82845PE Datasheet, PDF (129/176 Pages) Intel Corporation – 82845GE Graphics and Memory Controller Hub (GMCH) and 82845PE Memory Controller Hub (MCH)

Functional Description
5.4.1.6
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% percent 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 64 K 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 3 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.).
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.
Intel^® 82845GE/82845PE Datasheet
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