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Most applications provide ways of tailoring (usually
through resources or command line arguments) the colors of various elements
in the text and graphics they display. A color can be specified either
by an abstract color name, or by a numerical color specification. The numerical
specification can identify a color in either device-dependent (RGB) or
Color strings are case- insensitive. X supports the use
of abstract color names, for example, "red", "blue". A value for this abstract
name is obtained by searching one or more color name databases. Xlib
first searches zero or more client-side databases; the number, location,
and content of these databases is implementation dependent. If the name
is not found, the color is looked up in the X server's database. The text
form of this database is commonly stored in the file /usr/lib/X11/rgb.txt.
A numerical color specification consists of a color space
name and a set of values in the following syntax:
An RGB Device specification is identified by the prefix "rgb:"
and has the following syntax:
<red>, <green>, <blue> := h | hh
| hhh | hhhh h := single hexadecimal digits
h indicates the value scaled in 4 bits,
hh the value scaled in 8 bits,
hhh the value scaled in 12
bits, and hhhh the value scaled in 16 bits, respectively. These
values are passed directly to the X server, and are assumed to be gamma
The eight primary colors can be represented as:
For backward compatibility, an older syntax for RGB
Device is supported, but its continued use is not encouraged. The syntax
is an initial sharp sign character followed by a numeric specification,
in one of the following formats: #RGB (4 bits each) #RRGGBB (8 bits each)
#RRRGGGBBB (12 bits each) #RRRRGGGGBBBB (16 bits each) The R, G, and B
represent single hexadecimal digits. When fewer than 16 bits each are specified,
they represent the most-significant bits of the value (unlike the "rgb:"
syntax, in which values are scaled). For example, #3a7 is the same as #3000a0007000.
An RGB intensity specification is identified by the prefix "rgbi:" and
has the following syntax:
The red, green, and blue are floating point values between
0.0 and 1.0, inclusive. They represent linear intensity values, with 1.0
indicating full intensity, 0.5 half inten- sity, and so on. These values
will be gamma corrected by Xlib before being sent to the X server.
The input format for these values is an optional sign, a string of numbers
possibly containing a decimal point, and an optional exponent field containing
an E or e followed by a possibly signed integer string. The standard device-independent
string specifications have the following syntax:
(none, 1, none)
All of the values (C, H, V, X, Y, Z, a, b, u, v, y, x) are
floating point values. Some of the values are constrained to be between
zero and some upper bound; the upper bounds are given in parentheses above.
The syntax for these values is an optional '+' or '-' sign, a string of
digits possibly containing a decimal point, and an optional exponent field
consisting of an 'E' or 'e' followed by an optional '+' or '-' followed
by a string of digits.
For more information on device independent color, see
the Xlib reference manual.
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