DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 4.5 Pseudocolor
(8-bit) graphics systems
use a limited number of
integers, usually 0
through 255, to represent
colors. Each pixel
value is processed
through a color lookup
table (CLUT) to obtain
red, green, and blue
output values to be
delivered to the monitor.
Pseudocolor In a pseudocolor (or indexed color, or colormapped)
system, several bits – usually 8 – comprise each pixel in
an image or framebuffer. This provides a moderate
number of unique codes – usually 256 – for each pixel.
Pseudocolor involves “painting by numbers,” where the
number of colors is rather small. In an 8-bit pseudocolor
system, any particular image, or the content of
the framebuffer at any instant in time, is limited to
a selection of just 2 8 (or 256) colors from the universe
of available colors.
I reserve the term CLUT for
pseudocolor. In grayscale and
truecolor systems, the LUTs store
transfer functions, not colors. In
Macintosh, pseudocolor CLUT
values are roughly, but not
optimally, perceptually coded.
8 bits (code 0…255)
0
…
42 37 71
…
255
Each code value is used as an index into a color lookup
table (CLUT, colormap, or palette) that retrieves R’G’B’
components; the DAC translates these linearly into
voltage levels that are applied to the monitor. (Macintosh
is an exception: Image data read from the CLUT is
in effect passed through a second LUT.) Pseudocolor
CLUT values are effectively perceptually coded.
The CLUT and DACs of an 8-bit pseudocolor system are
sketched in Figure 4.5 above. A typical lookup table
retrieves 8-bit values for each of red, green, and blue,
so each of the 256 different colors can be chosen from
a universe of 2 24 , or 16777216, colors. (The CLUT may
return 4, 6, or more than 8 bits for each component.)
Pseudocolor image data is always accompanied by the
associated colormap (or palette). The colormap may be
38 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
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A
D
A
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CLUT
R’
G’
B’