Intel® 945G/945GZ/945GC/ 945P/945PL Express Chipset Family ...

Functional Description
10.5.5.2 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, and BS on a component by component basis.
10.5.5.3 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 Highlights (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.
OpenGL and D3D use a different vertex to select the flat shaded color. This vertex is defined as
the “provoking vertex”. In the case of strips/fans, after the first triangle, attributes on every vertex
that define a primitive are used to select the flat color of the primitive. A state variable is used to
select the “flat color” prior to rendering the primitive.
Gouraud shading is performed by smoothly interpolating the vertex intrinsic color components
(Red, Green, Blue). Specular Highlights (R,G,B), Fog, and Alpha to the pixel, where each vertex
color has a different value.
All the attributes can be selected independently from one of the shading modes by setting the
appropriate value state variables.
10.5.5.4 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 components by dithering.
Dithering is performed on blended textured pixels. In 32-bit mode, dithering is not performed on
the components.
10.5.5.5 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 (fewer 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. The higher the
density the lower the 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
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