301467-005 Datasheet, PDF (237/426 Pages) Intel Corporation – Express Chipset

Functional Description
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Pixel Accurate “Fast” Scissoring and Clipping Operation
The GMCH supports 2D clipping to a scissor rectangle within the drawing window. Objects are
clipped to the scissor rectangle, avoiding processing pixels that fall outside the rectangle. The
GMCH’s clipping and scissoring in hardware reduce the need for software to clip objects, and
thus improve performance. During the setup stage, GMCH clips objects to the scissor window.
A scissor rectangle accelerates the clipping process by allowing the driver to clip to a bigger
region than the hardware renders to. The scissor rectangle needs to be pixel accurate, and
independent of line and point width. The GMCH will support a single scissor box rectangle that
can be enabled or disabled. The rectangle is defined as an Inclusive box. Inclusive is defined as
“draw the pixel if it is inside the scissor rectangle”.
Depth Bias
The GMCH supports source Depth Biasing in the Setup Engine. The Depth Bias value is
specified in the vertex command packet on a per primitive basis. The value ranges from -1 to 1.
The Depth Bias value is added to the z or w value of the vertices. This is used for coplanar
polygon priority. If two polygons are to be rendered that are coplanar, due to the inherent
precision differences induced by unique x, y and z values, there is no guarantee which polygon
will be closer or farther. By using Depth Bias, it is possible to offset the destination z value
(compare value) before comparing with the new z value.
Backface Culling
As part of the setup, the GMCH discards polygons from further processing, if they are facing
away from or towards the user’s viewpoint. This operation, referred to as “Back Face Culling” is
accomplished based on the “clockwise” or “counter-clockwise” orientation of the vertices on a
primitive. This can be enabled or disabled by the driver.
Scan Converter
Working on a per-polygon basis, the Scan Converter uses the vertex and edge information is used
to identify all pixels affected by features being rendered.
Pixel Rasterization Rules
The GMCH supports both OpenGL and D3D pixel rasterization rules to determine whether a
pixel is filled by the triangle or line. For both D3D and OpenGL modes, a top-left filling
convention for filling geometry will be used. Pixel rasterization rule on rectangle primitive is also
supported using the top-left fill convention.
2D Functionality
The stretch BLT function can stretch source data in the X and Y directions to a destination larger
or smaller than the source. Stretch BLT functionality expands a region of memory into a larger or
smaller region using replication and interpolation. The stretch BLT function also provides format
conversion and data alignment.
Datasheet
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