Computer Graphics World

August 2009 17 Visual Effects ■ ■ ■ ■ converted the images to UV maps. "It was very freaky to see the mouth and eyes mov- ing on the UV map," Aithadi says. Furtility, the hair system MPC devel- oped for 10,000 BC (see "Making His- tory," March 2008), had to be upgraded to help the groomers work with hair that's thinner and not as long as mammoth hair, and to create hair that moves but retains a part. "I think the shots with Ginny are so good because the hair helps sell the whole thing," Aithadi says. For cloth simulation, they relied on Sy- fl ex to move three diff erent fabrics used in the team uniforms. More diffi cult than the kids' capes, though, was the cloth covering the stadium. "It has 50-meter-tall pieces of cloth that are so big you need small de- tails or you'll kill the scale," Aithadi notes. "But, the more detail you have, the more polygons you have. Oliver Winwood spent three months getting that simulation right, and then he had to add the wind, and the problem with wind is that you can quickly see a pattern when the mathematical wav- ing is too uniform." e tryouts, however, happen in an uncloaked structure. "All you can see is the wooden structure," Aithadi says, "the stairs inside the towers, everything. We had to build every beam. We ended up with 35,000 pieces of geometry." In addition to the CG digital doubles and the two stadiums—one bare and one dressed with cloth—MPC built the surrounding environment, a task that took the matte painters six months. ey started with stills taken by the produc- tion crew in Scotland and a panorama that served as a background, but that background was fi ve miles from the sta- dium. e matte painters needed to fi ll the space between. "We didn't want fl at depth," Aithadi says. "We wanted haze between trees and to have that haze changing every 200 meters." To give the artists a sense of scale, the crew built a 360-degree environment based on the stills and placed spheres at various depths within that environment, starting with a sphere the size of a foreground tree. "Because of the spheres, we could defi ne depth, which gave us the ability to create a 2D environment that looked 3D," Aithadi says. "Using those spheres in depth, we cre- ated level curves, and then created geom- etry at the right depth that we camera-pro- jected with photographs. e only problem was that the trees were fl at, so compositors manually rotoscoped tree edges to place haze behind them." Growing Up When Warner Bros. released the fi rst Harry Potter fi lm eight years ago, the actors play- ing the teenagers now were children. To create their witchcraft and wizardry, how- ever, the visual eff ects have been state of the art for each Potter fi lm, and the magical ef- fects have helped drive the fi lms' huge suc- cesses. But, only one, Harry Potter and the Prisoner of Azkaban, has received a visual eff ects Oscar nomination. Now that the children have moved to- ward adulthood, though—both the actors and the students in the stories—the eff ects have grown edgier and more sophisticated, too. So, perhaps this Potter will break the spell. at is, unless the Muggles take Pot- ter eff ects for granted. ■ Barbara Robertson is an award-winning writer and a contributing editor for Computer Graphics World. She can be reached at BarbaraRR@comcast.net. Water Zombies The lake inside the crystal cave where Harry and Dumbledore travel to retrieve a Horcrux churns with Inferi, corpses bewitched to do a dark wizard's bidding. Indus- trial Light & Magic (ILM) created these naked, soulless creatures, which Voldemort gave the task of protecting the Horcrux. "They look like people but skinnier," says visual effects supervisor Tim Alexander. "We have thousands in some shots." For shots with up to 100, primarily those that attack Harry and pull him into the water, animators keyframed the performances. To populate the entire under water cave with thousands of Inferi all tangled together and writhing, the crew used motion cycles created by the animators that they instanced onto cards driven by a particle simulation. "We rendered 800 frames of seven or eight cycles that were spread throughout the cave and randomly chosen at different times," Al- exander says. "When they're cards underneath the water, we built them so we could relight them. We didn't bake prelighting into them. We can treat them like sprites, but they have normals." For the water, ILM used the studio's PhysBam software to create a 3D simula- tion for swells, splashes, and ripples from the Inferi. But for the shallow water, CG artist and developer Chris Horvath created a GPU-based solver using a sur- face plane. "We were doing 1 K and 2K shallow-water sims in real time," he says. "It's the same Navier-Stokes equation used for 3D simulations, but we collapsed one of the dimensions in the assumption that the water is shallow." In the 3D sim, the Inferi are geometry. In the 2D sim, which is image-based, they are elements. Underwater, there is no simulation. The fi lmmakers created the illusion by fi lm- ing Daniel Radcliffe (Harry) underwater, to have his hair and clothes fl oat realisti- cally. ILM animated the Inferi as if they were moving in water, and then compos- ited Harry and the Inferi into swirling CG particulate matter lit by the glowing fi reballs that Dumbledore shoots at the zombie-like creatures. –BR

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