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n n n n CG Animation Witkin, David Baraff , and Michael Kass, de- veloped the studio's FizT (for cartoon physics tool) simulation system, originally developed for Monsters, Inc. to move the cloth in little Boo's T-shirt and the monster Sully's blue hair. Th e breakthrough came with unique methods they devised to trace and analyze intersections and solve problems caused when hair and cloth intersect, and especially when one part of a garment inadvertently fi nds itself stuck inside another and can't decide how to get back out. Th e three scientists published their fi ndings in a SIGGRAPH 2003 paper, and received Scien- tifi c & Engineering Academy Awards in 2006 for the cloth system they developed. After Monsters, Inc., Pixar's simulation art- ists continued using FizT to move hair and cloth, with only a few evolutionary changes Th e cool thing is that as a simulation artist, you might understand how to program, to write a simulation, and understand physics, but you also have to understand the fl ow and dynamics of cloth and hair to make it realistic. Th at re- quires a diff erent eye. Th e artists are the people who know when something looks wrong and can fi gure out how to make it right." Th ree senior scientists at Pixar, Andrew along the way, for each fi lm since. But, Me- rida's hair was too much. "Around the fall of 2008, Lena Petrovitch, Merida our hair lead on Brave, and Andy Witkin came together to work on a new system," Chung says, "Lena on the production side, Andy on the R&D side." Petrovitch, who had responsibility for hair modeling and simulation for Brave, had identi- fi ed a problem with FizT: To move the volume of hair in Merida's design without producing a tangled mess, every part of every curly strand had to see every other strand, which FizT could do, but not as fast as they would need. Merida's thick hair was too long and curly. 16 June/July 2012 processing," Chung says. "One of the solu- tions he came up with was that at the initial start of the simulation, the system could fi gure out which hairs cared more about each other and, depending on algorithms, could send groups of hairs to diff erent processors. Th at way, we could run the simulations somewhat independently while keeping track of whether the groups cared about each other." Th e team discovered that Merida's hair ran best on eight processors. Beyond that, the overhead caused by having the groups talk to each other outweighed the advantages of multi- processing. Witkin also devised a solu- tion to handle the volume of curls. "Before, we ran the simulation on ev- ery point of the hair, however many points were modeled," Chung says. "With the new method, we fi lter the curl to a core spring system, as if you had a core curve through the curl. Th at provided dy- "At that time, Andy was really into multi- Elinor's daughter is 5 feet 6 inches tall and has blue eyes. Her shape is athletic. She'd rather practice archery and ride her horse than sew a tapestry or play a lute. You can see her fl am- ing red hair when she's but a dot in the large, green landscape. Her hair exemplifi es her personality: curly and wild. When straightened, her curls are 4 feet long. To simulate the movement, Pixar scientists devel- oped a new system that the studio named Taz. Merida's everyday blue dress is rugged, with space at her elbows and shoulders for movement. She rips the tight dress that Elinor makes her wear for a ceremony, and breaks free. namics in a more stable way." Chung suggests imagining a curly telephone cord to understand the concept. If you swing the cord, it stays curly. "But, the motion is al- most as if a curve runs through the curl and that's what moves the curl," she explains. "So, what we created was a way to simplify a curl into a core curve, and simulated that. Th e motion was on this fi ltered core curve. Th e fun was taking that physi- cal model and fi guring out how to translate it into a point and spring system." Th e core curve solved another problem,