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Intel
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82854 Graphics Memory Controller Hub (GMCH)
128 D15343-003
The scissor rectangle accelerates the clipping process by allowing the driver to clip to a bigger
region than the hardware renders to. The scissor rectangle is pixel accurate, and independent of line
and point width. The GMCH supports a single scissor box rectangle.
6.4.2.5 Backface Culling
As part of the setup, the GMCH can discard polygons from further processing, if they are either
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.
6.4.2.6 Scan Converter
The Scan Converter takes the vertex and edge information identifies all pixels that are affected by
features being rendered. It works on a per-polygon basis, and one polygon may be entering the
pipeline while calculations finish on another.
6.4.2.7 Texture Engine
The GMCH allows an image pattern or video to be placed on the surface of a 3D polygon. The
texture engine performs texture color or chromakey matching texture filtering (anisotropic,
trilinear, and bilinear) and YUV to RGB conversion.
As texture sizes increase beyond the bounds of graphics memory, executing textures from graphics
memory becomes impractical. Every rendering pass would require copying each and every texture
in a scene from system memory to graphics memory, then using the texture, and finally overwriting
the local memory copy of the texture by copying the next texture into graphics memory. The
GMCH, using Intel's Direct Memory Execution model, simplifies this process by rendering each
scene using the texture located in system memory. The GMCH includes a cache controller to avoid
frequent memory fetches of recently used texture data.
6.4.2.8 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. Perspective correction involves a
compute-intensive "per-pixel-divide" operation on each pixel. Perspective correction is necessary
for realistic 3D graphics.
6.4.2.9 Texture Decompression
As the textures' average size gets larger with higher color depth and multiple textures become the
norm, it becomes increasingly important to provide support for compressed textures.
DirectX* supports Texture Compression/Decompression to reduce the bandwidth required to
deliver textures. The GMCH supports several compressed texture formats (DirectX: DXT1, DXT2,
DXT3, DXT4, DXT5) and OpenGL FXT1 formats.