Datasheet

Functional Description
R
184 Datasheet
5.4.2.10 Texture 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 RGB 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.
5.4.2.11 Anti-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 can render internally at
1600x1200 and then this image is down-sampled (via a Bilinear filter) to the screen resolution of
640x480 and 800x600. Full Scene Anti-aliasing removes jaggies at the edges as well as moiré
patterns. The GMCH renders the super-sampled image up to 2K x 2K pixel dimensions. The
GMCH then reads it as a texture and bilinearly filters it to the final resolution.
5.4.2.12 Texture 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 is used.