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Intel® 82845G/82845GL/82845GV GMCH Datasheet

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Functional Description

GMCH supports 7 types of texture filtering:

• Nearest (aka Point Filtering): Texel with coordinates nearest to the desired pixel is used. (This

is used if only one LOD is present).

• Linear (aka Bilinear Filtering): A weighted average of a 2x2 area of texels surrounding the

desired pixel are used. (This is used if only one LOD is present).

• Nearest MIP Nearest (aka 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 are 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

Maps. Trilinear MIP Mapping is used to minimize the visibility of LOD transitions across the

polygon.

• Anisotropic MIP Nearest (Anisotropic Filtering): This is used if many LODs are present. The

nearest LOD-1 level will be determined for each of four sub-samples for the desired pixel.

These four sub-samples are then bilinear filtered and averaged together.

Both D3D and OGL (Rev.1.1) allow support for all these filtering modes.

Multiple Texture Composition

The GMCH also performs multiple texture composition. This allows the combination of two or

greater MIP Maps to produce a new one with new LODs and texture attributes in a single or

iterated pass.

Flexible vertex format support allows multitexturing because it makes it possible to pass multiple

texture information in the vertex structure.

Cubic Environment Mapping

Environment maps allow applications to render scenes with complex lighting and reflections while

significantly decreasing processor load. There are several methods to generate environment maps

(e.g., spherical, circular, and cubic). The GMCH supports cubic reflection mapping over spherical

and circular since it is the best choice to provide real-time environment mapping for complex

lighting and reflections.

Cubic Mapping requires a texture map for each of the six cube faces. These can be generated by

pointing a camera with a 90-degree field-of-view in the appropriate direction. Per-vertex vectors

(normal, reflection, or refraction) are interpolated across the polygon and the intersection of these

vectors with the cube texture faces is calculated. Texel values are then read from the intersection

point on the appropriate face and filtered accordingly.

845GL Datasheet(PDF) 123 Page - Intel Corporation

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