6.3 Color Conversion Contexts and Gamut Mapping
When Xlib converts device-independent color specifications
into device-dependent specifications and vice-versa,
it uses knowledge about the color limitations of the screen hardware.
This information, typically called the device profile,
is available in a Color Conversion Context (CCC).
Because a specified color may be outside the color gamut of the target screen
and the white point associated with the color specification may differ
from the white point inherent to the screen,
Xlib applies gamut mapping when it encounters certain conditions:
-
Gamut compression occurs when conversion of device-independent
color specifications to device-dependent color specifications
results in a color out of the target screen's gamut.
-
White adjustment occurs when the inherent white point of the screen
differs from the white point assumed by the client.
Gamut handling methods are stored as callbacks in the CCC,
which in turn are used by the color space conversion routines.
Client data is also stored in the CCC for each callback.
The CCC also contains the white point the client assumes to be
associated with color specifications (that is, the Client White Point).
The client can specify the gamut handling callbacks and client data
as well as the Client White Point.
Xlib does not preclude the X client from performing other
forms of gamut handling (for example, gamut expansion);
however, Xlib does not provide direct support for gamut handling
other than white adjustment and gamut compression.
Associated with each colormap is an initial CCC transparently generated by
Xlib.
Therefore,
when you specify a colormap as an argument to an Xlib function,
you are indirectly specifying a CCC.
There is a default CCC associated with each screen.
Newly created CCCs inherit attributes from the default CCC,
so the default CCC attributes can be modified to affect new CCCs.
Xcms functions in which gamut mapping can occur return
Status
and have specific status values defined for them,
as follows:
-
XcmsFailure
indicates that the function failed.
-
XcmsSuccess
indicates that the function succeeded.
In addition,
if the function performed any color conversion,
the colors did not need to be compressed.
-
XcmsSuccessWithCompression
indicates the function performed color conversion
and at least one of the colors needed to be compressed.
The gamut compression method is determined by the gamut compression
procedure in the CCC that is specified directly as a function argument
or in the CCC indirectly specified by means of the colormap argument.
Christophe Tronche, [email protected]
