Step by Step: The Golden Compass
Jan 1, 2008 1:00 PM, By Ellen Wolff
A photoreal polar bear wearing elaborate armor is a signature image in director Chris Weitz's fantasy film for New Line Cinema, The Golden Compass (recently Oscar-nominated for Achievement in Visual Effects). While the bear, named Iorek Brynison, represents a fallen prince and is voiced by Sir Ian McKellan, he still had to look and move like a real animal. This requirement was especially evident in shots where the bear is shown in natural light in a street scene, reacting to the pivotal character Lyra (Dakota Blue Richards).
Creating believable polar bears was a prime assignment for the London-based visual-effects shop Framestore CFC. Lead R&D Developer Alex Rothwell says the job required writing significant custom code for hair simulation and for the way a polar bear's body jiggles when it moves. The team animated the bear with Autodesk Maya, and it initially tested commodity simulation tools. But off-the-shelf simulation software proved too slow to handle the subtle interactions that these shots required.
“We had to simulate the dynamics of the bear's hair to give it some natural, flowing movement — especially with the armor pressing against it,” Rothwell says. “Because the armor is quite thin, when the edges are jutting into the bear's skin, they have a small cross-sectional area. We needed a high enough density of dynamic hairs to pick up the detail of those thin plates colliding with the hair. We had 120,000 dynamic hairs on the body and 50,000 on the head, and each one of those was fully dynamic — it could be pressed and squashed. The total number of hairs on the bear is 4 million, interpolated between those 170,000 total guide hairs.”
Also at key moments in The Golden Compass, the young actress had to stroke the bear's fur. “Her finger, compared to the size of the whole bear, was extremely small. So we needed a high density of guide hairs on the bear's surface for those shots as well. We didn't want to create custom dynamic hair per shot, which is very time-consuming. We wanted a blanket solution where we could cover the whole bear, so we developed a standalone hair solver — from the ground up,” Rothwell says.
His team viewed documentaries of real bears in preparation for this assignment, and they had high-def footage captured specifically for their reference use. “When we analyzed the polar bears' movements, we saw that their fat was very loose — it almost seemed to float around their bodies like a weird fluid fat,” Rothwell says. “It was like a wave that pointed the bear's hair up and down and gave it a rippling effect. It picked up the lighting and really helped sell the movement and the weight of the bear. But we discovered that although that did contribute huge amounts to the look of the bear, as far as the hair was concerned, a lot more was going on. That's why we pushed our hair sim.”
Two pieces of proprietary code were needed to get the bear's fat and skin to ripple convincingly. “We had a ‘fat jiggle,' which was another standalone solution developed for this show,” Rothwell says. “It's a full physics-based simulation of fats moving on the bear's body. When a bear plants its foot very hard, there's a ripple of fat that travels from the leg upwards and over its back. We tried Maya's jiggle deformer and combinations of various kinds of particle expressions and constraints to try and get this effect, but nothing gave us this kind of propagating wave of fat traveling over the body. So we wrote a more complex model, and by tuning various parameters, we could generate this rippling effect. Then, in Maya, we could paint — per shot — the places where we wanted the fat to appear. We called it the ‘jiggle sim.' I'm afraid we weren't very inventive about coming up with clever acronyms.”
Once the fat depth was painted, Rothwell says, that was taken into account as part of the simulation. “Then if the director or our visual effects supervisor, Ben Morris, said it wasn't working in a certain area, we could go back and paint maps that would suppress the jiggle in that area,” he says. “To a degree, we could direct and control where the jiggle was happening. Sometimes we didn't even have to re-sim it — we could just paint it out, and then paint in an undynamic mesh.
“Another tool that we added for this show was a ‘skin slide' tool. After the fat jiggle had been calculated, we'd put the skin slide on. That allowed the skin to move over the underlying surface of the bear rig and not create any unsightly stretching.” The integration of the hair, fat, and skin simulations, Rothwell says,“are what ultimately produced the realism that you see onscreen.”
The bear's rusted, pitted armor completed his distinctive look, and the team achieved this using the texture-painting software Maxon BodyPaint 3D. The armor was a modeled piece of geometry that was painted and bump-mapped. “The complex color changes within the textures themselves are all hand-painted to look roughly forged,” Rothwell says.
To make these CG elements appear integrated within the overcast scene, Framestore's shader writers relied on high-dynamic-range images (HDRI) captured on set. “You need top-quality HDRIs when it's not a very strongly lit scene,” Rothwell says. “It wouldn't be easy to replicate overcast lighting with spotlights.”
Renders were done with Pixar RenderMan. “[Renders] were pretty heavy,” Rothwell says. “When you render that many hairs, and each individual hair is being shaded, it can be very time-consuming. But towards the end, we were getting full-frame bear renders back in 35 minutes.”
The CG team used Apple Shake and Autodesk Inferno for integration with the plate photography. They worked with clean background plates, although a stand-in had been used on set to establish eyelines for the actress. They used 2d3 boujou and RealViz MatchMover to track the plate.
Rothwell says the proprietary code developed for The Golden Compass will be applicable to other CG characters with jiggling fat. He recalls walking on the beach with his wife during preparation for this show and noticing some female joggers. “They weren't prime athletes. As they jogged past, I turned my head, and my wife asked what I was doing. I said, ‘I'm just looking at the way their fat is moving. Honestly, it's not what you think.'”
CREDIT ROLL
Director: Chris Weitz
DP: Henry Braham
Senior Visual Effects Supervisor: Michael Fink
For Framestore CFC
Visual Effects Supervisor: Ben Morris
Lead R&D Developer: Alex Rothwell
Creature Supervisor: Matthew Hughes
Animation Supervisor: Dadi Einarsson
Compositing Supervisor: Ivan Moran
CG Supervisors: Laurent Hugueniot, Andy Kind
