5.4 Raster Subsystem
This data is transformed bv the GEs using the top matrix of the current transformation
stack. If polygons fed to the GEs have more than 3 vertices, they are subdivided into
triangles. Triangles are the basic polygonal working units of the system. Once the
geometric processing has completed, the triangles (now in screen space) are passed
onto the Triangle Bus (T-Bus) for processing by the Raster Subsystem.
The Raster Subsystem resides on the Raster Manager board and contains the great
bulk of the custom VLSI processors and graphics system memory. The data received
by the RM across the T-Bus must be scan-converted into pixel data, then processed
into the frame buffer before control is handed off to the Display Subsystem to
generate displayable video signals.
Incorporating extensive parallelism in almost every stage, the Raster Subsystem
allows the system to stav busy with full-performance anti-aliasing and texture
mapping without generating bottlenecks. Anti-aliasing is performed with a method
called multisampling, in which images are rendered into a frame buffer with a higher
effective resolution than the displavable frame buffer. just how much higher is
determined by the number of sub-samples computed for each pixel. Thus, if each
pixel has eight sub-samples (positioned at eight of the 64 possible sub-pixel locations),
then the effective resolution is eight times the displayable resolution.
When the image has been rendered into these sub-samples, the multiple samples per
pixel are effectively blended to determine the color of the displayable pixel. Image
memory is interleaved among the parallel processors in such a way that adjacent