Issue link: http://digital.copcomm.com/i/612975
12 cgw n o v e m b e r . d e c e m b e r 2 0 1 5 to place previously designed and modeled trees, rocks, bushes, and so forth. When a set met the river, effects artists would place the river, and then the set dressers would place vegetation, rocks, and beach as appropriate around the riverbank. "In some cases, we could get away with no interaction between the water and the set," Reisch says. "In other cases, we needed to control where the water interacted. The set that interacted with the water was the same from shot to shot. The set dressers added rocks and so forth to change the look. I don't know how we could have done the 200 river shots without this modular approach." M O V I N G I M A G E S As it would in the real world, the vegetation added by the set dressers moves. Grass sways in the wind. Breezes whisper through the cottonwood trees. "We wanted all these elements in this enormously complex world – hero elements and procedural background elements – to be always moving," Cameron says. "This is true to the idea of nature as an antagonist that is rich and alive. At first we thought we'd do this only for assets close to camera. Then we realized we could use it for almost everything without increasing the cost." Proprietary simulation tools made this possible. "It's a hierarchical curve simulation tuned for dealing with lots of geometry," Cameron says. "We built rigging into the trees and used that rigging for simulation. We could also sim- ulate on top of the rigging and then rebind the geometry to the simulated rigging to have high complexity." The system is semi-implicit and spring-based. That is, it simulates on particles that move guide geometry around. The connection between the simulation data and how a tree moves is determined by weather level. "We have 20-odd terabytes of simulation for different wind levels for every asset," Cameron says. "For each of type of vege- tation, we run simulations with 15 different wind levels to create 30-second-long clips. When we set-dress a sequence, we pull on these clips as a starting point. We can say the weather is four on a scale from one to 15. We can say 'light summer breeze' or 'gusting wind,' and we've set the wind level even if we change trees." The leaves, however, move procedurally. "If we were to simulate down to the granular level, it would be very expensive," Cameron says. "So, we can activate procedures that have the same wind levels. The leaves on the trees, the hair on the characters all respect whatever the weather – the wind level – is at the moment. We blend everything together at render time. So, on a quiet day, you can see the grasses and the trees moving in a subtle way, and Spot's hair is moving, as well." Also adding richness and motion to the environment were flocks of birds, a herd of "cattle," and in one touching sequence, fireflies. "The fireflies emerge from a field of grasses," Cameron says. "In that shot, we had grass, flesh and skin simulation on Arlo, vegetation moving, and fireflies, too. It was tremendously fun to work on." The studio uses two types of crowd simulation. A custom, brain-based procedural system created in Houdini handled large crowds such as the flocks of birds, the cattle, and the fireflies. For small-scale crowds, the animation team used a clip-mixing tool. "We can scale up to a few hundred agents," Cameron says. "We have tools for managing and sequencing the pre-assem- bled clips. It's non-procedural animation." S P O T- O N A N I M A T I O N Turning CG models into be- lievable, emotional characters for the film took the skills of 84 animators. "The characters are very stylized, and that's on purpose," Jessup says. "We wanted Arlo to feel vulnerable in an authentic, awesome environment, so we have an idealized realism for the settings and a caricatured take on the characters." Arlo's challenge for the animators was the young dinosaur's weight. To meet the challenge, they referenced elephants: At the height of his back, Arlo is 10 to 12 feet tall, a height similar to that of an adult elephant. Using Presto, Pixar's proprietary animation soware, and a Wacom Cintiq tablet, lead animators would draw atop footage of elephants EFFECTS ARTISTS USED A PROPRIETARY CROWD SYSTEM TO ANIMATE THE FIREFLIES.