910GL datasheet(241/426 Pages) INTEL

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Functional Description

Datasheet

241

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.

12.6.4.4

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 component by dithering.

Dithering is performed on blended textured pixels. In 32-bit mode, dithering is not performed on

the components

12.6.4.5

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. Higher fog

density produces 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.

12.6.4.6

Alpha Blending (Frame Buffer)

Alpha Blending adds the material property of transparency or opacity to an object. Alpha

blending combines a source pixel color (RSGSBS) and alpha (AS) component with a destination

pixel color (RDGDBD) and alpha (AD) 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 are supported.

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