A challenge to the Digital Domain team was to figure out the look and feel of the terracotta warriors. Courtesy of Digital Domain. All images © Universal Studios.

It's been seven years since adventurer Rick O'Connell last took on the evils of the ancient world in The Mummy Returns (opening today from Universal). Now back for a third entry in the series, The Mummy: Tomb of the Dragon Emperor, the film posed unique challenges to the visual effects crew at Digital Domain, which split the work on the film with Rhythm & Hues.

VFX Supervisors Joel Hynek and Matthew Butler of Digital Domain had previously worked together on xXx with director Rob Cohen, who takes over this installment of the franchise from Stephen Sommers. The film sees O'Connell, played by Brendan Fraser, and his family trying to stop Jet Li's revived Dragon Emperor and an army of terracotta warriors from conquering the world.

The first major challenge the film posed, according to Hynek, was to figure out how the terracotta warriors were going to look and move. Cohen's original idea was to create a look that Hynek described as "a liquid solid," and Digital Domain set about figuring out a way to make that happen.

"With the Dragon Emperor himself, he needs to be able to be animated to the level of seeing him deliver dialogue," adds Butler. That led to an investigation of facial movements that involved studying ways to make the face move like that of a human but also to still look like stone. The idea soon evolved to one in which the terracotta surface would crack as the warriors moved, and then be reformed in some manner.

While the physics of the destructive end was relatively simple to work out, the regeneration was not and many ideas were floated as to how it could work. "Maybe he's got something hot in the core that just fills in the gap," Butler adds.

But the results Digital Domain came up with still fell short.

"The director and the studio didn't seem to think that it was expressive enough," Butler says. They pulled back on the cracking idea and, late in the process, Cohen came up with the idea of the "under mummy." The idea was that the Dragon Emperor and his army were put into a terracotta shell, and when the exterior cracks you can see their decaying bodies underneath. This under mummy offered sufficient levels of expression, while making the terracotta warriors look tougher and suitably grotesque as the villains of the movie.

Another issue was the availability of Li, whose character appears in synthetic terracotta form for most of the movie. Butler says they always knew that "we were potentially going to be on our own" as far as having access to Li.

The idea of the "under mummy," in which the mummy's decaying body could be seen through cracks in the terracotta shell, helped give the Dragon Emperor more expressiveness. Courtesy of Digital Domain.

To ensure they had what they needed to create a terracotta version of the actor, Hynek says they used motion capture to record Li's movements and facial motions. They used a Witness camera setup, with three video cameras running in synch and in phase shooting right along with the film camera. This allowed the crew to capture very accurately the motion of Li's performance in 3D.

"It's not the first this camera has been used, but I think it is the first time they've been used in synch and in phase, which was important for getting very accurate results," Hyneck suggests. Much of this footage ended up not being used, especially after the under mummy was incorporated into the movie, and keyframe animation and motion capture of other performers was used to amp up Li's final performance.

In creating the film's 3D characters, Hynek says they primarily used Maya and Houdini for animation and Nuke -- originally a proprietary Digital Domain package but now owned by The Furnace in London -- for compositing. "We love it, because it has a 3D camera component to it that makes it very easy to incorporate 3D animation into it and you can still modify it and use a lot of the 3D aspects," Hynek says.

They also used Storm, a proprietary 3D volumetric renderer, and Track for tracking. "That came in incredibly useful here because, of course, the camera was never holding still, and we also used it then to track things on to Jet Li's face," Hynek says.

In addition to the terracotta army, the script called for a rival army of desiccated mummies called the Foundation. Butler says Massive was used to control the armies, which at times numbered near 5,000 terracotta warriors and 1,600 in the Foundation army.

Cohen, however, wanted each soldier to look different. "So our team leader had to come up with ways of having 20 different bodies and different heads," Butler says.

For the Foundation, in particular, that made the work a real challenge.

"These guys, they're decrepit but they have hair and they have clothing, of course -- all those bloody annoying things that make it very difficult for us to achieve," Butler offers.