The dimension of hue is not our only option for describing colour. For many scientific purposes, systems are used that dispense with hue and chroma, and instead use orthogonal coordinates. Two types of systems of orthogonal coordinates, CIE Lab space, and a series of related colour spaces used for video systems, will be mentioned briefly here.

Most books on colour science and many websites give good accounts of CIE Lab space, and the details and history of its derivation from colourimetric data from large numbers of individuals (for example see the account at the site by efg's Computer Lab). While the specification of Lab colour eliminates the dimension of hue, its graphical representation in the Adobe Colour Picker in practice offers the digital painter a means of selecting colours according to the dimensions of hue, relative chroma and lightness (Figure 7.14).


Figure 7.14. CIE Lab Colour space. Left: Gamut of RGB colours in Lab space viewed in ab plane, using ColorSpace. Note that the additive complimentary pairs are not exactly opposite each other in CIE Lab space. Right: Graphical representation of Lab space in the colour picker in Photoshop CS2, showing relationship to hue, relative chroma and lightness.

Bruce MacEvoy has published colour wheels showing representative colourimetric measurements of a large number of watercolour pigments plotted using in a modified version of the CIE Lab system, and more subsequently in the more recent CIECAM system (link). As colour is measured in these systems in colorimetric units, unknown colours or mixtures strictly speaking can not be plotted on these diagrams by artists lacking spectrophotometers, although positions could of course be guessed roughly based on the plotted pigments. The opposing hues or "visual complementaries" in the CIE Lab system are not exact additive, pigmentary or psychological complements.

YUV, YIQ, and YCbCr colour spaces, devised for video systems, also use orthogonal coordinates instead of hue (Figure 7.15). YCbCr, which I have been using for the illustrations of image colours in space throughout this site, is a transformation of YUV that conveniently results in the RGB gamut, when viewed from above, forming a regular hexagon with the screen primaries evenly spaced and opposite their additive complementaries, as in the RGB-CMY hue circle (Figure 7.15).

Figure 7.15. Plan views of RGB gamut in (A) YUV, (B) YIQ and (C) YCbCR colour spaces. In YCbCr the screen primaries are evenly spaced, as in the RGB-CMY hue circle, though in the reverse order.

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