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4.2 Color management
4.2.1 Basic understanding
The translation between RGB values specified in applications and the color displayed on-screen is controlled by
pixel values serving as indices to a color map. The application obtains the pixel values from the X window sys-
tem.
RGB values
RGB values are the red, green and blue intensity values that are used to define a color. According to the X.11
protocol these values are represented as 16-bit, unsigned numbers, with 0 the minimum intensity and 65535 the
maximum intensity. Most hardware however represents each value as a number in the range from 0 to 255.
Color map
A color map consists of a set of numbered entries – so called color cells – defining pixel values. Each color cell
contains a single RGB triple – a combination of RGB values. The color map will be filled at runtime.
X.11 supports two ways for handling color maps:
•
shared color cell
Colors that can be used by different X clients are called shared colors. A color that was once allocated can not
be changed. When an X client is requesting for a shared color the X server searches the color map for exactly
the requested color (exact match). If the color is not found a new color will be allocated in the color map.
•
private color cell
A color that can only be used by one client is considered to be a private color. Only the client that allocated it
can use a private color. A client is able to change the contents of a private cell. As long as entries are avail-
able in the color map, private colors can be allocated.
Color depth
On a black-and-white screen, each pixel is represented with a single binary digit, i. e. a color depth of 1. Many
color workstations have a color depth of 8bit, each pixel on such a screen is represented by eight bits, allowing
8
2
= 256 on-screen colors. A color depth of 24 bits corresponds to 2
cover all shades of colors the human eye can distinguish.
Barco – TransForm A - X Terminal – DOC-3265-2 – user's manual – Revision 09 – September-2006
24
= 16.7 million (16M) colors and is said to
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4. Operating
4-3
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