Computer Graphics World

July / August 2017

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44 cgw j u ly . a u g u s t 2 0 1 7 F E A T U R E opportunity to place tons of lights. Lights in the infield. Lights in pit row. "At the end of the race, when Cruz jumps over Storm, you can see all those lights reflected in the hoods," White says. "It was like a celebration." Lighting Volumes Early in production, the effects crew worked closely with RenderMan teams in Seattle and at Pixar in Emeryville, California, to be sure they could create the volumetric effects on the scale needed for the film. "We knew that about 70 percent of our work would involve volumetrics in some way," Reisch says. "Mud, dust, fireballs, even white water. We would have a quarter mile of dirt track with tire smoke and cars kicking up dust. With the first release of RIS, the focus was on modeling the light paths, on getting all those multiple bounce refractions. But aer Finding Dory, we needed to improve volumes because there is so much data. Terabytes of data. And, overlapping volumes. We needed to be sure RIS could handle all this efficiently." With the Reyes renderer, effects artists diced the volumes into micro-voxels, which represented volumes. With the path-tracing RIS, the team devised a more probabilistic solution. "With the path-tracer, we calculate that a certain amount of a volume along a ray has a certain percentage of likelihood of getting absorbed or reflected," Reisch explains. "And for thin volumes like dust, we focused on de-noising our volume renderer as well. We don't get a lot of samples when the volume is thinnest and darkest, so we needed to be sure they were noise-free." In the past, effects artists would have overcranked the density to make dust feel thicker and more detailed, or they might have disconnected it from the lights. "But, that was always a cheat," Reisch says. "We stepped back to approach dust the right way, to get detail without having to overcrank it. We are really pleased with the way light scatters within a volume now. We get a beautiful glow and backlight." Reisch, along with Julian Fong, a senior soware engineer, and Magnus Wrennige, a principal soware engineer on the RenderMan team, will reveal some of their techniques during a Production Session at SIGGRAPH. "Mud was the biggest challenge on the show," Reisch says. "Mud is such a difficult thing to do. It's between a solid and a liquid. And in the Thunder Hollow Crazy 8 sequence [the demolition derby], mud was front and center. We used Hou- dini's fluid simulation, but a lot of thought went into how we set it up with varying viscosity to create a clumpy feel." Aer spending time with reference material, the crew decided they needed multiple components to give a CG volume a muddy feel. "There are areas that are almost like dirty water, other areas with a chunky flow along the surface that has wrinkly detail, and another area with sharp aggregate-like pieces of rock that track along with the fluid surface," Reisch says. With that in mind, Effects Artist Stephen Marshall worked on making mud for six months before mud oozed into any shots. "We'd take a two-pass approach," Reisch says. "We'd have a top layer, very liquidy, that would fill back in aer a car ran though and created a tread mark. A lower-resolution base simulation of the puddle would cover the ground and any trenching as the cars went through it and pushed the mud aside. On top of that, we'd have some high-res- olution clustered splash simulations. We put the detail and computational resources on those simulations, on the stuff that hit against the cars. That way we could make adjustments on only one cluster of a splash LIGHTING ARTISTS HAD TO AIM MORE LIGHTS AT STORM'S DARK SNOUT TO SEE HIS MOUTH MORE CLEARLY. PIXAR HAD TO PREVENT RIS FROM ADDING DISTRACTING REFLECTIONS TO EYES.

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