How do you fill a stadium with 3500 people – while letting the director shoot with a moving camera, from any angle – without busting a low budget? That was the question put to Vancouver visual effects studio Rainmaker, by Andy Finkman, director of DreamWorks Pictures and Lakeshore Entertainment’s She’s the Man. In the film, the star, Amanda Bynes, a soccer player, is on the football pitch performing in the big game.

“Andy wanted to do something big,” says Mark Breakspear, visual effects supervisor. “The crowd was an integral part of the performance.”

Finkman had rejected the tried and true method of duplicating crowds by stitching together film of 300 extras moved in blocks around the stadium. That method required a fixed camera. “He would have had to cut from people running around on the pitch to a boring, locked off shot,” says Breakspear. “And creating a CG crowd would have been prohibitively expensive, especially with people clapping and cheering.”

Rainmaker, however, had been experimenting with techniques that could solve Finkman’s problem. “We’re not reinventing the wheel,” Breakspear says. “We’re using existing techniques in a unique way to solve a problem in time and budget.” A quick test proved their point and the studio got the job. Here’s what they did:

First, the director filmed the shot. “He could do wide shots panning on the feet, raking shots, perpendicular shots,” Breakspear says. “He didn’t have to worry about locking the camera off. We told him to imagine the place was full of people.” In fact, the stadium was empty but for a few people selling hot dogs.

Second, Rainmaker filmed around 100 people in a studio against a greenscreen. While the people reacted as if they were watching a soccer game, the visual effects crew shot them with two cameras, one straight on and one from the side.

Then, the crew created geometric stand-ins in NewTek’s Lightwave from elements cut from the greenscreen shots ‘ simple approximations for blocks of crowds. “They aren’t CG and they aren’t flat cards,” says Breakspear. “They have arms and legs. We use clip mats on 3D objects that are kind of like spheres, so the picture denotes what’s left of the object. We get the sense of them being 3D.” They rendered the elements in Lightwave using the hotdog sellers as lighting reference.

And lastly, the crew inserted people at the correct angle from the camera’s point of view. “We tracked the blocks of people mapped to the CG geometry back into the scene,” Breakspear says, noting that the studio used 2d3’s boujou for this job. “It gave the impression of a full crowd, but they weren’t tiles.”

There was one problem. After the director saw the result, he wanted to change the color of the clothes people were wearing ‘ they were too similar. So, Matthew Krentz, a Rainmaker compositor devised a plug-in for eyeon Software’s Digital Fusion that uses a procedural fractal algorithm to change the colors. “It blew us away,” says Breakspear. “He could separate out their clothes and change colors on the fly in realtime. We could add variation to the crowd or make it one color. We can also do this on practical shots.”

Now, Rainmaker has been rearranging, extending and applying the techniques in various ways for such other films as Blades of Glory and The Da Vinci Code. “There’s no reason why we couldn’t substitute any of our textured geometry with more complicated geometry,” says Breakspear. “We could match geometry to people walking up and down steps, convert textures into cycles, and change colors. It’s more a way of thinking than a specific technique.”