Computer Graphics World

August 2009 14 n n n n Visual Effects Navier-Stokes fluid-flow sim, a variation of the same equations we ran earlier." Because this simulation is two-di- mensional, though—the slice is only a plane—the simulation can run at 2k, even at 4k resolution, which adds a tremendous amount of detail. e result of the simu- lation is a set of images that contain den- sity, temperature, texture, fuel, and velocity data. "ese are combined by our renderer to produce the final image," Horvath says. "ey're volume-rendered from near to far." Although the simulations run sepa- rately for each plane, the underlying par- ticle simulation provides continuity. Horvath created the images using a clev- er hack that treats texture maps like large input data arrays and a final display like a large output data array. "is was before CUDA was available," he says, "so we used OpenGL, the GL shading language GLSL, and GPGPU, which tricks OpenGL into using textures as computation planes. And instead of using shaders to draw things, we used them to compute things. Because e verything is stored in a texture, we can dis- play everything immediately. You can have our simulation on screen in front of you and hit a key to switch between different data planes [the different sets of images]." Horvath used EXR files to store the temperature and density data, and GTO files to store the simulation controls. For most of the shots, the TDs ended up us- ing 128 slices. "ere's no reason to choose 128," Horvath says. "ere's nothing spe- cial about that number. It's just that pro- grammers are geeks and like to use pow- ers of two. Typically, the number of slices required is a feature of how much depth complexity we need." To simulate a fireball, for example, the technical directors used only eight slices, and the simulation took only 10 minutes. But, for the parting of the so-called Red Sea shot, during which Harry and Dumb- ledore row across the lake between walls of fire, the TDs needed to run six simulations that they layered from back to front. "at's the only shot in which the main body of the fire is multiple elements," Hor- vath says. "at's because we needed more than a thousand slices, and that was too big for the renderer to hold in memory. It was our nightmare shot." Fiery tunnels aside, for the most part, by using the GPU fire-simulation process, the crew could look at a fully rendered fire in about half a day. "It was great," Alexander says. "We had never done photorealistic fire, although we'd tried many times in the past. e standard approach, rendering a heavy-duty 3D particle sim volumetrically, can take days on end. And, now we had su- per highly-detailed photorealistic fire that we could change twice a day." Cinesite: Magical Explosions All told, Cinesite created 474 shots, of which 204 had CG elements and the rest were cleanups. Andy Robinson was the compositing supervisor, and Ivor Middle- ton, the CG supervisor. To the studio, Hermione's lovebirds in scene 63 were the most important. "We've mostly done environments for Harry Potter, so having CG creatures to work on, as well, was nice," Middleton says. "ey're little lovebirds that reflect Hermione's mood." At the beginning of the shot, as Harry comforts a distressed Herm- ione, the birds twitter around her head. But when Ron Weasley enters with another girl, the birds turn into angry little darts. "ey become aggressive and zoom toward Ron," Robinson says. "When he exits, they hit the door and burst into an explosion of feathers." To create the feathers, the 3D artists made small planes, rather than using hairs. "We didn't need to build individual barbs," Middleton explains. "We could simulate the feathers with texture maps and an anisotropic shader." Once the birds hit the door, the dynamics switch on. Lead techni- cal director Holger Voss turned the birds into the explosion of feathers using a com- bination of Autodesk Maya's rigid-body dynamics and nCloth, which handled in- dividual feather dynamics. e second explosion that Cinesite created for the film involved inanimate objects—a stained-glass window in Hog- warts' great dining hall. "ey didn't shoot anything for it," Robinson says. "e shot was an afterthought." e shot takes place near the end of the film. e Death Eaters, who are invading Hogwarts, explode the stained glass win- dow. Plates slide off the tables, and cups smash onto the floor. e magic candles move with the shock wave, and the flames go out. Although Cinesite had created the magic ceiling and magic candles for the great hall, Cinesite created magic candles for the great hall, and then for an explosive scene near the end of the film, blew them out. For that sequence, the studio digitally detonated a stained-glass window and shattered the dinnerware.

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