Computer Graphics World

JULY 2010

Issue link: https://digital.copcomm.com/i/14320

Contents of this Issue

Navigation

Page 31 of 83

n n n n Visual Effects tween a Komodo dragon and rhinoceros; and a spirit dragon. “Making the designs for these characters or- ganic but close to the animated characters in the series was a very difficult dance,” Helman says. “We started with Appa. To work out a way for us to understand how a six-legged creature would walk, we used the outlines from the 2D artwork to create a creature and rig it. We wanted to be able to pose the crea- ture in a specific way to create 2D artwork.” Te design prototyping team would then ILM’s design prototyping team created, rigged, and rendered simple 3D models of Appa, then printed 2D artwork of the six-legged creature in various poses that the art department painted and combined with backgrounds. the ocean. Yet, as with the other elements, the director wanted the water bending anchored in reality. “Our challenge with water was the multiple levels of scale,” Hammack says. “But, we de- cided to push forward with existing tools for water simulation and put our development resources into the other elements.” Early in the film, as Aang learns how to bend water, the water needed to look un- controlled, as if the young avatar struggled. “Night wanted people to feel that struggle through the motion of the water,” Hammack says. “Later, we show ultimate control. So tell- ing the story through water was another chal- lenge. Te fluid engines want to do real phys- ics, and when we fight against that, it’s harder for the engine. Te level of instability we add is difficult; it takes much more computation. So, we relied heavily on our fluid-simulation expert Lee Uren. Ryan Hopkins led the team that produced the wave at the end. And, Dan Pearson set up the water-rendering pipeline.” Because the wave was so big, the simulation team decided to build an underlying structure for the fluid simulation using sculpted geom- etry. To render it, they used a technique simi- lar to that for the big waves in Evan Almighty, Poseidon, and Pirates of the Caribbean. “We have a complicated fluid simulation underneath that we turn into a mesh and put displace- ment shaders on top,” Pearson says. “On top of that, we render points for the splashes, some mist and other atmospherics, and use volume renders, as well, for the mist. It’s a complicated pipeline with lots of independent simulations that we have to chain together—fluid sims, to particle sims, to more particle sims.” Because the large wave was so deep, the wa- ter could be opaque. Te big bubble and the tentacles of water that Aang bends, however, are transparent. “We used geometry as source 30 July 2010 and target objects feeding geometry into the fluid simulation and telling it to try to reach the shape of the geometry,” Hammack says. Animators worked with geometric shapes to clock out the choreography and timing for the water bending, using as reference videos they found on YouTube of astronauts popping wa- ter balloons in zero gravity. “It took a big leap of faith for Night to look at our gray-shaded tentacles or blobs and trust the result,” says Tim Harrington, animation supervisor. “But, he soon realized it was part of the choreogra- phy process.” Once Shyamalan approved the choreography, the animators sent the geom- etry to the fluid-simulation crew and on into Pearson’s water-rendering pipeline. “For the giant tentacles, we had an under- lying structure driven by a mesh, and we streamed particles on top,” Pearson says. “We had a technique where we could mesh the par- ticles directly into a surface. We could combine these animated meshoids, as we called them, with simulations, and then generate a mesh with everything altogether. At that point, we had a surface with characteristics based on the radius and weight of each particle and how much influence each particle had.” Tat surface is what they rendered. “When you store simulations on grids, you have a lot of data to carry around,” Pearson says. “So, we could do the process once, turn it into a mesh that we could store and re-render efficiently using [Mental Images’] Mental Ray and [Pix- ar’s] RenderMan because we were storing the interface, not the internal structure.” Appa, Momo, and Dragons Although the animators helped choreograph water-bending shots, their main task was creat- ing about 200 shots with CG creatures: Appa, a six-legged flying bison-like animal that Aang rides; Momo, a little flying lemur; a hybrid be- pose the 3D model, render it, and deliver it to the art department, where artists placed Appa against a background and painted on the plas- tic render. “He’s in the right scale, and the rig tells you what the limbs are doing,” Helman says. “Tat gives the director an idea of how the creature moves before he commits to cam- era moves, and without having to wait for an entire production cycle.” To create the six-legged walk, the anima- tors referenced elephants and polar bears. “We came up with a modified four-legged walk,” Harrington says, “where the pairs of front legs work together but are slightly offset. It’s a solu- tion that’s believable to audiences.” Appa’s face was a separate challenge. “He has a face that’s almost human,” Harrington says. “He looks like a giant bison, but if you look at cows and bison to try to figure out how to bring life to the eyes, you don’t see a lot go- ing on in their heads. So, we looked at gorillas. Tey seem slightly more intelligent. For Momo, the advantage and the problem were the same. Te team could look at real le- murs for reference, but lemurs don’t fly. “From our conversations with Night at the beginning of the project, we knew it was really important that the characters have a purpose,” Harrington says. “Night would say that Momo is like a street kid, always looking for an opportunity, maybe looking for food or a warm place to stay. If there is danger, he might bolt and then show up again when it’s safe.” Like Aang who befriends him, Momo is, apparently, the last of his kind. For flying reference, the animators dis- covered giant fruit bats from the Philippines that were roughly the same size as Momo, and based Momo’s mechanics on those crea- tures. “Anytime we put wings on a creature, it becomes a logistical nightmare,” Harrington says. “And Momo has to walk on all fours like a lemur, so we didn’t know what to do with the wings at first, how to fold them up, where to put them. We ended up folding them in a line along his forearm so they looked appeal- ing when closed and believable that he could open them and fly.”

Articles in this issue

Archives of this issue

view archives of Computer Graphics World - JULY 2010