Hairy calculations: picturing tresses in a truer light.

Computer animators find hair tricky to portray realistically because it contains so many strands and because these strands respond in complex ways to light, wind, head motion, and each other. For blond hair in particular, some of the hair's most important optical properties have been missing from software simulations that render cartoon tresses. So, animators have had to play hairdresser, tediously adding a sunny, diffuse glow to unrealistically dull coiffures.

Now, computer scientists have devised a way to trace light rays through a realistic computer model of light-colored hair. The method enables computers to automatically generate a golden color and sheen.

"The nice thing is that now we can use realistic parameters for the hair. We don't have to fake it anymore," says Stephen R. Marschner, who developed the new approach with Jonathan T. Moon. Both are at Cornell University.

Moon is scheduled to present the work Aug. 3 in Boston at the SIGGRAPH 2006 computer-graphics conference.

Real hair reflects some light and lets some pass through. In 2003, a research team that included Marschner depicted hairs in a simulation with sufficient realism to include the transmission of light through the shaft. Before that work, rendering programs treated hairs as opaque plastic cylinders, "like a bunch of little colored wires," Marsehner says.

Although adding transparency improved the naturalness of computer-generated black and dark-brown hair, blond hair still looked lifeless. In the 2003 model, light that had passed through a hair didn't illuminate other hairs.

In reality, light-colored hair lets most of the light pass through. Moreover, the light refracts as it leaves each hair shaft, much of it spreading outward and perpendicular to the shaft. So, the light that hits one patch of blonde hair also provides a soft glow to a wider area of a hairdo.

Unlike the 2003 algorithm, the new one includes illumination caused by transmitted and refracted light, Marschner notes.

The resulting improvement in light hair's look shows that "you really do have to care about this light that goes through the hair and bounces off other fibers many times," Marschner says. The technique also applies to fur, grass, and other fibrous materials, he adds.

The calculation time for rendering multiple scattering is a couple of hours per movie frame, which might be acceptable for films (SN: 1/26/02,p. 56) but is much too slow for computer games, says Marschner.

Animators used aspects of the 2003 advance to render the hair of computer-animated doubles for Naomi Watts in last year's film King Kong. Studios will probably take advantage of some features of the new computer improvement too, Marschner suggests.

Software engineer Ivan Neulander of Rhythm & Hues Studios, a Los Angeles-based animation company, says that the new approach is probably too slow for entertainment-industry artists to use routinely. On the other hand, he suggests, it "could serve as a benchmark against which simpler, fake models can be measured."