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.
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.
"You put constraints or phantom objects to control the hair in simulation," adds Steve Goldberg, the visual effects supervisor. "One of the first things we worked on was how it fell off her as she walked. They played a lot with the friction on the ground and tangential forces to get the hair follow with her in a believable way."
Kelly Ward, the senior software developer, whose specialty is modeling and simulation, got a head start in improving Dynamic Wires. She was then joined by Maryann Simmons, who concentrated on integration, and Tom Thompson, who worked on the simulation guides and turning them into 100,000 hairs ready for rendering.
"The key to our success on the film was thinking of hair in terms of volume rather than a shell-like surface or individual strands," says Ward.
Meanwhile, Hide Yosumi, a character TD, built an animation rig attached to her head as well as a separate prop hair rig in Maya . "We needed to create a new system of keyframe control for the animators," Yosumi suggests. "Then we took that animation and put some simulation on top of it. Not surprisingly, the prop hair rig was used throughout most of the film, when Rapunzel's hair takes on a life of its own, winding its way to a shadowed part of the Tower or another brightly-lit room and then through a field of grass. It required a lot more light and a lot more integration."
The scenes where it was straight simulation were done through draw overs. A few line gestures would be enough to indicate weight and silhouette. There were 173 curves that were simulated or animated.
And what was the most difficult moment to render?
When Rapunzel wraps her hair around the chair with Flynn Rider and slowly moves it. Talk about hair-raising.
Bill Desowitz is senior editor of AWN & VFXWorld.