845GL datasheet(126/193 Pages) INTEL

126 / 193 page

Functional Description

126

Intel® 82845G/82845GL/82845GV GMCH Datasheet

Depth Buffer

The Raster Engine can read and write from this buffer and use the data in per fragment operations

that determine whether resultant color and depth value of the pixel for the fragment are to be

updated or not.

Typical applications for entertainment or visual simulations with exterior scenes require far/near

ratios of 1000 to 10000. At 1000, 98% of the range is spent on the first 2% of the depth. This can

cause hidden surface artifacts in distant objects, especially when using 16-bit depth buffers. A

24-bit Z-buffer provides 16 million Z-values as opposed to only 64K with a 16-bit Z-buffer. With

lower Z-resolution, two distant overlapping objects may be assigned the same Z-value. As a result,

the rendering hardware may have a problem resolving the order of the objects, and the object in the

back may appear through the object in the front.

By contrast, when W (or eye-relative z) is used, the buffer bits can be more evenly allocated

between the near and far clip planes in world space. The key benefit is that the ratio of far and near

is no longer an issue, allowing applications to support a maximum range of miles, yet still get

reasonably accurate depth buffering within inches of the eye point.

The GMCH supports a flexible format for the floating-point W buffer, wherein the number of

exponent bits is programmable. This allows the driver to determine variable precision as a function

of the dynamic range of the W (screen-space Z) parameter.

The selection of depth buffer size is relatively independent of the color buffer. A 16-bit or 24-bit

Z/W buffer can be selected with a 16-bit color buffer. Z buffer is not supported in 8-bit mode.

Stencil Buffer

The Raster Engine provides 8-bit stencil buffer storage in 32-bit mode and the ability to perform

stencil testing. Stencil testing controls 3D drawing on a per pixel basis, conditionally eliminating a

pixel on the outcome of a comparison between a stencil reference value and the value in the stencil

buffer at the location of the source pixel being processed. They are typically used in multipass

algorithms to achieve special effects (e.g., decals, outlining, shadows, and constructive solid

geometry rendering).

Projective Textures

The GMCH supports projective textures. These textures require three floating-point texture

coordinates to be included in the FVF format. Projective textures enable special effects

(e.g., projecting spot light textures obliquely onto walls, etc.).

4.4.1.6

2D Engine

The GMCH provides 2D hardware acceleration for block transfers of data (BLTs). The BLT engine

provides the ability to copy a source block of data to a destination and perform operations on the

data using a pattern, and/or another destination. The Stretch BLT engine is used to move source

data to a destination that need not be the same size, with source transparency. Performing these

common tasks in hardware reduces processor load and, thus, improves performance.

845GL Datasheet(PDF) 126 Page - Intel Corporation