be used for video games and computer-animated movies. One
trick for speeding up the computation is to exploit the reversibility
of light rays. If one was to trace every ray emitted by every illu-
minated surface, only a tiny fraction of those would actually end
up passing into the virtual “camera,” and therefore almost all of
the computational effort would be wasted. One can instead start
a ray at the camera, trace it backward in time, and see where it
would have come from. With this technique, there is no wasted
effort.l/This photorealistic image of a nonexistent countertop was pro-
duced completely on a computer, by computing a complicated ray
diagram.
Section 12.1 The ray model of light 775