DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Ashdown, Ian, Color Quantization
Bibliography, Internet,
Figure 4.6
Display modes
24 b (3 B)
Truecolor
16 b (2 B)
Hicolor
8 b (1 B)
Pseudocolor
640×480
800×600
1152×864
A truecolor or hicolor image can be approximated in
pseudocolor through software application of a fixed
colormap. Alternatively, a colormap quantization algorithm
can be used to examine a particular image (or
sequence of images), and compute a colormap that is
optimized or adapted for that image or sequence.
Display modes
A high data rate is necessary to refresh a PC or workstation
display from graphics memory. Consequently,
graphics memory has traditionally been implemented
with specialized “video RAM” (VRAM) devices. A lowcost
graphics adapter generally has a limited amount of
this specialized memory, perhaps just one or two megabytes.
Recently, it has become practical for graphics
adapters to refresh from main memory (DRAM); this
relaxes the graphics memory capacity constraint.
Modern PC graphics subsystems are programmable
among pseudocolor, hicolor, and truecolor modes.
(Bilevel and grayscale have generally fallen into disuse.)
The modes available in a typical system are restricted by
the amount of graphics memory available. Figure 4.6
sketches the three usual modes available in a system
having one megabyte (1 MB) of VRAM.
The top sketch illustrates truecolor (24 bits per pixel)
operation. With just 1 MB of VRAM the pixel count will
be limited to 1 ⁄3 megapixel, 640×480 (“VGA”). The
advantage is that this mode gives access to millions of
colors simultaneously.
To increase pixel count to half a megapixel with just
1 MB of VRAM, the number of bits per pixel must be
reduced from 24. The middle sketch shows hicolor
(16 bit per pixel) mode, which increases the pixel count
to 1 ⁄2 megapixel, 800×600. However, the display is now
limited to just 65536 colors at any instant.
To obtain a one-megapixel display, say 1152×864, pixel
depth is limited by 1 MB of VRAM to just 8 bits. This
forces the use of pseudocolor mode, and limits the
number of possible colors at any instant to just 256.
40 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES