Graphics Reference
In-Depth Information
characteristics is a matter of choice (just as choosing the coordinate axes to use
on a plane is a matter of choice; any pair of perpendicular lines can work!). So
some people choose to describe colors in terms of hue, saturation, and “value,”
while others prefer to describe mixes of red, green, and blue. We'll say much
more about these in Section 28.13.
Careful physiological experiments have revealed much of the structure of the
eye; Deering [Dee05] presents a good summary of the results of this work in the
context of understanding what the retina can detect, which provides a guide to
what is worth rendering in the first place. The key thing, from the point of view
of understanding color, is the presence of two kinds of receptors: rods and cones.
Rods are sensitive to visible light of all wavelengths, while the three types of
cones are sensitive to different wavelengths of light: The first has its peak response
at 580 nm, the second at 545 nm, and the third at 440 nm (see Figure 28.4).
Detailed observations of the response curves for the receptors (including rods) are
described by Bowmaker and Dartnall [BD80]. These are often described as “red,”
“green,” and “blue” receptors, even though the red and green peaks occur at wave-
lengths commonly described as yellow, with the red peak being an orangy yellow
and the green peak being a greener yellow. (To be more precise, a monospectral
light of 580 nm wavelength causes, in most viewers, the percept “orangy yellow.”)
A better set of names is “long wavelength,” “medium wavelength,” and “short
wavelength” receptors, and the names L , M , and S are often used for these. We'll
generally use “red,” “green,” and “blue,” however, to avoid the need to convert
from wavelength to color.
One can read this graph by saying, for instance, that a certain amount e of light
at 560 nm will cause a response in a red receptor, but that one would need twice as
much light at wavelength 530 nm to generate the same response in that red recep-
tor. (Of course, these lights provoke very different responses in the green and blue
receptors, too.) Furthermore, the effects of different lights on the red receptor are
additive: Sending in both e light at 560 nm and 2 e light at 530 nm will generate
the same red-receptor response as sending 2 e at 560 nm. If we use f (
λ
) to indicate
λ
and use I (
λ
) to indicate the incom-
the red receptor's response at wavelength
ing light's intensity at wavelength
λ
, then the total response from the receptor
will be
420
534 564
B
G
R
400
500
Wavelength (nm)
600
Figure 28.4: The approximate spectral response functions of the three types of cones in
the human retina; the labels R, G, and B are misleading, because the peaks of the R and G
curves both correspond to monospectral lights that most people describe as in the “yellow”
range.
 
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