JG82852GMSL7VP Datasheet, PDF (132/176 Pages) Intel Corporation – Intel® 852GM/852GMV Chipset Intel® 852GM/852GMV Chipset Hub (GMCH)

Functional Description
R
5.4.3.3.
5.4.3.4.
5.4.3.5.
5.4.3.6.
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 (R, G, B), 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 in the GMCH helps to hide color quantization errors for 16-bit color buffers. 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-bit or 6-bit component by
dithering. Dithering is performed on blended textured pixels. In 32-bit mode, dithering is not performed.
Vertex and Per Pixel Fogging
Fogging is used to create atmospheric effects such as low visibility conditions in flight simulator-type
games. It adds another level of realism to computer-generated scenes. Fog can be used for depth cueing
or hiding distant objects. With fog, distant objects can be rendered with fewer details (less polygons),
thereby improving the rendering speed or frame rate. Fog is simulated by attenuating the color of an
object with the fog color as a function of distance, and the greater the distance, the higher the density
(lower visibility for distant objects). There are two ways to implement the fogging technique: per-vertex
(linear) fogging and per-pixel (non-linear) fogging. The per-vertex method interpolates the fog value at
the vertices of a polygon to determine the fog factor at each pixel within the polygon. This method
provides realistic fogging as long as the polygons are small. With large polygons (such as a ground plane
depicting an airport runway), the per-vertex technique results in unnatural fogging.
The GMCH supports both types of fog operations, vertex and per pixel or table fog. If fog is disabled,
the incoming color intensities are passed unchanged to the destination blend unit. If fog is enabled, the
incoming pixel color is blended with the fog color based on a fog coefficient on a per pixel basis using
the following equation before sending to the destination blend unit.
Alpha Blending
Alpha Blending in the GMCH adds the material property of transparency or opacity to an object. Alpha
blending combines a source pixel color and alpha component with a destination pixel color and alpha
component. For example, this is so that a glass surface on top (source) of a red surface (destination)
would allow much of the red base color to show through.
Blending allows the source and destination color values to be multiplied by programmable factors and
then combined via a programmable blend function. The combined and independent selection of factors
and blend functions for color and alpha is supported.
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Intel^® 852GM/852GMV Chipset GMCH Datasheet