Mathematical Topics from 3D Graphics - 3D Math Primer for Graphics and Game Development - page 353

stone and an important bit of graphics history. Actually, describing Blinn-
Phong as “history” is wishful thinking—perhaps the most important reason
to study this model is that it still is in such widespread use! In fact, it's the
best example of the phenomena we mentioned at the start of this chapter:
particular methods being presented as if they are “the way graphics work.”
Different lighting models have different goals. Some are better at sim-
ulating rough surfaces, others at surfaces with multiple strata. Some focus
on providing intuitive “dials” for artists to control, without concern for
whether those dials have any physical significance at all. Others are based
on taking real-world surfaces and measuring them with special cameras
called goniophotometers, essentially sampling the BRDF and then using
interpolation to reconstruct the function from the tabulated data. The no-
table Blinn-Phong model discussed in Section 10.6 is useful because it is
simple, inexpensive, and well understood by artists. Consult the sources in
the suggested reading for a survey of lighting models.
10.1.3 A Very Brief Introduction to Colorimetry
and Radiometry
Graphics is all about measuring light, and you should be aware of some
important subtleties, even though we won't have time to go into complete
detail here. The first is how to measure the color of light, and the second
is how to measure its brightness.
In your middle school science classes you might have learned that every
color of light is some mixture of red, green, and blue (RGB) light. This is
the popular conception of light, but it's not quite correct. Light can take
on any single frequency in the visible band, or it might be a combination of
any number of frequencies. Color is a phenomena of human perception and
is not quite the same thing as frequency. Indeed different combinations of
frequencies of light can be perceived as the same color—these are known
as metamers. The infinite combinations of frequencies of light are sort of
like all the different chords that can be played on a piano (and also tones
between the keys). In this metaphor our color perception is unable to pick
out all the different individual notes, but instead, any given chord sounds
to us like some combination of middle C, F, and G. Three color channels is
not a magic number as far as physics is concerned, it's peculiar to human
vision. Most other mammals have only two different types of receptors (we
would call them “color blind”), and fish, reptiles, and birds have four types
of color receptors (they would call us color blind).
However, even very advanced rendering systems project the continuous
spectrum of visible light onto some discrete basis, most commonly, the
RGB basis. This is a ubiquitous simplification, but we still wanted to
let you know that it is a simplification, as it doesn't account for certain