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Step by Step: Penelope

Jul 1, 2007 12:00 PM, By Ellen Wolff

It's been nearly a decade since the Academy Award for Visual Effects was bestowed upon What Dreams May Come, which featured a memorable CG tree that appeared to grow before the viewer's eyes. Generating that tree was accomplished through a complex software approach called “L-systems,” and since then, the evolution of “organic” CG procedures has seemed slow. But with the upcoming release of Penelope by IFC Films, we'll see some digital organics on the screen once again.

The London-based studio Double Negative Visual Effects has animated a tree that appears to age a century in a single scene. The boughs twist, and winds send leaves flying as the seasons change. This fantasy film, directed by Mark Palansky for Stone Village Pictures and produced by Reese Witherspoon, used the iconic image of a family tree to tell its tale. Picture frames of family members adorn the tree, and the camera flies through the branches to see these pictures — with the tree evolving all the while.

“The first thing that happened was that our visual effects supervisor Jody Johnson photographed lots of trees — not to determine the tree's shape, but whether the branches split or were twisted. The director had a specific, fantastic tree in mind,” says Double Negative 3D supervisor Pieter Warmington.

Once the tree's style was determined, model makers built physical versions that were shot with motion control for the close-ups among the branches. A 2ft.-high clay maquette was also built, showing how the tree would look when it was old. This helped Double Negative previz a CG tree, Warmington says. “We could reference something physical that we could match our CG tree with later on,” he says. “Once we started making the full-size CG tree, the textures, leaf shapes, and colors had to match the physical model so it would look like the same tree.”

The tree itself was created in Autodesk Maya as a polygonal model, but to make it grow Double Negative used a Maya plug-in called xFrog (developed by Greenworks Organic-Software). Other procedural software packages can be used to generate realistic-looking trees, but Warmington says, “They don't grow. xFrog has a simple slider, which, when you move it, makes the image go from ‘no branch’ to a branch, and then leaves come out. The only downside to the way xFrog works is that because it's a procedural system, you can't really design the shape of the tree as you might like it. You've got control over the shape of the branches, but you can't really control exactly where you want a specific branch to be. When you build an entire tree procedurally, if the director asks you to move a branch to the left, you can't do that.”

Instead of growing the entire tree with xFrog, Double Negative used it to grow the sub-branches, twigs and leaves, and added them to the polygonal tree model. Warmington used Maya's curve tools to change the shape of the branches as necessary. “The gross animation that needed to be directed was done as a blend shape,” he says. “I could edit the blend shapes in a traditional animation style.”

But it took a while. “We could easily build a tree that looked good at the end of the shot, but when we wound the animation back, the branches didn't look particularly interesting in their previous positions. So it was a case of designing beginning, middle, and end stages, and then animating between them. Then it would look like one branch at the beginning grows into a believable shape at the end,” Warmington says.

Another major challenge involved making the xFrog leaves tumble convincingly from the tree. Double Negative's research and development programmer Peter Kyme figured out how to detach each leaf from the tree and animate it separately. “It wasn't easy,” Kyme says. “We wrote a plug-in that would attach to xFrog and track each leaf, determining the scaling, orientation, and position of each leaf in space.”

“Because the leaves had to animate separately,” Warmington says, “they had to be in one of two states: They were either attached and growing or detached and blowing — and also reacting to gravity.” Double Negative built upon the particle system approach it used to generate a shower of papers for World Trade Center — using a particle for every leaf generated by xFrog. “Once Peter came up with the system of using a particle for every leaf, I could then use standard particle systems and Pete's field of gravity to make the leaves drop off,” Warmington says.

“We could add variations to it, so all the leaves wouldn't drop at once,” Warmington says. “We could use our existing expertise writing expressions for particles, and use Maya's own fields to control the particles.” At this point, after nearly three months of work, he says, “It took just a few days to get the final animation.”

“[The lesson of Penelope] is that anything that's going to be procedural needs to have tweakability,” Kyme says. “You just don't have that level of control with purely procedural tools. There are strategies you can adopt to allow a bit of customization and directorial control, but there's always a heavy trade-off and no easy workarounds.”

But the end result, Warmington says, may inspire people to incorporate growing CG into future films. “Once writers and directors see that this technology is available, they'll realize they can try things that they might never have thought were possible,” he says. “They'll write stories that can integrate these ideas, rather than being laughed out of the room whenever they suggest it.”

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CREDIT ROLL

Director: Mark Palansky

Visual Effects Supervisor: Jody Johnson

3D Supervisor/Senior Technical Director: Pieter Warmington

Research and Development Programmer: Peter Kyme

Visual Effects Producer: Ellen Walder

Digital Compositor: Christoph Salzmann

Senior Texture Artist: Alban Orlhiac

© 2008 Penton Media, Inc.