82845PE Datasheet, PDF (127/176 Pages) Intel Corporation – 82845GE Graphics and Memory Controller Hub (GMCH) and 82845PE Memory Controller Hub (MCH)

Functional Description
5.4.1.5
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 R_S, G_S, B_S 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.
Intel^® 82845GE/82845PE Datasheet
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