BACKGROUND: A Cornell University professor named Steve Marschner and his graduate student, Jonathan Moon, have developed the first practical computer animation method to use physically realistic rendering for blond hair and still get the right color. Computers can create 3D structures resembling hair, but the process of “rendering,” in which the computer figures out how light will be reflected from those structures to create an image, requires complex calculations that take into account the scattering of light between hairs.
THE PROBLEM: Dark hair has a sheen, and real blond hair has a glow. These are the result of the scattering of light off individual strands of hair. This effect is very difficult to reproduce digitally. It requires a laborious process known as 'path tracing' to keep track of all that hair-to-hair scattering. To save time, current methods use approximations, which work well for dark hair, but just don't do justice to glowing blond tresses. The problem is compounded because when light hits a mass of blond hair, it not only reflects off the surfaces of each strand, it also passes through the hairs and emerges in diffused form, and then is reflected and transmitted yet again, and so forth.
THE SOLUTION: Marschner and Moon developed a new process that traces rays from the light source into the hair, using approximations of the scattering to produce a map of where photons of light are located. The program then traces a ray from each pixel of the image to a point in the hair, using the map to determine how much light should be present in that spot. The result? Blond hair, simulated in 2.5 hours, that is as realistic as the path tracing method, which required 60 hours of computation. Marschner is now extending his work to realistically simulate how hair moves by coming up with a better geometric model for generating random strands in space.
The Optical Society of America contributed to the information contained in the TV portion of this report.