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8 cgw n o v e m b e r . d e c e m b e r 2 0 1 7 F E A T U R E CLOTHES MAKE THE SKELETON Because all the skeletons wear clothes, their costumes were as important in forming their silhouettes as their bones. "We weren't bound by tissue for these characters," Santos says. "We were bound by what they wear." Pixar can trace its research and development into cloth simulation back to little Boo's T-shirt in the film Monsters, Inc. Pixar's Fizt physics tool, originally developed by David Baraff and Andy Witkin, has evolved to accommodate the needs of subsequent films, but this film's bony characters and huge scope presented new challenges. To illustrate, Emron Grover, simulation TD, shows early sim results – clothing snagged on a skeleton's knee, cloth bunched up around a bony hand. "The cloth simulation was incred- ibly difficult," he says. "We needed a better collision detection system to solve cloth hanging in front of a collision sphere. It's an extremely mathematically difficult problem. There are hundreds of thousands of little triangles. The math could not figure it out." To help iron out that problem, Pixar brought David Eberle, who had been at PDI, onto the team. Eberle, now a senior simulation soware engineer at Pixar, created what Grover calls a continuous collision system. The system supports simulation of tetrahedral meshes with their invertible element model, allowing for coupled simulations between volume and cloth meshes. "Fizt can do detangling, and that has made our simulator extremely robust," Grover says. "We wanted that ability in Eberle's continuous collision system. Eberle's system gives us almost flawless collisions – it's amazing what it can do. And on top of that, it's 100 percent faster." To help simplify the calculations, the team gave each skeleton a collision body for the simulation. That particularly helped Hector, whose torn, unbuttoned jacket reveals the bones beneath. "We fused the rib cage into one mesh but kept some detail between his ribs," Grover says. "Lee [Unkrich] and Adrian [Molina, co-director] wanted to see the bones, to feel the bones. So, we attach negative pieces of cloth that the cloth on top collides with. That makes the cloth on top stretch and not fall between the bones. We also fused the radius and ulna and the fingers, but we le the patella." For Imelda, the simulation team used force fields to constantly push her dress outward and create her large shape. Sim- ilarly, forces created a fake trapezius, the large muscles in the back that move the shoulders on some skeletons. Cloth pillows filled negative spaces to create volume between bones. "Usually, a character's musculature beneath the clothing creates the shapes we're used to seeing," Grover says. "But, we had large gaps." TAILORS AND TINKERERS To create the costumes worn by the skele- tons in the Land of the Dead and the human characters in Santa Cecilia, tailors at Pixar created costumes in 3D but used 2D prin- ciples when they tessellated the clothing. They cut seams in the same places they would normally be cut in a 2D pattern. PLAY THAT GUITAR Communicating to the audience that the animated character Miguel was actually playing a guitar became a challenge for the animators and character technical directors. "We knew that having him play the guitar right, having his fingers interact with the strings, would be one of our most difficult things," Santos says. "But, the director wanted it to look authentic." Characters Supervisor Christian Hoffman asked Pixar Anima- tion Supervisor and Short-Film Director Dave Mullins for help. "I wanted to make it as easy as I could for the animators be- cause this would be complicated," Hoffman says. "Dave plays the guitar. He gave me a whole packet of information." The first step was to have a rigging lead develop an inverse kinematic system for the fingers. "We usually don't set up inverse kinematics on the fingers because that gives us 10 new IK solvers, but I knew we'd need that for this," Hoffman says. Next, they optimized the workflow for animation. "If an animator positions the hand to play a particular chord on the fret board and wants to slide the hand, the fret spaces change," Hoffman says. "So, we automatically adjust the fingers. They get closer or farther apart." The riggers also had the strings vibrate appropriately when Miguel strums the guitar. "You can get really geeky about the vibration and harmonics, and we didn't go there," Hoffman says. "We have first harmonics. We don't go to second harmonics. But, in addition to strumming and string vibration, an animator could pluck a string and it would deform in a slightly linear manner. On the frets, when Miguel's fingers press down, the area above doesn't vibrate. If the string was vibrating when he presses down, when he lis a finger, it would still vibrate. We had simple simulations the animators could fire off that would calculate the vibrations." The ability to convince the audience that Miguel is actually playing the guitar helped particularly during the scene in which Miguel shows his passion for music. "When I saw that, I thought, 'Wow,' that's a touchstone," says Darla Anderson, producer. "I love how quiet and simple that scene is, and yet it holds such depth, complexity, and emotional centering."