Computer Graphics World

August 2009 12 n n n n Visual Effects follows his swim to the surface, we see an orange glow on the water. e visual effects wizardry has only just begun. On the island, Dumbledore is directing a raging firestorm that swells to 100 feet high and 200 feet in diameter. Waves of fire swirl around him. And then, like Mo- ses parting the Red Sea, Dumbledore says a magical word, and the churning wall of flames splits to create a safe passage across the lake for the two wizards. Tim Alexander led a crew of approxi- mately 60 people at ILM who worked on the sequence's 160 shots for about a year to create the crystal cave, the CG water, the CG Inferi, and the CG fire. When they be- gan, they did not have the ability to create such a fire, so Chris Horvath took on that problem in November 2007. is year, SIGGRAPH accepted the technical paper he and Willi Geiger submitted on the pro- cess titled "Directable, High-Resolution Simulation of Fire on the GPU." "I wanted to emulate the ability of really skilled compositors to paint with filmed elements," Horvath says. "So the original intention was to come up with something that would allow us to shape the use of filmed elements with particle simulation." With this in mind, Horvath first tried to bend and stretch sprite elements along a particle path. It didn't work. Horvath believed that might have been because the filmed elements he had to work with didn't have the long trailing streams of fire he needed. So, because he didn't have the pho- tographed elements he wanted, he decided to use CG elements instead. And thus, the science project began. "e goal was to not have anything partic- ulate," Horvath says, "no sprites and none of the artifacts that had been present in the past. at was the basis of our technique, which is essentially image-based simulation." e result is a two-stage process that technical directors used to create the fire- storm. e process begins with a 3D par- ticle simulation using Geiger's FakeFlow system. "It does a coarse, quick, low-reso- lution enforcement of the Navier-Stokes non-divergence component and adds a little viscosity," Horvath explains. "One of the things we learned is that a little of this goes a long way. We used tiny fluid grids—64 cubes at the most, and some- times 32—but that gave us a base fluidity as a starting point." e simulation results provided the flowing motion that Horvath needed, but not the detail. "If we had an infinitely large computer that could hold a 2k x 2k x 2k grid, we could theoretically perform a full- blown fluid sim, but it would take days and days and days," Horvath says. "So we found a way to split the grid into slices and exploit parallelism in the GPU." is is where the magic happens. First, since the only thing that matters in visual effects is what the camera sees, the second part of the process is oriented to the frus- tum—the camera's field of view. Rather than running a 3D simulation, Horvath stacked, in effect, a series of rectangular, two-dimensional slices of a 3D grid. He spaced these simulation planes equally apart and faced them to the camera from the foreground to the background, to pro- duce high resolution close to the camera and appropriate resolution farther back. "All the particles from the coarse [3D] sim- ulation are drawn on each plane," Horvath explains. "But a weighting function deter- mines their size. If they're close to the plane, they're drawn strong. If they're far, they fade off, and the size of that fall-off is important. e particles become the input to another The goal in creating the firestorm surrounding Dumbledore was to create highly detailed and directable fire that didn't look like it was made of particles. Let It Snow For a wintery sequence that de- manded more than the fake flakes caught on camera, Cinesite drew on proprietary image-based track- ing software called Motion Warper to dust the actors with snow. The studio started by photographing the fake snow to create tiny ele- ments. Motion Warper then tracked the live-action image and applied the elements. "It does pixel analysis on the whole image," says Ivor Middleton, CG supervisor. "It isn't just a cor- ner-pin application. We were able to snow up Hagrid, with his huge beard and furry cloak blowing in the wind, more or less straight out of the box with a little tweaking. It's some- thing we've also used to track scars and for face replacement." –BR

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