DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

The browser-safe palette forms
a radix-6 number system with
RGB digits valued 0 through 5.
216 6 3
=
fixed, independent of the image, or it may be specific to
the particular image (adaptive or optimized).
A popular choice for a fixed CLUT is the browser safe
palette comprising the 216 colors formed by combinations
of 8-bit R’, G’, and B’ values chosen from the set
{0, 51, 102, 153, 204, 255}. This set of 216 colors fits
nicely within an 8-bit pseudocolor CLUT; the colors are
perceptually distributed throughout the R’G’B’ cube.
Pseudocolor is appropriate for images such as maps,
schematic diagrams, or cartoons, where each color or
combination is either completely present or completely
absent at any point in the image. In a typical CLUT,
adjacent pseudocolor codes are generally completely
unrelated; for example, the color assigned to code 42
has no necessary relationship to the color assigned to
code 43.
Conversion among types
In Figure 4.1, traversal from left to right corresponds to
conversions that can be accomplished without loss.
Disregarding pseudocolor for the moment, data in any
of the other four schemes of Figure 4.1 can be
“widened” to any scheme to the right simply by
assigning the codes appropriately. For example,
a grayscale image can be widened to truecolor by
assigning codes from black to white. Widening adds
bits but not information.
A pseudocolor image can be converted to hicolor or
truecolor through software application of the CLUT.
Conversion to hicolor is subject to the limited number
of colors available in hicolor mode. Conversion to truecolor
can be accomplished without loss, provided that
the truecolor LUTs are sensible.
Concerning conversions in the reverse direction, an
image can be “narrowed” without loss only if it contains
only the colors or shades available in the mode to its
left in Figure 4.1; otherwise, the conversion will involve
loss of shades and/or loss of colors.
CHAPTER 4 RASTER IMAGES IN COMPUTING 39