Simulating the "glow" of blond hair in computer graphics is apparently much harder than creating lifelike brown or black hair. Cornell researchers report that they have a new method to render blonde locks much quicker with a better end result. Researcher Steve Marschner's previous CG hair styling tricks appeared in King Kong while his skin rendering techniques used in Lord of the Rings won him a technical achievement award from the Academy of Motion Picture Arts and Sciences. From the Cornell Chronicle Online:
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 and (grad student Jonathan) 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.
