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MillimeterShrek 4-D
Jul 1, 2003 12:00 PM, by Ellen Wolff
Riding the Waves
Fans of the Oscar-winning, computer-animated movie Shrek won't get to see the sequel until 2004, but there's a tidbit available now to tide them over — a 13-minute special-venue film called Shrek 4-D. Playing at Universal's theme parks in California, Florida, and Japan, Shrek 4-D is a digitally projected, stereoscopic motion picture from the original movie's creators, PDI/DreamWorks. It features the voices of Mike Myers, Eddie Murphy, Cameron Diaz, and John Lithgow, who reprise their movie roles. But this time around, the theater seats are programmed to move along with the animation on screen. And to add additional sensory elements, the venue is equipped to blow wind and spray water on the audience at key moments.
The narrative of Shrek 4-D is designed to take advantage of this, especially when Shrek's bride Fiona finds herself adrift on a runaway river raft. Shrek and his sidekick Donkey try to rescue her by flying overhead aboard Dragon, whose wing dips into the water in an attempt to reach Fiona. Hovering nearby is also the ghostly presence of the late, un-lamented Lord Farquaad who is seeking revenge. With all of the film's main characters in this climactic scene, it is, says PDI's Paul Wang, “A big ‘money’ shot.”
Wang, who served as the film's visual effects supervisor and digital art director, notes that the frames in this sequence are extremely dense, requiring both character and environmental animation. For the characters, the studio benefited from being able to reuse the digital models from the original film. They were then animated using PDI's inhouse animation system, which runs on PCs equipped with the Red Hat Linux OS.
The most challenging aspect of the shot was to believably animate the raging river that looms large in the scene. “We build systems for many things — like crowds and dust — which can be changed slightly to match different looks for different movies,” Wang notes. “But water is something you almost always have to reinvent for every kind of water because it doesn't scale well. In this particular shot we see water both closeup and far away. We were dealing with different scales of water at the same time, which is difficult.”
Proprietary code was written to create the waves, the foam, and the various types of splashes that occur throughout the scene. “There's underwater foam and then two or three foam layers on top of the water,” Wang explains. “There's also foam that turns into spray when it hits the land and rocks. We needed rocks in the middle of the river to really sell the violence of the water. Then there's the reflective surface that you'd generally see in the ocean.”
PDI's software development strategy included particle system simulation to generate various layers of foam. “What we did that was different was that we used a clustering technique,” Wang says. “The simple way to do it would be to just have particles that swirled as they flowed down the river. We had particles that attached themselves into patterns that we called clusters. So they look like little pieces of flotsam, but they're dynamic shapes in the water. Instead of having thousands of particles, we have thousands of clusters.”
A key component to making this work was to have these clusters floating on the surface of the water in a plausible way. “The water surface is what's called a height field. Basically we have noise running through this water — the noise is white where the amplitude is high. This creates surface waves,” Wang explains. “That's not rocket science; it's been done before. But what we did was to put the clusters on top of this water, and wherever there's a peak in the water, the particle clusters flow down from the peak into the troughs as if pulled by gravity. It seems like a subtle thing that you wouldn't notice, but it really makes a visual difference. You feel that the clusters of foam are moving in ways that make sense, like they're little boats riding the waves. We didn't have it at first, and when we put it in, it was very believable. We've all seen the ocean and we know if the physics are wrong.
“The reason we picked this technique of foam and surface was because it's procedural, so we wouldn't have to do a lot of hand animation. But we could also make adjustments in the places where the director thought something looked odd. We used little tricks of the trade — like softening the edges of the foam into the color of the water. Things like that help with the integration.”
Of course there are limits to the tricks that can be done when a film will be projected stereoscopically, admits Wang. “Most things that would normally be classified as a 2D ‘paint fix’ couldn't be done because you can't paint the exact same thing for both the left eye and right eye views. Unless it's a really subtle thing, you need to go back and do a 3D fix-it and then re-render. You can't ‘fix it in post.’”
When it came to rendering, Wang says, “We had about twelve layers — usually there's about six. But it made the process much more modular; we could change something without re-rendering the entire shot.” He notes that since the original Shrek, PDI has made improvements to its proprietary renderer, and it was tested on Shrek 4-D. “Things are a little bit cleaner, and it handles displacement really well, so we can have very small details. We can capture even tiny little pieces of bark on Fiona's wooden raft.”
All those details — especially the tiny bits of spray and foam — added to the experience, especially since the audience is being sprayed simultaneously with real water. “Every time we'd show [DreamWorks co-founder] Jeffrey Katzenberg our stuff he'd say, ‘More! We'll have a sprayer, let's use it.’ He was right,” Wang says. “When I've watched this scene with audiences, everybody screams.”
Credit Roll
Simon Smith — Director
David Lipman — Producer
Paul Wang — Visual Effects Supervisor/Digital Art Director
Raman Hui — Animation Director
Bob Whitehill — Head of Layout
Matt Baer, Rick Glumac — Effects Animators
Scott Peterson, Mahesh Ramasubramanian — Software Development
Continue the discussion on “Crosstalk” the Millimeter Forum.
