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166
Intel
82865G/82865GV GMCH Datasheet
Functional Description
Depth-Bias
The GMCH supports source depth biasing in the setup engine, the depth bias value is specified in
the vertex command packet on a per primitive basis. The value ranges from -1 to 1. The depth bias
value is added to the z or w value of the vertices. This is used for coplanar polygon priority. If two
polygons are to be rendered that are coplanar, due to the inherent precision differences induced by
unique x, y and z values, there is no guarantee which polygon will be closer or farther. By using
depth bias, it is possible to offset the destination z value (compare value) before comparing with
the new z value.
Backface Culling
As part of the setup, the GMCH discards polygons from further processing, if they are facing away
from or towards the user’s viewpoint. This operation, referred to as “Back Face Culling” is
accomplished based on the “clockwise” or “counter-clockwise” orientation of the vertices on a
primitive. This can be enabled or disabled by the driver. This is referred to as “Back Face Culling.”
5.4.1.2
Scan Converter
The Scan Converter takes the vertex and edge information that is used to identify all pixels that are
affected by features being rendered. It works on a per-polygon basis.
Pixel Rasterization Rules
The GMCH supports both OpenGL and D3D pixel rasterization rules to determine whether a pixel
is filled by the triangle or line. For both D3D and OpenGL modes, a top-left filling convention for
filling geometry is used. Pixel rasterization rule on rectangle primitive is also supported using the
top-left fill convention.
5.4.1.3
2D Functionality
The stretch BLT function can stretch source data in the X and Y directions to a destination larger or
smaller than the source. Stretch BLT functionality expands a region of memory into a larger or
smaller region using replication and interpolation. The stretch BLT function also provides format
conversion and data alignment.
5.4.1.4
Texture Engine
The GMCH allows an image, pattern, or video to be placed on the surface of a 3D polygon. The
texture processor receives the texture coordinate information from the setup engine and the texture
blend information from the scan converter. The texture processor performs texture color or
ChromaKey matching, texture filtering (anisotropic, trilinear and bilinear interpolation), and
YUV-to-RGB conversions.
Perspective Correct Texture Support
A textured polygon is generated by mapping a 2D texture pattern onto each pixel of the polygon. A
texture map is like wallpaper pasted onto the polygon. Since polygons are rendered in perspective,
it is important that texture be mapped in perspective as well. Without perspective correction,
texture is distorted when an object recedes into the distance.