82845G Datasheet, PDF (121/193 Pages) Intel Corporation – Intel 82845G/82845GL/82845GV Graphics and Memory Controller Hub (GMCH)

Functional Description
4.4.1.2
4.4.1.3
4.4.1.4
Zone Rendering
Zone Rendering Technology is a unique mechanism that addresses memory bandwidth limitations
by reducing the required memory bandwidth for graphics. The 3D graphics engine divides the
frame buffer into rectangular zones and then sorts the triangles into memory by zone. The 3D
graphics engine then completely processes the zone, writing the pixel data to memory and then
proceeds to the next zone. By processing only a single zone of the frame buffer at a time, the use of
on-chip memory (cache) is highly optimized and each pixel in each scene is drawn only one time.
As a result, the system memory bandwidth required to render each scene is greatly reduced.
Depth-Bias
The GMCH supports source Depth biasing. 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. By using Depth
bias, it is possible to offset the destination z value (compare value) before comparing with the new
z value.
Backface Culling
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.
Scan Converter
The Scan Converter takes the vertex and edge information 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.
2D Functionality
The alpha 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.
Texture Engine
The GMCH allows an image, pattern, or video to be placed on the surface of a 3D polygon. The
texture processor performs texture color or chromakey matching, texture filtering (anisotropic,
trilinear and bilinear interpolation), and YUV-to-RGB conversions.
Intel^® 82845G/82845GL/82845GV GMCH Datasheet
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