ColourSystem Class Reference

List of all members.

Detailed Description

The colour mapping is specified by the (x,y) chromaticities of the three primary colours of the system, and the chromaticity and luminance of the system's white point. (From this, the luminances of the primaries can be derived.) See Foley & van Dam, p586.

See ColourSystem.h for various predefined colours systems and illuminants.

Sources: http://www.inforamp.net/~poynton/ColorFAQ.html, http://www.efg2.com/lab/Graphics/Colors/Chromaticity.htm

Notes.

CIE Y matches the eye's spectral efficiency curve thus it measures the colour's *luminance*, in candela.

X and Z are orthogonal to luminance; varying them does not affect the luminance (or perceived brightness) of the colour, just its hue.

We can find the luminance of an RGB colour by simply transforming it back to XYZ space, and then taking the Y component. Thus if w = toXYZ[1], then dot(w, clr) is clr's luminance. The Luminance() method is provided for this.

To convert a chrominance/luminance (x, y, Y) specification into an XYZ colour requires:

 form XYZ corresponding to x, y
 clr = [x, y, 1.0 - y - x]
 normalise to a luminance of 1.0
 clr /= y;
 multiply through by luminance
 clr *= Y; 
 

Beware: converting a (say) unit luminance chroma to RGB will give you colours that are equally perceptually bright. This may not be what you want: if you try to display such a range of chromas, for instance, the white point will get mapped to [1, 1, 1], but bluish hues will be mapped so that the blue component is much bigger than 1, because of our relative insensitivity to blue.

In such cases, you probably want the brightest-possible device colour corresponding to each specified chroma, so...

 form XYZ corresponding to x, y
 clr = [x, y, 1.0 - y - x]
 convert to RGB
 clr = cs.XYZToGamut(clr);
 normalise to brightest possible RGB
 clr /= MaxCmpt(clr);
 

Definition at line 21 of file ColourSystem.h.

Constructor & Destructor Documentation

Member Function Documentation

Member Data Documentation

• ColourSystem.cc
• ColourSystem.h