Intel® 945G/945GZ/945GC/ 945P/945PL Express Chipset Family ...

10.5.3 4X Faster Setup Engine
Functional Description
The setup stage of the pipeline takes the input data associated with each vertex of a 3D primitive
and computes the various parameters required for scan conversion. In formatting this data, the
GMCH maintains sub-pixel accuracy. For the 82945G/82945GC/82945GZ GMCH, it was
redesigned to run at 1 cycle/attribute.
10.5.3.1 3D Primitives and Data Formats Support
The 3D primitives rendered by the GMCH are points, lines, discrete triangles, line strips, triangle
strips, triangle fans, and polygons. In addition to this, the GMCH supports the Microsoft DirectX
Flexible Vertex Format (FVF) that enables the application to specify a variable length parameter
list. This obviates the need for sending unused information to the hardware. Strips, Fans, and
Indexed Vertices, as well as FVF, improve the vertex rate delivered to the setup engine
significantly.
10.5.3.2 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, the 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 supports 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”.
10.5.3.3 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 assurance of 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.
241