Shrek 4-D

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.”

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