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564 22. Visual Perception
pastels to fully saturated “pure” colors. The third property, hue, corresponds most
closely to the informal sense of the word “color” and is characterized in a manner
similar to colors in the visible spectrum, ranging from dark violet to dark red.
Plate XI shows a plot of the hue-saturation-lightness (HSV) color space. Since
the relationship between brightness and lightness is both complex and not well
understood, HSV color spaces almost always use brightness instead of attempting
to estimate lightness. Unlike wavelengths in the spectrum, however, hue is usu-
ally represented in a manner that reflects the fact that the extremes of the visible
spectrum are actually similar in appearance (Plate XII). Simple transformations
exist between RGB and HSV representations of a particular color value. As a
result, while the HSV color space is motivated by perceptual considerations, it
contains no more information than does an RGB representation.
The hue-saturation-lightness approach to describing color is based on the
spectral distribution at a single point and so only approximates the perceptual
response to spectral distributions of light distributed over space. Color percep-
tion is subject to similar constancy and simultaneous contrast effects as is light-
ness/brightness, neither of which are captured in the RGB representation and as
a result are not captured in the HSV representation. For an example of color
constancy, look at a piece of white paper indoors under incandescent light and
outdoors under direct sunlight. The paper will look “white” in both cases, even
though incandescent light has a distinctly yellow hue and so the light reflected off
of the paper will also have a yellow hue, while sunlight has a much more uniform
color spectrum.
Another aspect of color perception not captured by either the CIE color spaces
or HSV encoding is the fact that we see a small number of distinct colors when
looking at a continuous spectrum of visible light (Plate X) or in a naturally oc-
curring rainbow. For most people, the visible spectrum appears to be divided into
four to six distinct colors: red, yellow, green, and blue, plus perhaps light blue and
purple. Considering non-spectral colors as well, there are only eleven basic color
terms commonly used in English: red, green, blue, yellow, black, white, gray,
orange, purple, brown,andpink. The partitioning of the intrinsically continuous
space of spectral distributions into a relatively small set of perceptual categories
associated with well defined linguistic terms seems to be a basic property of per-
ception, not just a cultural artifact (Berlin & Kay, 1969). The exact nature of the
process, however, is not well understood.
22.2.3 Dynamic Range
Natural illumination varies in intensity over 6 orders of magnitude (Figure 22.12).
The human vision system is able to operate over this full range of brightness lev-