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Animated Blondes Have More Fun
Computer Scientists Reveal Secrets Behind Making Animated Blonde Hair Lifelike

December 1, 2007 — Computer scientists created a modeling technique that makes it more than 20 times faster to animate true-to-life blonde haired characters. Because blonde hair does not absorb much light, in real life it ends up as many different shades, which makes for complicated modeling. The new technique uses a new algorithm that interprets where the light is coming from, maps the amount of light that should be available at each point in the hair, and creates an accurate depiction of the color at each point on the head.

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Lots of movies use computer-aided graphics to make non-real characters look realistic. But making realistic hair is still a challenge for Hollywood. Computers easily help make fake characters look real. But, it turns out, making realistic blonde hair, is one of the hardest things to create in movies.

"The kind of problem that we’ve been looking at, is how to simulate light that reflects from hair to hair to hair that’s responsible for that sort of glow or soft look of light colored hair," Steve Marschner, Ph.D., Computer Scientist at Cornell University said.

Now, computer scientists at Cornell University have developed a way to give animated blondes real life looking hair.

"You see the right kinds of highlights, you see them in the right places, moving in the right ways, and also you get the right soft appearance," Dr. Marschner said.

Blonde hair doesn’t absorb much light, so most hair ends up as several shades of blonde -- a process the new method easily recreates.

"It is basically a tool for measuring light reflection," Dr. Marschner said.

A computer program charts out light rays traveling around a hair sample. Then, it maps out where the light landed inside the hair, and determines where and how much light goes into the final image. A process made faster and more realistic than ever.

"Our hair model has become probably the most often used tool for rendering hair in, in applications where it has to be very realistic," Dr. Marschner said.

Used also for very real non-blondes -- like a hairy, life-like King Kong.

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.

Note: This story and accompanying video were originally produced for the American Institute of Physics series Discoveries and Breakthroughs in Science by Ivanhoe Broadcast News and are protected by copyright law. All rights reserved.