custom vertex shader program can be used to manipulate the 3D vertices prior to rasterization. Once transformed and lit, the vertices undergo clipping and rasterization resulting in fragments. A second custom shader program can then be run on each fragment before the final pixel values are output to the frame buffer for display.
The graphics pipeline is well suited to the rendering process because it allows the GPU to function as a stream processor since all vertices and fragments can be thought of as independent. This allows all stages of the pipeline to be used simultaneously for different vertices or fragments as they work their way through the pipe. In addition to pipelining vertices and fragments, their independence allows graphics processors to use parallel processing units to process multiple vertices or fragments in a single stage of the pipeline at the same time.
See also [edit]
References [edit]
- ^ Graphics pipeline. (n.d.). Computer Desktop Encyclopedia. Retrieved December 13, 2005, from Answers.com: [2]
- ^ Raster Graphics and Color 2004 by Greg Humphreys at the University of Virginia
