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Hair has always been treated like a character by Glen Keane: "an outward manifestation" of the character's personality and problem. But Keane and Disney met their match with Rapunzel in Tangled (opening today). With 70-feet of luxurious blond hair, "this is a story about a girl with enormous potential -- she has something inside her that has to get out," Keane explains. "It's physically exploding out of her and is her life force, so the hair has to be compelling…

"[It] has to have rhythm and the way it moves; it has to have volume; it has to have twist; the swoop has to have a definitive shape; and there are a lot of artistic choices for a technical crew that had never been challenged that way before. This wasn't just pushing a button and making sim happen; you've got to also artistically position the hair. It was an amazing team to watch grow. I asked Eric Daniels, who worked with me on Long John Silver doing the mechanical arm, to oversee the hair team to bring artistic qualities into that."

That's because early on hair was treated solely as a technical challenge, so Keane lectured the technical team on the artistic importance, and from then on it became a hybrid solution, like so many aspects of Tangled.

The result is a breakthrough achievement for the way CG hair is rendered, using a blend of simulation and animation techniques to create a new 2D/3D aesthetic in keeping with Keane's vision. So no more worries about touching hair or cloth and out of control collisions.

"I was the liaison between the character TDs and Glen and the directors [Nathan Greno & Byron Howard] to speed up the process and have it look the way they wanted," Daniels suggests. "There were hundreds of parameters that you could tweak and every time the TD would make a change, it would take hours to get the results back. They would have to really think hard: Should I change this and move that a little higher? Make this friction a little lower? Or make the bend frequency a little higher? I can't tell you how collaborative this hair thing has been. It had to be spread across the entire studio. The character animators had to be able to control the hair in a lot of situations, like when she's using it as a whip. But they also had to be able to leave the hair for us. And then the way we would animate the hair would vary from shot to shot."

This necessitated the revamping of the simulation engine developed for Bolt called Dynamic Wires, which added volume, sensuous twists, graceful turns, breaking strands and Rapunzel's trademark swoop in the front. The technical team animated 147 different tubes representing the structure of the hair, which would then be rendered into a final image with up to 140,000 individual strands of hair.

In addition, several new tools were written for hair interaction, motion control and interpolation, volume rendering and a new shading algorithm design to provide more of a real world look to the hair. In fact, they rewrote their RenderMan shader system from scratch. There was even a new hair rig for greater control in animation.

And when Rapunzel's magical hair glowed, that was created by the effects department in collaboration with Look Dev TD Lewis Siegel. The glow was based on an occlusion pass and they also did a fiber-optic effect where light went right down the individual strands and washed through the entire hair. But then that glow had to interact with the characters and environments. The lighting department, for instance, was given a composite graph comprised of effects and animation that was incorporated into its graph. Lighting would also use point clouds combined with hand-placed lights to generate, say, diffuse lighting from the hair onto the characters.