"We spent a lot of effort trying to get the physics of the hair -- both the motion and look," says Technical Supervisor Mark Hammel. "We figured if we could get that realistic then we could concentrate on more artistic control. We broke down shading models to have individual shading controls to pull highlights and shadows in ways that depart from reality."

"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.

Thus, the two main breakthroughs were in the areas of volume transmission (forward scattering) and volume diffuse (backward scattering). The former is particularly important because that pertains to light that enters the hair once, travels through the strands and comes out the other side for backlighting. To achieve these volumetric breakthroughs, they applied techniques of Iman Sadeghi, a UC San Diego grad student, and translated them into a non-ray tracing technique more suitable to a production environment. They could not only render more quickly but also have greater artistic control.

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.