MILK looks exactly the same as white paint鈥攐r at least it does in
computer graphics animations. So animators spend hours doctoring shots to make a
glass of milk look convincing enough that you want to pick it up and take a
sip.
When you shine light into a glass of milk, some photons bounce off the
surface but most continue, bouncing back and forth off the microscopic fat and
protein molecules suspended in the liquid before being reflected. This
鈥渟ubsurface scattering鈥� gives milk a soft look that makes it easily
distinguishable from a glass of white paint鈥攁t least to the naked eye. The
same effect gives marble its soft translucent sheen.
Plotting the path of each photon would be way too difficult. So Henrik Wann
Jensen, a graphics expert at Stanford University, wondered if he could simplify
the problem. He found the answer in a nine-year-old issue of Medical Physics,
which analysed subsurface scattering in human skin (vol 19, p 879).
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The key is that light travels a relatively short distance once it enters a
translucent solid or suspension. You can see this if you shine a laser pointer
at a glass of milk. Only a small part of the milk turns pink. And you can
simplify the problem by modelling the spread of light inside the solid or
suspension as a fuzzy sphere.
From this Jensen developed a general set of rules that works for any
material. His lighting model can also cope with rays of light coming from more
than one direction. So far, he can make computer-generated milk, marble and skin
appear strikingly realistic.
To make the model, the researchers shine a tightly focused beam of white
light on a sample of milk or marble and photograph it over a wide range of
exposures to measure the scattered and reflected light. These readings are
incorporated into the model, which can then reproduce the effect. The results
are very realistic, says Jensen. People can even see the difference between
whole and skimmed milk.
Jensen and his colleagues Marc Levoy, Stephen Marschner and Pat Hanrahan have
used the technique to create highly convincing images of Michelangelo鈥檚 David.
鈥淲ith this technique we may be able to see David the way Michelangelo saw him
when he created the statue,鈥� says Jensen.
