A second application is the ability to draw hollow polygons, a useful tool for
visualizing the structure of solid models.  By drawing the outline of each facet into the
stencil, then performing Z-buffered drawings of the whole facet while using the
stencil as a mask, the true joining edges of an object's surface can be displayed alone,
highlighted, or with background-color filled to give a hidden-line representation.

Most significantly, the stencil mechanism allows Constructive Solid Geometry pixel
algorithms to be implemented in a parallelized environment.  The flexible testing and
updating constructs designed into the Image Engines allow the construction of unions
and intersections of primitive shapes that have all the attributes of texture mapping,
transparency, and aiiti-aliasing.


3.3.20 Arbitrary Clipping Planes
 

The Geometry Subsystem supports the definition of six planes in 3D space.  Geometric
primitives can be clipped against these planes in addition to the normal six planes
that describe the current viewing volume, providing an ideal mechanism for viewing
the cross section of model components or volumetric data.

Alterniitivelv, the distance between a primitive and any plane can be calculated.  This
distance can be used as a texture mapping coordinate, which will then produce a
contour map that can be applied to any 3D model for improved visualization.
 

3.3.21 Pixel Read, Write, and Copy
 
The PowerVision system offers an unprecedented level of power in pixel moving
operations.  Proprietary circuits in the Scan Conversion and Raster Subsvstems allow a
variety of pixel formats to be supported.  Images ranging from one to 32 bits in depth
can be manipulated.  Further, zooming and shrinking can be performed on rectangular
areas to an arbitrary fractional power.  Operations take place at a peak rate of 20
million pixels per second.  Arithmetic and logical operations performed bv the Image
Engines enable image processing to be performed on these rectangular areas.
 
3.3.22 Sphere Rendering
 
The PowerVision graphics subsvstem supports high-speed rendering of high-quality
spheres.  Its intelligent frame buffer allows a sphere to be rendered as a "2.5 D" image,
that is, as a two-dimensional array of pixel data with a depth value associated with
each pixel.  These images are rendered using the standard Z-buffer algorithm.  Using
these techniques, the PowerVision graphics supercomputers draw spheres at 10M
pixels per second.

For applications that render spheres using polygon primitives, the increased polygon
performance of the PowerVision svstem allows spheres to be rendered at a rate of
7800 128-triangle spheres per second.

Sphere rendering is not incorporated in to the Graphics Library; it is a separate sphere
library