Computer Graphics World

September 2009 20 n n n n CG I•Stereo to high-resolution geometry that the team rendered through Arnold. More specifically, although ODE easily handles volumes, such as buildings, to han- dle organically shaped food, like bananas and hamburgers, as well as buildings, the Cloudy team gave the dynamics engine the ability to calculate the movement of con- vex hulls. "ODE is fast and stable when you use spheres and boxes," Davies says. "But, it takes a lot of manual setup to approximate an arbitrary-shaped object, such as a tur- key leg. You can't easily represent it with a sphere or box, but if you can divide it into sections and each is a convex body, you can approximate the exact shape of many pieces. By using convex shapes, we could build any arbitrary shapes we needed. We got less interpenetration between objects, the simulation was faster, and it looked more realistic." To begin, the effects team used operators in Houdini to divide objects into convex shapes via a program called Qhull, which shatters volumes into convex hull parts based on predefined patterns. e hamburger bun, for example, became 24 rigid bodies bound together loosely. "Fortunately, it all happens procedurally in Houdini," Davies says. "If you give it a shard, it returns a convex shape that wraps around it. We compiled those and fed them into the simulator." A "shard" might be pieces of pipes, dry- wall and studs, and other internal geom- etry for a house, or a hierarchy of shapes that make up a banana or a hamburger. "When you see a rough approximation of a banana, it looks like armor, with all these rigid shapes parented together in a chain," Kramer says. "To rain hamburgers, we had many semi-rigid convex bodies stuck to- gether with flexible constraints and layers for the buns, meat, lettuce, tomato, and pickles. Each was its own collision object, and all the layers had breakable constraints. When a hamburger hits the ground, the bun might pop off and a tomato might roll away on its edge. It was all based on physics and dynamics." Mathematical glue held the rigid-body parts together during the simulation; the joints between could be breakable or soft. Constraints specified how much force would break the glue for breakable joints to, for example, shatter a building into pieces, pop apart the layers of a hamburger, or, for soft joints, bend a noodle. "Once the simulation is done," Davies says, "we have tools for mapping the high- resolution data onto the rigid-body repre- sentation. If something bends or crumples, we used a polygonal bind to have the high- resolution geometry reflect what was going on in the sim." In other words, the high- resolution banana is bound and deformed to match the movement of the low-resolu- tion, rigid-body "armor." When the bouncing burgers numbered in the thousands, the effects artists would drop 20 or so, then bake the resulting ani- mation cycles out and attach them to par- ticles later, again using Houdini operators. "e particles would tell the system, 'Hey, I've hit the ground,' and that would trigger a new animation state," Kramer explains. One of the most dramatic sequences in the film revolves around a tornado made of spaghetti—and meatballs—that rains down on people in a restaurant. "Each strand of spaghetti was a chain of rigid-body boxes," Davies says. "Using our [Houdini] tool sets, we built collid- ers for the characters and the tables in the restaurant. e rigid-body boxes in the chains could interact with the characters and tables, and could pile up on each oth- er. For each chain, we generated a curve, and then generated noodle geometry for each curve that we sent to the renderer. I think the result looked convincing—tech- nically and artistically." And, funny. "is film never got old for me," Kramer says. "Even after seeing shots many times, I still laughed. e crew would be cracking up in dailies every day." One reason might have been that the directors encouraged them to come up with gags. "We had such creative freedom," Bredow says. "We had ideas come straight off a tech- nical director's desk and into the film." He gives one example in which animators had posed Flint, who nearly electrocutes him- self while working in a high-voltage area. e technical director took that cartoony pose, put a skeleton that he designed in- side, and lit it to expose the skeleton. "e directors and production designer flipped," Bredow says. "It was like Wile E. Coyote. ey loved it." n Barbara Robertson is an award-winning writer and a contributing editor for Computer Graphics World. She can be reached at Barbara RR@comcast.net. TDs created the spaghetti tornado using chains of rigid-body boxes to facilitate collisions with other objects, and then rendered a curve drawn through each chain with noodle geometry.

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