Section 4            VGXT Graphics Subsystem
 
The VGXT offers much improved texture performance for advanced visualization and
simulation applications over the earlier VGX graphics subsystem.  The primary
difference between the VGX and the VGXT lies in the Raster Subsystem.  While the
system architecture of the VGXT is identical to that of the VGX, the VGXT employs
new Image Engines (IEs) that have been leveraged from the high-end SkyWriter Host
Integrated Computer Image Generator.


4.1 Pixel Processors
 

The new IE ASICs have been designed to perform all the same functions as the VGX
processors, yet improve both texture performance and quality.  By hard-wiring
algorithms that had previously been microcoded, the pixel fill rate for bilinear
MIPmapped texture has been improved from 17M pixels per second on the VGX to
50M pixels per second on the VGXT.  The addition of this functionality almost doubles
the complexity of this processor, yet it still fits into the existing system architecture.
Each VGXT system has 20 IE processors running in parallel.  Systems with the optional
MultiBuffer/TX(tm) card have twice as many processors (40) and double the fill rate of
the standard VGXT.

All performance numbers quoted are derived from MultiBuffer/TX systems.


4.2 Texture Performance
 

To maintain compatibility with VGX systems, all the VGX texturing methods are
supported on VGXT.  They include:

o   Point Sampling

o   Bilinear Interpolation

o   MIPmap Point Sampling

o   MIPmap Linear Interpolation

o   MIPmap Bilinear Interpolation

Using a single MIPmap layer, Bilinear Interpolation performs its sampling and
blending in the level of the MIPmap closest in size to the polygon being textured.  This
blends the four texels around the S and T location of the pixel being textured to
produce the final texel color to be assigned to the pixel.

The VGXT has been optimized for this method of texture mapping and can perform
bilinear MIPmapping at 50M pixels per second.  All the other methods also perform at
this speed, so unless it is necessary to avoid MIPmapping to use larger textures, this
should be the preferred mode of texturing.

If even higher-fidelity texture mapping is desired or needed, the VGXT also supports
Trilinear interpolation in the MIPmap.  This is the highest quality texture mapping
method available and mav be found on most of the newer, high-end image
generators.  Trilinear interpolation performs two bilinear interpolations in the two