It was hard to picture the semi-opaque milk in Shrek I.

Shrek, some say the most difficult shot to produce was that of a small glass of milk. By the time Shrek 2 came out in 2004, vastly improved software for rendering milk meant that the guards in the sequel went crazy for the stuff, even going so far as dumping boiling milk on a walking gingerbread man.
    Milk was previously difficult to model realistically because it is translucent. In the first Shrek, it was modeled as an opaque fluid, which meant the light bounced straight off its surface, making it look like paint.
    To build a realistic model of milk, in 2001, Henrik Wann Jensen at the University of California, San Diego, and colleagues added reflections from light scattering beneath the milk’ s surface. They used a technique that was later used to make Gollum’ s skin look eerily realistic in The Lord of the Rings trilogy. Now, insights gained during this progress are being put to work in the dairy industry, in the name of quality control.
    To model just how light moves under the surface of a substance, Jensen specifies the substance, ability to scatter, absorb, refract and spread light. He deduces what values each property should have for a given substance by shining a spot of light onto a sample and measuring how the light intensity fades from the centre of the spot. Software then uses those properties to create a realistic model of the light moving and scattering beneath the surface.
    Now Flemming Moller, a researcher at Danish food-ingredient company Danisco, is borrowing Jensen’ s technique to help determine particle sizes in drinking yogurt and to measure the size of air bubbles and ice crystals in ice cream—important for quality control and standardization. Like Jensen, he shines a spot of laser light on the yogurt or ice cream. As he has already correlated how the resulting pattern varies with particle and air bubble size, he can determine them from the shape of the spot. This allows Moller to test the products’ quality without having to sample the food invasively, something that always carries a risk of contamination. It also removes the need to dilute the samples, which is necessary for standard light-based tests.
    The technique is not used routinely at Danisco but Moller hopes it will become widespread. "This work has been an eye-opener," he says. "I thought that computer graphics were very simple—you sit down and it’ s a lot of nerds. I was very surprised that there was a lot of science behind it. "
    Compliments aside, Jensen has since updated the milk model so that it can be programmed to vary the sub-surface scattering and reflection according to the relative fat and protein composition of the milk. The primary light-scattering particles in skimmed milk are clumps of protein, but whole milk also contains fat globules. Jensen’ s model uses this to work out how to vary the way milk looks according to the fat and protein composition. He found that skimmed milk looks bluish, because protein molecules scatter blue light preferentially and whole milk looks white, because fat globules scatter all frequencies equally.
    He can also reverse the process to determine the fat and protein content of a sample of milk—and therefore the type of milk just by shining light on it. He does this by running multiple milk simulations, tweaking the fat and protein content with each run until the optical properties of the simulated milk—and therefore the fat and protein content—match that of the real thing. Moller hopes to use the same technique to more precisely determine particle size in a sample.
    Jensen believes that such models will have other applications. By measuring how pollutants affect the optical properties of seawater, a model similar to the milk model could be used to monitor and interpret changes in the oceans, he says. And a model of the atmosphere might allow changes in its composition to be tracked.