more than 16 million colors.  The DACs are multiplexed to handle the input from the
five parallel pixel pipes. fn color-'ndex mode, pixel data packets are used as indices
into a 12-bit-in 24-bit-out color map before being sent to the DACS.  This map defines
4096 simultaneously visible colors from a palette of 16.7 million.  The pixel-mapping
feature of the color-index mode allows screen colors to be quickly modified by simply
changing the values stored in the color maps.

The PowerVision graphics subsystem uses RAMDAC technology to provide gamma
correction under application control.  Any 8-bit to 8-bit mapping can be specified and
is applied both to RGB and color index pixels.

Alpha Channel

The PowerVision graphics subsystem gives users the capability to output the contents
of tl-ie Alpha buffer.  This is useful for video applications such as compositing, which
require coverage information on a per-pixel basis to blend different images together in
the same scene.


3.2 PowerVision Configuration
 

3.2.1 MultiBuffer
 
The basic PowerVision system contains a single raster board.  System expendability
enables the addition of one more raster board to produce a system with double the
number of Span and Image Memory Processors.  The MultiBuffer option doubles the
pixel fill rate of the standard svstem and provides additional frame buffer bit planes.

On a MultiBuiffer system, the accumulation buffer functionality for anti-aliasing,
motion blur, and so on is executed and managed directly in the hardware, whereas the
standard VGX systems simply emulate the accumulation buffer in software.
Additional memory on the MultiBuffer board may also be used to hold more resident
(non-paged) textures.

The VGXT svstems also have a MultiBuffer option, the MultiBuffer/TX, which serves
the same purpose, but which has new, texture-accelerated Image Engine processors.
 

3.2.2 Video Configurations
 
Broadcast Quality NTSC and PAL

Broadcast quality video signals are generated with the broadcast video option
daughter board.  This board is a daughter module that sits on top of the standard
PowerVision display board and provides both composite video signals and genlock.
Video signals are available for RGB (RS170), NTSC, and PAL formats (NTSC and PAL
versions are different and must be selected at time of purchase).

Programmable Video Output Formatter

The Video Output Formatter (VOF) is a microprogrammable processor that provides
timing signals to control the operation of the interleaved parallel Multimode Graphics
Processors and to manage the timing of SYNC, BLANK, and other signals that
structure the scanning pattern of the display device.