Issue link: https://digital.copcomm.com/i/398245
s e p t e m b e r . o c t o b e r 2 0 1 4 c g w 2 3 V I S U A L E F F E C T S selves when they can." As a result, the solver is more forgiving when ani- mators move a CG character in extreme ways. It's also fl exible. "We can use it for gener- alized elastic bodies, and we can also apply a muscle activation fi eld," Jacobs says. "We can specify a UV direc- tion per element for fi elds we want to intersect that element. When we apply an activation force, it will cause a single tetrahedron or many to contract in the direction the fi eld is running." The result is analogous to the way muscle fi bers behave. "Muscles are mostly water, so when we squish them, they bulge in transverse ways," Jacobs says. "And because our elements conserve vol- ume, when we squish them, they transversely bulge." Artists working with the system can assign material properties, such as stiff ness, density, and volume conser- vation, to simulate muscle, fat, and tendons, all of which helped add jiggles and wiggles to the organic part of the Griever in The Maze Runner. "He's meaty and slug-like," Jacobs says. "We gave him a structure like a seal, which is a fat animal with a support structure under the fat. We baked out the animation onto the skeleton and drove the simulation from the skeletal motion. Onto each bone, we attached muscles and fat, and applied constraints – parameters that determine when muscles fl ex based on some criteria. We don't have tons of muscle detail, but we did get wave propagation that you can see in some shots. He settles, pounces a foot, and you see a shock wave through his body." The simulator took care of the shock wave using proper- ties the team set. "We defi ned him as so on the inside with a fat layer that was very volume conserving, and outside, he has a thin layer of stiff er material constrained to the underlying anatomy. So, he moves a little like a water balloon. With a water balloon, you squeeze one end and the water fi lls the other. With the Griever, it's not so direct because we split the volume into hundreds and thousands of tetrahedra." Running simulations like this always carry a computa- tional cost, though. "Because of the compu- tational cost, we wanted to parallelize it as much as possible, so I based our system on a Crawford Doran dependence graph," Jacobs says. "Each node on the graph represented a diff erent stage of running the simu- lation to produce the fi nal simulation. We could specify which nodes happened in parallel." A er the system's suc- cessful application on the Griever in The Maze Runner, the developers created a stand-alone tool that includes the solver and Maya plug-in. And, they have ex- panded the system. "For The Maze Runner, we ran the system only on the or- ganic part of the Griever," Ja- cobs says. "Now, the system can represent rigid bodies, as well." – Barbara Robertson CG FIRE ENVELOPS THE DIGITAL CREATURE, CAUSING IT TO SCREAM IN TERROR.