3.5. Raster Subsystem
 
In support of advanced shading and lighting effects, the SkyWriter Raster Subsystem
incorporates an array of parallel Image Engines - custom VLSI ASICs which perform various
interpolation functions.  These functions include the interpolation of colors across faces,
transparency across faces, texture and depth (to perform hidden surface removal).
 
3.5.1 Color Interpolation
 
When performing Gouraud shading, the Image Engines interpolate a color value for a given
pixel within a polygon to enable smooth pixel to pixel color transitions.  Advanced lighting
models enable the display of surface highlights and lighting features such as time-of-day or
local light sources.

3.5.2 Color Blending
 

When multiple occulting surfaces have partial transparency, the Image Engines also perform
color blending between each set of surfaces depending upon the alpha values associated with
each vertex or even pixel (when texture mapping with alpha).  These alpha values represent a
degree of translucency from 0 to 1 and enable the simulation of windows, cockpit canopies,
smoke, clouds, and other effects.
 
3.5.3 Z Buffer
 
SkyWriter incorporates a 24 bit Z buffer in each graphics pipeline to provide high quality
hidden surface removal during rendering without the need to sort the polygons being rcndered,.
Thus, this process can Outperformed with little or no necessary programming considerations in
the application software traversing and rendering the database.
 
3.5.4 Texture Mapping
 
SkyWriter has been designed from the start to be able to display complex, texture mapped
scenes at high (30-60Hz) frame rates.  Keeping compatible with the earlier PowerVision
systems was one of the design goals, but dramatically improved Performance and image
quality was even more important.

Textures may be derived from actual photographs or generated synthetically.  They may contain
full or partial transparency at any point and support full 24 bit RGB true color.  The following
sections detail the various methodologies utilized to achieve high quality, realistic imagery.