Hair Today....

The barber shop I go to is old school. A long, narrow room with yellowed newspaper clippings taped to the wall, an irritating 13-inch TV permanently set to ESPN, and a revolving barber pole on the sidewalk out front.

The shop only has two real concessions to the 21st Century. The 13-inch TV is color and copies of Dr. Dobb's Journal mysteriously appear every month, mixed in among the fishing and hunting magazines.

With this in mind, I'd be curious to know what makes Steve Marschner, an assistant professor of computer science at Cornell University, and Jonathan Moon, a Ph.D. student working with Marschner, experts in hair. Let's face it, a Ph.D. from Cornell is okay, but it isn't the same thing as a certificate from the Mystros Barber Academy or license from OG's School of Hair.

Still, Marschner and Moon know their hair. They know it so well, in fact, that they've come up with a new algorithm for more accurately simulating computer-generated hair.

Marschner and Moon's algorithm begins by tracing rays from the light source into the hair, using some approximations of the scattering and producing a map of where photons of light can be found throughout the volume of hair. Then it traces a ray from each pixel of the image to a point in the hair and looks at the map to decide how much light should be available there.

The result, in a test rendering of a swatch of blond hair, appears almost identical to a rendering by the laborious path-tracing method. Path tracing for the test required 60 hours of computation, while the new method took only 2.5 hours, the researchers report.

"The model that's been around since the '80s works for black hair, and a model we introduced in 2003 in collaboration with workers at Stanford gets brown hair right and makes blond hair better," said Marschner. "Using that model with our new work provides the first practical method to use physically realistic rendering for blond hair and still get the right color."

The problem is that light traveling through a mass of blond hair is not only reflected off the surfaces of the hairs, but passes through the hairs and emerges in a diffused form, from there to be reflected and transmitted some more.

The only method that can render this perfectly is "path-tracing," in which the computer works backward from each pixel of the image, calculating the path of each ray of light back to the original light source. Since this require hours of calculations, computer artists resort to approximations.

"People do something reasonable for one bounce and then assume it reflects diffusely," Marschner explained. In other words, he said, they assume that hair is opaque. "In light-colored hair it's important to keep track of the hair-to-hair scattering," he said.

Marschner now plans to look for better ways to generate the geometric model of hair that underlies the rendering and to simulate realistically the way hair moves. "Tools that generate hair generate random strands in space, and it's unclear whether the arrangement is realistic," he explained.

Actually, as you can tell from my accompanying photo, I have other concerns. Like now that Marschner and Moon can help me more realistically render hair, how about helping just grow it.

Posted by Jon Erickson at 08:55 AM  Permalink