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simulation Waggoner says. "The characters have rigs with simple spheres that interact with the grass. The spheres, sized to fit each character, are part of the rig in the sense of saying where the feet are. We use them as input, as colliders in the simulation." "When we run a rigid-body simulation," she continues, "we tell it where the ground is and where the character is walking. The spheres fitted to the characters' feet emit a trail of special spheres. These spheres have timing that allows them to float up at a certain velocity." During rendering, the grass deforms according to the animation of these special spheres, which creates the appearance of grass coming up slowly over time behind a character's footsteps. The simulation artists created another interesting technique, this one to animate the pages in books. "A book has a lot of pages, so there are a lot of collisions," Waggoner says. "People might think we'd use a cloth simulator, but very few simulators, even our proprietary simulators, can handle that amount of collisions. We used our rigid-body pipeline – the same engine we used for the balloons in Up. I think that might be surprising." To move the pages in the books, the artists created sheets of paper with cubes – that is, rigid bodies – linked together with joints. The rigid-body engine then moved the cubes based on parameters that specified joint velocity and stiffness. "I don't remember how many rigid-body pages there were in a book, but we rendered 100 pages," Waggoner says. "So, it was a large number." Lighting and Rendering One major change for the studio happened in the lighting department. Monsters University is the first film at Pixar to use raytracing throughout for rendering. "We felt RenderMan was ready, and we felt we could help the RenderMan team by using it in-house and pushing and collaborating with them, and improving it, as well," Bakshi says. "We wanted to raytrace everything and simplify the lighting setup. Our films have looked visually rich and the lighting looks beautiful, but it was the lighting artists' job to imitate the effect of global illumination. That imposed a complexity in the lighting setup." To reduce that complexity, a team of nine researchers, technical directors, and artists created a physically plausible, energy-conserving lighting system with importance sampling. Technical Director Christophe Hery, who had received a Scientific and Engineering Award for point-based rendering, was the driving force behind the new raytraced lights and global illumination system. Included in the system is a method he devised for raytracing hair with importance sampling. With the new system, rather than dozens of point lights, the artists use any of six "smart" lights to illuminate a scene quickly and accurately. Three are shapes: square, disk, and sphere. Three represent large-area lights: sun, window, and sky. In addition, the artists could apply maps to the lights – even paint on them 16 ■ CGW Ju l y / Au g u s t 2 0 1 3 to direct rays to particular areas or colors. "The goal wasn't to be more efficient, although we were," Bakshi says. "The goal was to simplify the lighting task. It came at some cost – a technological cost to develop the software and lights, and the cost of raw computing power." Video: Go to "Extras" in the July/August 2013 issue box ■ NEW, "SMART" raytraced lights made lighting easier and global illumination possible throughout. To render Monsters, Inc., Pixar's renderfarm had 3,500 processors, a huge number at the time. Eleven years later, to render Monsters University, Pixar doubled the size of the renderfarm used on Brave to 24,000 processors. "Once we said that's what we needed, it was such a big investment that we had to stick to our render-time targets," Bakshi says. "We kept the sets not too geometrically complex. We counted the number of hairs, and if there were too many, we'd figure out how to make the same look with fewer number of hairs. To fit within the renderfarm capacity, we imposed this rigor on all the assets we built. And, we did a lot of optimization along the way. We had to make sure we could render the movie." Bakshi calculates that it took 100 million CPU hours to render the film, with each frame taking, on average, 29 hours to render. The result is a prequel that is orders of magnitude more visually complex that the original. Pixar has always been a measure of the state of the art of computer graphics, and Monsters University is no exception. As with any good prequel, everyone knows how this story ends. It ends with Blinn's law. Attributed to computer graphics pioneer Jim Blinn, it states, "As technology advances, the rendering time remains constant." A completely tricked-out college campus. Four hundred students. Hair. Raytracing. Simulation. Global illumination throughout. 24,000 CPUs. Amazing. ■ CGW Barbara Robertson is an award-winning writer and a contributing editor for CGW. She can be reached at BarbaraRR@comcast.net.