Intel D15343-003 manual Texture Chromakey, Anti-Aliasing, Texture Map Filtering

Functional Description

6.4.2.10Texture Chromakey

Chromakey is a method for removing a specific color or range of colors from a texture map before it is applied to an object. For nearest texture filter modes, removing a color simply makes those portions of the object transparent (the previous contents of the back buffer show through). For linear texture filtering modes, the texture filter is modified if only the non-nearest neighbor texels match the key (range).

Chromakeying can be performed for both paletted and non-paletted textures, and removes texels that fall within a specified color range. The Chromakey mode refers to testing the ARGB or YUV components to see if they fall between high and low state variable values. If the color of a texel contribution is in this range and chromakey is enabled, then this contribution is removed from the resulting pixel color.

6.4.2.11Anti-Aliasing

Aliasing is one of the artifacts that degrade image quality. In its simplest manifestation, aliasing causes the jagged staircase effects on sloped lines and polygon edges. Another artifact is the moiré patterns, which occur as a result of the fact that there is very small number of pixels available on screen to contain the data of a high-resolution texture map.

Full scene anti-aliasing uses super-sampling, which means that the image is rendered internally at a higher resolution than it is displayed on screen. The GMCH renders internally at 1600x1200, reads the image as a texture, and finally down-samples (via a Bilinear filter) to the screen resolution of 640x480 and 800x600. Full scene anti-aliasing removes jaggies at the edges.

6.4.2.12Texture Map Filtering

Many texture-mapping modes are supported. Perspective correct mapping is always performed. As the map is fitted across the polygon, the map can be tiled, mirrored in either the U or V directions, or mapped up to the end of the texture and no longer placed on the object (this is known as clamp mode). The way a texture is combined with other object attributes is also definable.

The GMCH supports up to 12 Levels-of-Detail (LODs) ranging in size from 2048x2048 to 1x1 texels. (A texel is defined as a texture map element.) Included in the texture processor is a texture cache, which provides efficient MIP-mapping.

The GMCH supports seven types of texture filtering:

•Nearest (also known as Point filtering): Texel with coordinates nearest to the desired pixel is used. (This is used if only one LOD is present.)

•Linear (also known as Bilinear filtering): A weighted average of a 2x2 area of texels surrounding the desired pixel is used. (This is used if only one LOD is present.)

•Nearest MIP Nearest (also known as Point filtering): This is used if many LODs are present. The nearest LOD is chosen and the texel with coordinates nearest to the desired pixel are used.

•Linear MIP Nearest (Bilinear MIP mapping): This is used if many LODs are present. The nearest LOD is chosen and a weighted average of a 2x2 area of texels surrounding the desired pixel is used (four texels). This is also referred to as Bilinear MIP Mapping.

•Nearest MIP Linear (Point MIP mapping): This is used if many LODs are present. Two appropriate LODs are selected and within each LOD the texel with coordinates nearest to the desired pixel are selected. The Final texture value is generated by linear interpolation between the two texels selected from each of the MIP Maps.

•Linear MIP Linear (Trilinear MIP mapping): This is used if many LODs are present. Two appropriate LODs are selected and a weighted average of a 2x2 area of texels surrounding the desired pixel in each MIP Map is generated (four texels per MIP Map). The Final texture value is generated by linear interpolation between the two texels generated for each of the MIP

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