Intel D15343-003 manual Backface Culling, Scan Converter, Texture Engine, Texture Decompression

Intel® 82854 Graphics Memory Controller Hub (GMCH)

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.

6.4.2.5Backface 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.

6.4.2.6Scan 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.

6.4.2.7Texture 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.

6.4.2.8Perspective 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.

6.4.2.9Texture 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.

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