Intel
82865G/82865GV GMCH Datasheet
171
Functional Description
Depth Buffer
The Raster Engine is able to 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 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 Z/W 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 two, simultaneous projective textures at full rate processing, and four textures
at half rate. 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.).