This feature is sometimes used by older games to produce
fade-out and flashing effects.
TrueColor and DirectColor visuals store an actual RGB value in the video framebuffer.
The individual red, green, and blue components are looked up in separate
colormaps, or color lookup tables (CLUT), to determine the value of the signal to
send to the monitor on the screen. TrueColor uses nonwritable colormaps to ensure
linear intensity changes, while DirectColor uses a writable colormap to provide
color-correction capabilities.
Most current desktop and notebook systems use a 24-bit RGB visual (DirectColor/
TrueColor), but small-form-factor devices such as phones, PDAs, and tablets may use
a more restrictive visual due to memory and power constraints.
Table 9-1. X visuals
Monochrome
visual class
Color visual
class
Colormap
details
Typical number
of panes Notes
StaticGray (0) StaticColor (2) Nonwritable 1-16
GrayScale (1) PseudoColor (3) Writable 4-16
??” TrueColor (4) Separate map for
each RGB channel;
nonwritable
(linear ramp)
24 Most commonly
used visual on
modern displays
??” DirectColor (5) Separate map for
each RGB channel;
writable
24
??” ARGB (6) Same as True-
Color
32 Alpha channel
enables variable
transparency;
used with COMPOSITE
extension
9.
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