845GL datasheet(124/193 Pages) INTEL

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Functional Description

124

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

4.4.1.5

Raster Engine

The Raster Engine is where the color data (e.g., fogging, specular RGB, texture map blending, etc.)

is processed. The final color of the pixel is calculated and the RGBA value combined with the

corresponding components resulting from the Texture Engine. These textured pixels are modified

by the specular and fog parameters. These specular highlighted, fogged, textured pixels are color

blended with the existing values in the frame buffer. In parallel, stencil, alpha and depth buffer tests

are conducted that will determine whether the Frame and Depth Buffers will be updated with the

new pixel values.

Texture Map Blending

Multiple Textures can be blended together in an iterative process and applied to a primitive. The

GMCH allows up to four texture coordinates and texture maps to be specified onto the same

polygon. Also, the GMCH supports using a texture coordinate set to access multiple texture maps.

State variables in multiple texture are bound to texture coordinates, texture map, or texture

blending.

Combining Intrinsic and Specular Color Components

The GMCH allows an independently specified and interpolated “specular RGB” attribute to be

added to the post-texture blended pixel color. This feature provides a full RGB specular highlight

to be applied to a textured surface, permitting a high-quality reflective colored lighting effect not

available in devices that apply texture after the lighting components have been combined. If

specular-add state variable is disabled, only the resultant colors from the map blending are used. If

this state variable is enabled, RGB values from the output of the map blending are added to values

for RS, GS, BS on a component by component basis.

Color Shading Modes

The Raster Engine supports the flat and Gouraud shading modes. These shading modes are

programmed by the appropriate state variables issued through the command stream.

Flat shading is performed by smoothly interpolating the vertex intrinsic color components (Red,

Green, Blue), Specular (RGB), Fog, and Alpha to the pixel, where each vertex color has the same

value. The setup engine substitutes one of the vertex’s attribute values for the other two vertices

attribute values thereby creating the correct flat shading terms. This condition is set up by the

appropriate state variables issued prior to rendering the primitive.

Gouraud shading is performed by smoothly interpolating the vertex intrinsic color components

(Red, Green, Blue), Specular (RGB), Fog, and Alpha to the pixel, where each vertex color has a

different value.

All the attributes can be selected independently to one of the shading mode by setting the

appropriate value state variables.

Color Dithering

Color Dithering helps to hide color quantization errors. Color Dithering takes advantage of the

human eye’s propensity to “average” the colors in a small area. Input color, alpha, and fog

components are converted from 8-bit components to 5- or 6- bit component by dithering. Dithering

is performed on blended texture pixels. In 32-bit mode, dithering is not performed on the

components.

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