301467-005 Datasheet, PDF (242/426 Pages) Intel Corporation – Express Chipset

Functional Description
R
12.6.4.7
12.6.4.8
12.6.4.9
Microsoft DirectX* API and SGI OpenGL* API Logic Ops
Both APIs provide a mode to use bitwise ops in place of alpha blending. This is used for rubber-
banding (i.e., draw a rubber band outline over the scene using an XOR operation). Drawing it
again restores the original image without having to do a potentially expensive redraw.
Color Buffer Formats: 8-, 16-, or 32-bits per Pixel (Destination Alpha)
The Raster Engine supports 8-bit, 16-bit, and 32-bit Color Buffer Formats. The 8-bit format is
used to support planar YUV420 format, which used only in Motion Compensation and Arithmetic
Stretch format. The bit format of Color and Z will be allowed to mix.
The GMCH supports both double and triple buffering, where one buffer is the primary buffer
used for display and one or two are the back buffer(s) used for rendering.
The frame buffer of the GMCH contains at least two hardware buffers: the Front Buffer (display
buffer) and the Back Buffer (rendering buffer). While the back buffer may actually coincide with
(or be part of) the visible display surface, a separate (screen or window-sized) back buffer is used
to permit double-buffered drawing. That is, the image being drawn is not visible until the scene is
complete and the back buffer made visible (via an instruction) or copied to the front buffer (via a
2D BLT operation). Rendering to one and displaying from the other remove the possibility of
image tearing. This also speeds up the display process over a single buffer. Additionally, triple
back buffering is also supported. The instruction set of the GMCH provides a variety of controls
for the buffers (e.g., initializing, flip, clear, etc.).
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 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.
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Datasheet