contact.  This allows for sophisticated spot-lighting effects and projection of camera
data onto synthetic terrain.  There is no performance penalty incurred during standard
texture projection.


5.6.9 Atmospheric Effects
 

A programmable fog/haze function in RealityEngine allows users to set an arbitrary
fog color, which may then be blended in a depth-dependent fashion with the color of
each pixel to be affected by the fog.  This may or may not be the entire scene, and may
be varied on a channel-by-channel or even object-by-object basis.

The fog is computed by an exponential function that the user may control by loading
the various function parameters into the system fog table.  This table may be reloaded
as necessary on the fly.


5.6.10 Flexible Video
 

RealityEngine has an innovative, flexible display subsystem capable of scanning the
digital frame buffer out to video at varying resolutions and pixel timings.  High-speed
digital-to-analog converters (DACS) combined with custom circuitry enable software
programmability of video output formats without requiring any hardware or PROM
changes.

Programmable Video Timing

To enable the broadest range of applications possible, from Video Production to
Visual Simulation, RealityEngine incorporates advanced circuitry for generating a
programmable pixel clock.  The pixel clock controls the rate at which the frame buffer
is scanned out to a video signal.  The standard ultra-high-resolution system (with one
additional RM raster board) offers 1600 x 1200 pixels at 60Hz non-interlaced, while
the standard high-resolution system runs at a resolution of 1280x1024 pixels, which
mav be output at either 60 or 72Hz non-interlaced.

Some applications require different timings, however.  Some military displays require
the RS-343 standard.  Stereoscopy for molecular modeling requires 120Hz interlaced to
output both a left-eye view and a right-eye view at 60Hz each.  Using this hardware,
Silicon Graphics has developed a number of standard formats included in every
RealitvEngine svstem.

Should a need arise for some other, non-standard video timing format, the
programmable pixel timing firmware can be modified to add the new timing.  This
service is available from Silicon Graphics on an Non-Recurring Engineering (NRE)
basis.

Standard Formats

To provide users with as much built-in flexibility as possible, RealityEngine systems
are delivered with a broad rane of software-selectable video formats, from NTSC to
l600xl200 ultra-high-resolution to HDTV The IRIS 4D/VGX and VGXT systems were
the first general purpose real-time graphics systems to provide support for HDTV,