Issue link: https://digital.copcomm.com/i/19812
n n n n Simulation Riggers attached 173 “tubes” to Rapunzel’s head that animators and hair TDs could position and group into clusters. The simulation engine moved those tubes using parameters set on individual tubes or clusters, and the renderer instanced them into as many as 140,000 strands. Flynn’s hair and the horse’s mane used the same system. would put the hair into an initial pose, run the animation to see how the hair performed, and then the hair technical dir ectors would tweak the animation and add constraints that affect- ed the final simulation. Although Rapunzel’s 70 feet of hair has as many as 140,000 strands when r endered, the animators and TDs worked with a small num- ber by comparison: 173 dynamic wires, which looked like tubes and stretched, if necessary, the full 70 feet. Te simulation engine drove these tubes, not the 140,000 strands. Te render- ing engine interpolated the tubes to cr eate the number of strands needed for a shot. “Tanks to Kelly [Ward], the cur ves (tubes) matched, with about 90 to 95 per cent accuracy , the shape and volume of the hair once r endered,” Canal says. Riggers attached the tubes, which acted much like guide hairs in a typical hair simu - lation system, to Rapunz el’s CG head. Each tube came with betw een 30 and 40 contr ols that the animators and hair TDs could use to position and perform the hair, but they could add more on the fly if needed. “Or, they could reduce the controls to five, if that was enough for a nice smooth sway ,” Canal says. A ddi- tional tools could freeze any length of hair that didn’t need to move. Te crew could also gr oup the tubes into clusters, with all the tubes in a group following one tube’s lead, and animate ho w much the clustered tubes would follo w the leader. “We could blend betw een animation and physical simulation,” Canal says. If Rapunzel grabbed her hair , for example, animators might place the hair in her hand in the appr opriate frame, and then the sim - ulation engine would take o ver. To ease that 28 November 2010 transition, animation and simulation—using settings supplied by the TDs—would control the movement for varying percentages of the tubes in the cluster. Low-res geometr y—cubes and cylinders— with the same volume as objects, such as chairs and tables, that Rapunz el interacted with be - came collision objects for the simulation engine. “Interaction is very expensive, so the simpler the geometry the better, and the faster the simula - tion,” Canal says. “She uses her hair as a pillow, to open her closet and drawers, as a weapon. It was challenging and fun. Te hair TDs had to come up with ne w ideas and ne w constraints for how things interact with the hair.” When Rapunzel’s hair interacted with her skirt, the crew would usually simulate her hair first, using the hair as a volume for the skir t simulation. “S ometimes, though, it was the other way around,” Canal says. “We’d simulate the skirt and use it as a collision object that we fit into the hair engine.” For cloth, the cr ew used an evolution of the cloth engine built for Bolt. “It worked very well,” Canal says. “ We had complex garments with multiple layers in some cases, but it was fast. I t took only sec - onds per frame depending on ho w many lay- ers of cloth we had.” Goldberg describes a par ticularly difficult cloth simulation that the cloth team was able to handle: “It’s when Flynn and Rapunzel are in the woods and something frightens her . Glen K eane had her climb on F lynn, arms around his neck, legs ar ound his torso . He drew this as a joke in dailies, but the animator went ahead and animated it in CG. Te cloth team not only had to simulate her skir t, but they had to have it function in a way that was still family-friendly. It’s a wonderful shot.” To affect the simulated motion of Rapun- zel’s hair in this shot and all others, the TDs set v arious parameters. “Te simulation en- gine Kelly Ward and the team built was v ery realistic, but sometimes those results were not what we would expect to see or like, ” Canal says. If the hair felt too heavy, for example, the TDs would lighten the mass. A ppeal was al - ways more important than accuracy. Parameters also contr olled the interpola - tion; that is, the number of hairs instanced from the 173 tubes. “ When we say 140,000 individual hairs, that ’s for close-ups, ” Canal says. “For other shots, we had less.” Texture maps controlled the hair’s look, col- or, shininess, and so for th, but lighters could also affect the look using a separate set of pa- rameters. When Rapunzel sings a special song, her hair has a magical glow. “Her hair i s like Animators would perform Rapunzel grabbing her hair in shots such as this, and then the hair TDs would blend that motion with simulation.