Issue link: https://digital.copcomm.com/i/5474
December 2009 20 n n n n CG Characters•Environments For skin texture reference, the crew did photo shoots under controlled lights of young people with the most perfect skin they could find. "We discovered that even someone with nearly flawless skin still has lots of imperfec- tions in displacement and color. ey have nodules, bumps, pink around their eyes, and blotchy layers," Williams says. Painters added these imperfections to the texture maps and created a pore structure for the aliens that looked realistic. All this helped make their skin come alive. As for the color, even though the aliens had blue skin, the crew put red blood in their veins, and did so without turning their skin purple. "Before, we had more of an analytical approxi- mation for subsurface scattering," Williams says. "We went to an absorption-based subsur- face scattering routine. e system we use now does proper frequency-based scattering." Because they used the actual wavelength for red transmission through the nose, ears, and pores of the skin, the red blood didn't cause the blue skin to turn purple. ey also added a little red to the skin tone. en, they applied some of the same techniques and shaders writ- ten in Pixar's RenderMan to the plants. Deep in the Jungle "We cross-pollinated the efforts," Williams says. "e plant shader now uses the skin shader." e plants, however, aren't blue, even though they started that way. Blue light from a blue sky bouncing off blue plants onto blue-skinned characters created unin- teresting images. "We needed to have other colors hitting the characters' skin to give them the kind of complexity that helps make them look real," Williams says. At night, as the characters walk through the jungle, the plants glimmer with biolu- minescence. e CG artists used subsurface scattering to cause thick plants to glow like a wax candle. "Some plants just have a glow- ing moss over them," Saindon says. "It de- pended on the plant and how [Cameron] felt it should look." To create the rain forest, the Weta artists started with FBX files from Lightstorm that they imported into Maya scenes. "We had simple representations for where the trees and plants were," Saindon says. "Jim moved and placed things where he wanted for camera angles. So, we did a one-to-one match at first to get a layout that he specifically liked." Because the plants needed to be dynamic, all of them are models created using a rule- based growth system. Although they average 100,000 polygons, some have as many as one million polygons. "e plant-growing tools were almost like a modeling tool," Williams says. "Once we grew a plant, we could instantly create variants by changing the seed value for the random func- tions." e variants might change the number of branches and sub-branches, the height, the silhouette, the age, or other parameters. e crew planted the jungle using painting techniques to place trees, shrubs, and grass. "It's similar to [Maya's] Paint Effects, but we aren't creating geometry," Saindon says. "e system is taking pre-existing geometry and placing actual full-res models at correct angles on the ground." ey also used Massive's software to grow forests. When artists planted seeds on a ter- rain, Massive would simulate a forest grow- ing and competing for light and space. Bigger trees grew quickly, smaller plants died, and shade-loving ferns grew around the base of the large trees. "We'd create large areas, and then on a shot- by-shot basis, would sculpt scenes to play well for the camera and the depth of the scene," Williams explains. "All of our show is done inside Maya, and everything in the jungle is 3D, so when you move the camera around in Maya, you get a real 3D sense." To light and render the massive jungle, Weta implemented two techniques: stochastic prun- ing and spherical harmonics. e stochastic pruning threw away unnecessary geometry on the fly as a plant moved away from camera. "It might take a fern with a million polygons and push it back to a few pixels when it's in the distance," Saindon says. Spherical harmonics, a technique used for real-time rendering in video games, made it possible to light the rain forest. "Basically, we store coefficients for angles," Saindon says. "We calculate the harmonics for each indi- vidual plant, all the lighting angles, and store that on the geometry. at allows us to drop simple lights into the scene and still get proper occlusion from each plant. e plant does its own self-occlusion using its own harmonics, seeing what should be occluding what, and stores the information. at means we can light an entire jungle with one light. We could get complex lighting with a very simple setup. We couldn't have done the movie without it." Even so, the data processing requirements for the show were enormous. In addition to the characters, Weta created volumetric ex- plosions, fireballs, 3D water simulations, and other effects. "Joe [Letteri] set down the hard line," Williams says. "He told us not to plan on cheating anything." At one point during postproduction, the studio was generating 110gb of data an hour. "Jim Cameron's expectations are extremely high, and he demands a lot," Rosenbaum says. "e scope of CG movies is getting so large and the time constraints too tight, that people tend to compromise, but Jim doesn't compro- mise. He insists on a high standard. When I worked on e Abyss, it took us six months to create 90 seconds with the pseudopod. We went into it with the same question we had on this film: How the hell will we do this? And we had the same mind-set: We'll put our heads together and figure it out. He's always one to push a VFX company. And he certainly did it on this one." n Barbara Robertson is an award-winning writer and a contributing editor for Computer Graphics World. She can be reached at BarbaraRR@comcast.net. Jake Sully (actor Sam Worthington) prepares to inhabit the avatar body resembling a Na'vi, seen forming in the tank behind. The color palette for the film reflects James Cameron's fascination with the underwater world.