Computer Graphics World

May 2009 33 Visual Effects n n n n Also, at one point in the film, the Enter- prise enters a debris field made of parts of damaged ships from a small fleet. To create the Armstrong, Newton, Defiant, Excelsior, and Mayflower, modelers used five differ- ent designs, but borrowed paint jobs, that is, texture maps, that they had scaled down from the Enterprise and the Kelvin. Modelers did not create the damaged planets, however. To demolish these CG objects, the crew used simulation software. One planet implodes because of black-hole forces. e second succumbs to a pressure wave when the sun goes supernova. The Trouble with Rubble e big issue in destroying something as huge as a planet is generating broken bits in many scales—from the entire globe, through continents, boulders, and, finally, grains of sand. To break apart the planet itself, ILM developed a new tool called Fracture, a simulation system built on the studio's Physbam libraries, which more typically create fluid simulations. "It's a one-frame simulation," explains CG supervisor Joa- kim Arnesson of Fracture. "You run a sim- ulation on one frame. It breaks something apart and calculates the stress and fracture patterns. And then you have geometry you can run a sim on." Here's how it works: First, the technical di- rectors convert a sphere into a volume; that is, create a voxel grid. en they sow random seed points—Voronoi noise—mathemati- cally inside the volume, or some part of the volume, randomly or by using a field. Each seed point defines the center of a cell. "e seed points become a cell structure based on forces," Arnesson says. "When you run a stress test on the volume, you get a pattern that looks sort of like broken rocks. e structure fractures along these cell noise lines." e result is 3D cells, randomly shaped and sized depending on how many seeds the TDs put into the volume and how they distributed the seeds. "We could stretch the cell noise and control where we had more seeds," Arnesson says. "If we wanted small pieces, we used more seeds." en, to move these pieces, the TDs pushed the cells along the pipeline to the creature dev department, where a crew applied forces in a rigid-body simulation system. "e cells moved based on weight, mass, and forces, and collided with each other," Arnesson says. As these chunks of the planet collided, they needed to break apart, and smaller pieces needed to crumble off the larger ones. "We can simulate between 500 and 10,000 pieces with the rigid-body simula- tor," Arnesson says. "So, to simulate hun- dreds and millions of rigid bodies, we used particle systems procedurally tied into the rigid-body simulation." To create the dust, sand, and other small pieces, the creature dev artists sent key tim- ing data provided by the rigid-body simu- lation, and the planet chunks, back to the technical directors for the particle simula- tion. "e rigid-body simulator can record the stress factor on a per-CV or per-face basis," Arnesson says. "e particle engine can look at that data and know a particu- lar face was triggered by a break-off, so it's time to emit particles from that face." Arnesson also singles out two other com- puter graphics innovations at the studio for this show. One is a volume shader for Pixar's RenderMan that the TDs can drive from a field, and which they used primarily for explosions. "We've had similar shaders before," Arnesson says. "But this is a step better and definitely faster; RenderMan opens Zeno and evaluates a field to render a volume." e second is a new, procedural Render- Man plug-in based on a data-flow graph. Created by Dan Piponi, the new node- based interactive system gives the technical directors more flexibility than before. "We can input particles, do operations on them, and, at the end, render them as points, curves, spheres, or blobs," Arnesson says. "Or, for example, we can do something different for some of the particles based on distance. And, we can see and adjust the results very easily." e TDs used the system to generate a black hole by having it render curves that follow a particle simulation. Similarly, by using the particle renderer, they procedur- ally built a plasma drill that stretches from a CG mining platform in space to the surface of the Vulcan planet 10,000 feet below. Buckle In e actors playing Kirk, Sulu, and Olson space-jumped onto the digital drill platform by skydiving on wires in a greenscreen set, and then ILM sent the digital doubles down the digital umbilical cord toward the planet. Paul Kavanagh led the team of anima- ILM matched a small wedge of this drilling platform, built in the parking lot of Los Angeles' Dodger Stadium, with computer graphics, completed the pie-shaped platform, and then surrounded it with a completely digital environment.

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