6.10.4 Supplied White Point Adjustment Procedures

White point adjustment procedures provided by Xlib are as follows:

XcmsCIELabWhiteShiftColors
This uses the CIE L*a*b* color space for adjusting the chromatic character of colors to compensate for the chromatic differences between the source and destination white points. This procedure simply converts the color specifications to XcmsCIELab using the source white point and then converts to the target specification format using the destinations white point. No client data is necessary.
XcmsCIELuvWhiteShiftColors
This uses the CIE L*u*v* color space for adjusting the chromatic character of colors to compensate for the chromatic differences between the source and destination white points. This procedure simply converts the color specifications to XcmsCIELuv using the source white point and then converts to the target specification format using the destinations white point. No client data is necessary.
XcmsTekHVCWhiteShiftColors
This uses the TekHVC color space for adjusting the chromatic character of colors to compensate for the chromatic differences between the source and destination white points. This procedure simply converts the color specifications to XcmsTekHVC using the source white point and then converts to the target specification format using the destinations white point. An advantage of this procedure over those previously described is an attempt to minimize hue shift. No client data is necessary.

From an implementation point of view, these white point adjustment procedures convert the color specifications to a device-independent but white-point-dependent color space (for example, CIE L*u*v*, CIE L*a*b*, TekHVC) using one white point and then converting those specifications to the target color space using another white point. In other words, the specification goes in the color space with one white point but comes out with another white point, resulting in a chromatic shift based on the chromatic displacement between the initial white point and target white point. The CIE color spaces that are assumed to be white-point-independent are CIE u'v'Y, CIE XYZ, and CIE xyY. When developing a custom white point adjustment procedure that uses a device-independent color space not initially accessible for use in the color management system, use XcmsAddColorSpace() to insure that it is added.

As an example, if the CCC specifies a white point adjustment procedure and if the Client White Point and Screen White Point differ, the XcmsAllocColor() function will use the white point adjustment procedure twice:

Once to convert to XcmsRGB
A second time to convert from XcmsRGB

For example, assume the specification is in XcmsCIEuvY and the adjustment procedure is XcmsCIELuvWhiteShiftColors. During conversion to XcmsRGB, the call to XcmsAllocColor() results in the following series of color specification conversions:

From XcmsCIEuvY to XcmsCIELuv using the Client White Point
From XcmsCIELuv to XcmsCIEuvY using the Screen White Point
From XcmsCIEuvY to XcmsCIEXYZ (CIE u'v'Y and XYZ are white-point-independent color spaces)
From XcmsCIEXYZ to XcmsRGBi
From XcmsRGBi to XcmsRGB

The resulting RGB specification is passed to XAllocColor(), and the RGB specification returned by XAllocColor() is converted back to XcmsCIEuvY by reversing the color conversion sequence.

Next: Gamut Querying Functions

Christophe Tronche, ch@tronche.com