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138 Chapter 9: Color
When using 24-bit RGB values on a 32-bit system, multiple (sometimes partial) pixels
occupy each memory word. This slows write speed by a factor of 75 to 90% and
is so inefficient that many 24-bit systems actually devote 32 bits of framebuffer memory
to each pixel; this wastes 8 bits per pixel (25% of the memory) but significantly
speeds up write operations.
ARGB visuals use that extra 8 bits of data to represent transparency. This information
is used by the COMPOSITE extension when building the screen image from the
component images.
The default visual class??”the class of the root window??”is usually determined by the
X server based on the number of bits per pixel, but it can be requested on the X
server command line using the -cc option and a visual class number from
Table 10-1:
$ X -cc 3 -depth 24
In many cases, the X server will ignore the -cc option (or, as the Xserver manpage
politely notes, this option is ???Not obeyed by all servers???).
Your X sever may have an astounding number of TrueColor or
DirectColor visuals defined; these are intended for use as OpenGL
contexts.
9.3 Gamma
Cathode ray tubes, which were the first video output devices available and the earliest
form of computer display monitor, have a nonlinear response to input.
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