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FOR most computer-generated human models, every day is a bad hair day. But a new technique mimicking the biology of real hair has succeeded in giving these models lustrous locks.

To render virtual hair as an image, a computer needs to know how each strand will reflect light 鈥� and there are over 100,000 strands on a full human head. Since 1989, computer animators have relied upon a model that treats each strand as an opaque cylinder. The overall effect of this model is a head of hair with a single highlight 鈥� a glossy sheen that runs straight across the hair when the character is brightly lit.

鈥淭his old model tends to look a little bit too flat,鈥� says Steve Marschner of Cornell University in Ithaca, New York. That鈥檚 because real hair, especially brown, red or other light-coloured hair, is not opaque but translucent. In reality, lighting falling on hair produces a second highlight, varying with hair colour, beyond the main highlight. This second highlight is missing in today鈥檚 graphics because the old model fails to account for hair鈥檚 translucency.

So his team studied the structure of a single fibre of hair and quantified how it scatters light. 鈥淭he outside of the fibre is made up of overlapping, microscopic scales,鈥� says Marschner. 鈥淭hey look a lot like roof shingles.鈥� These overlapping scales mean that the surface of the strand is not a true cylinder. 鈥淭he double highlight turns out to be caused by the cellular structure of the surface of the hair fibre,鈥� he says.

Based on these studies, the team developed a new model that incorporates three components: light reflected from a hair strand鈥檚 surface; light that enters the strand and is reflected by the internal surfaces; and light that passes right through the strand. 鈥淥ur new method lets us make computer-generated images of hair that quantitatively resemble images of real hair, which was not previously possible,鈥� says Marschner.

Pierre Poulin, an expert on computer rendering of hair at the University of Montreal in Canada, agrees. 鈥淭he final renderings are quite stunning.鈥�