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Intel
82865G/82865GV GMCH Datasheet
169
Functional Description
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 which 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 (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 is 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 (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 textured pixels. In 32-bit mode, dithering is not performed on the
components.
Vertex and Per Pixel Fogging
Fogging is used to create atmospheric effects (e.g., 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