Issue link: http://digital.copcomm.com/i/259450
C G W Ja n u a r y / Fe b ru a r y 2 014 ■ 11 texture of coins, and get that sense of movement." The need for a new solver wasn't due solely to the number of coins – the simulation engine needed to move the coins in ways most rigid-body dynamics solvers don't address. "Rigid-body solvers are great for destruction, for smashing things to bits," Saindon says. "But, they don't work well for things that don't move. They like to be always moving. We needed to have the coins sit still and not jitter. Rigid-body solv- ers don't like to do that." Even with the new solver, the team discovered that creating believable motion was a surprising challenge. They put their digital coins on a 30- to 40-degree slope, sent them sliding down, and thought they had created a good simulation. "Peter [Jackson] said, 'No, this isn't what the gold coins would do,'" Saindon says. The team tried again, and again. Jack- son wasn't happy with the result. So, they brought fake gold coins from the set onto the motion-capture stage and videoed them. The coins were plastic coated with metal to look like gold. "We dumped five tons of this fake gold on a 30-degree ramp," Saindon says. "It didn't move. We dumped only a bucket. The coins sat there. We kicked them and only a few moved. It didn't do at all what we expected. So we went back to the drawing board." By increasing the resolution of the simulation result and tweaking the weight and the friction, they persuaded the pile of digital coins to move as gold coins really would. "It's like sand on a beach," Saindon says. "You knock off a little and a little moves, but it isn't a landslide like snow on a mountain. It's heavy. It has more friction." To render the sequence, the team used Pixar's PR Render- ■ (TOP) BILBO SEARCHES for a gem among 10 million CG gold coins with rigid-body dynamics. (Bottom) Weta Digital raytraced the dwarves' gold forge using PR RenderMan. a fantastic job, we didn't add the trees." To accomplish the shape change, the rigging crew created a bear rig, a digital-double rig that referenced the actor, and a third rig that could blend between the two. "The rig had to access the transformation across multiple channels," Aitken says. "There's the shape of him, and also his tex- tures. His fur has to shrink away. His human hair grows out. His teeth change. His claws become fingernails. What really clinched it, though, was the keyframe animation. You get the sense that he struggles through the process. You see the man appear in the bear early on, and at the end, see a hint of the bear dying out in the man." To facilitate the transformation's timing within the shot, animators could control the change with eight sliders that affected specific regions. The movement of hip from bear to human, for example, might take place a few seconds after another part of the body had already transformed. Typically, Weta Digital's creatures with human forms have a common UV layout to easily apply textures from one crea- ture to another. But in this case, the shapes and sizes were different enough that the crew needed to do texture migra- tion. "We set up a version of the transforming creature to have a common place where we could migrate the human textures into a bear-texture space," Aitken says. "Our fur system already supported the ability to animate the length of fur with texture maps. So, we used the maps to shrink his bear fur and grow his human hair." It might seem like a lot of time and energy for a quick shot that occurs in moonlight, but a quick check of the cast list for film three suggests that Mikael Persbrandt might be back. "I don't think we've seen the last of the bear," Aitken hints. – Barbara Robertson