Graphics Reference
In-Depth Information
1280
1024-pixel, 17 -inch display with an ordinary mouse—clicking on a par-
ticular pixel is virtually impossible.
We don't mean to suggest that perception should influence every decision
made in graphics; in Chapter 28 we'll see the risks that arise from treating
light throughout the rendering process in a way that captures only our three-
dimensional perception of color rather than the full spectral representation. How-
ever, in many situations where the range of brightness is small, the logarithmic
nature of the eye's sensitivity is not particularly important, and common practice
therefore often involves such things as averaging pixel values that represent log
brightnesses; such techniques often serve their purposes admirably.
×
Because computer graphics is actually in use all around us, we have to make con-
cessions to common practice, which has generally evolved because it produced
good-enough results at the time it was developed. But after a discussion of com-
mon practice, we'll often have a stand-back-and-look critique of it as well so
that the reader can begin to understand the limitations of various approaches to
graphics problems.
in Graphics
Because we will start our study of graphics with a discussion of light, it's use-
ful to have a few rough figures characterizing the light encountered in ordinary
scenes. Visible light, for instance, has a wavelength between approximately 400
and 700 nanometers (a nanometer is 10
10
−
9
m). A human hair has a diameter
×
10
−
4
m, so it's about 100 to 200 wavelengths thick, which helps
give a human scale to the phenomena we're discussing.
of about 10
×
A single photon (the indivisible unit of light) has an energy
E
that varies with the
wavelength
λ
according to
E
=
hc
/λ
,
(1.1)
10
−
34
J
sec
is
Planck's constant
and
c
10
8
m
where
h
≈
6.6
×
≈
3
×
/
sec
is the
speed of light; multiplying, we get
×
10
−
25
Jm
λ
1.98
E
≈
.
(1.2)
Using 650 nm as a typical photon wavelength, we get
10
−
25
Jm
1.98
×
10
−
19
J
E
≈
10
−
9
m
≈
3
×
(1.3)
650
×
as the energy of a typical photon.
An ordinary 100 Watt incandescent bulb consumes 100W, or 100 J
sec
,but
only a small fraction of that—perhaps 2% to 4% for the least efficient bulbs—is
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