RG82855GMESL72L Datasheet, PDF (147/213 Pages) Intel Corporation – Intel® 855GM/855GME Chipset Graphics and Memory Controller Hub (GMCH)

Functional Description
R
6.4.3.3
6.4.3.4
6.4.3.5
6.4.3.6
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.
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. 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.
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.
Datasheet
147