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MillimeterStep by Step: The Mummy: Tomb of the Dragon Emperor
Aug 1, 2008 12:00 PM, By Ellen Wolff
In Universal Pictures' third installation of The Mummy franchise, The Mummy: Tomb of the Dragon Emperor, we get to experience an especially creepy mummy up close and personal. Actor Jet Li is represented in a multi-layered computer-animated performance as a warrior trapped inside a terra-cotta shell. In moments of rage, his terra-cotta face cracks and falls away, revealing a grotesque "under-mummy" that also resembles the actor and delivers dialogue. This warrior’s curse is that he’s doomed to be encased forever in that shell, so terra cotta repeatedly re-grows over his face. The hot clay glows and smokes as it painfully re-seals.
“We basically had two hero characters,” says Nordin Rahhali, CG supervisor at Digital Domain (DD) in Venice, Calif. “The under-mummy resembles a dessicated, burnt Jet Li, and the terra-cotta outer shell resembles Jet Li in a more stylized way. The idea that director Rob Cohen had was that it would be like a Russian nested doll.”
Digital Domain’s effects crew began by amassing reference photography of a terra-cotta statue on set. “This gave us lighting cues and textures,” Compositing Supervisor Lou Pecora says. “We also shot a clean pass of the set and then a pass with Jet Li in it. You notice that when an actor is in a scene, the movement of the camera is more believable.” The actor’s image was then rotoscoped from the plate with the company's software Nuke (now marketed by The Foundry). Then DD’s proprietary tool Track was used to track the camera movement in the plate. “It’s better to rotoscope an environment rather than use greenscreen,” Rahhali says. “You can’t beat the real lighting cues that make a CG character sit in a scene.”
The digital version of Jet Li was modeled and animated in Autodesk Maya and rendered in Pixar RenderMan. “We made a model look like Jet first, and then we started dessicating it,” says Rahhali. To create multiple layers of the dessicated under-mummy, Digital Texture Lead Stan Seo created highly detailed textures on a Z-Brush model. “Displacement maps were then taken from Z-Brush and put onto a lower resolution cage to add back all that detail in Renderman,” Rahhali says. CG Lighting Supervisor Hanzhi Tang took Seo’s textures and created the shading model. “He added all the specular hits, the ambient occlusion, and beautiful subsurface scattering on the thinner pieces of skin,” Rahhali says.
The facial animation of the under-mummy drives the look of the crumbling terra-cotta shell, but a significant amount of simulation had to be added. “After we ran through the animation process, the shot would go through an effects process,” says Rahhali. “We have a pipeline that brings all the character geometry into [Side Effects] Houdini. Once it was in Houdini, our effects lead, Brian Gazdik, worked on all the cracking and shattering. A simulation was created on the surface of the character’s face. It was an interesting technique because based upon the deforming geometry, the animation was driving the surface simulation. The director wanted to have something that would crack and re-seal but still reveal the subtleties of the animation happening underneath. So our effects people would take this deforming geometry—the animation coming from Maya—and calculate stress maps to determine how much of an area of the face was moving during a particular length of time. Through that, they would run the simulation so that we’d have areas that were fully intact. Only when they hit a certain threshold of stress or flex would it crack along a seam. It was a physically based simulation that had artistic user input on where events would happen. It was a mix of articulate hand animation and some meticulous procedural work. Procedural work only goes so far—there was a lot of hand tweaking.”
While the crumbling terra cotta required significant particle simulation, the re-sealing of the character’s face with molten terra cotta was another challenge. “We wanted to come up with something that was physically based that you could believe was super-heated terracotta,” says Rahhali. “It had to be based on something you could relate to, like a hot piece of clay that’s glowing. These effects—including fine, wispy smoke and embers coming off the face—were pretty much all Houdini effects and all hand done.” DD rendered effects with Houdini’s Mantra renderer, though the company used its proprietary voxel renderer Storm (a Houdini plug-in) to render the smoke effects.
“In resealing the face,” says Pecora, “the director specifically wanted to see imperfections burning off, which happens when you’re firing clay. He wanted to see embers come flying off the face help the audience understand the physics of what was going on. When the face reseals and you see the expression on the under-mummy’s face, that has to translate to the outer terra-cotta shell.”
“There were so many layers required to give us what we needed in compositing,” says Pecora. “They got combined through a customized Nuke macro that we affectionately called ‘Pimp my Mummy.’ It was put together by one of our look development compositors, Chia-Chi Hu. It combined all the layers and allowed us access to color sliders and color controls. We had one input node that all had the layers in it—that got fed into Pimp My Mummy, which split all those layers and assigned controls to each of them so we could dial the subsurface scattering separately, without having to dig through the layers. We spent a lot of time in compositing addressing a lot of the lighting issues by adjusting the different passes—all within the interface that Chia-Chi put together for this show.”
This allowed Digital Domain to finesse the final look as efficiently as possible. “There were times when we had our supervisors or the director sit with an artist and dial in the controls interactively. That kind of time saving was invaluable," says Pecora. "You can’t do that by just iterating and iterating. You can cover seven or 10 iterations with one interactive session. What used to be just the toy of the cappuccino-steeped ‘Flame elite’ is now in the hands of us humble compositors!”
Credit Roll:
Director: Rob Cohen
Visual Effects Supervisors: Joel Hynek, Matthew Butler
Computer Graphics Supervisors: Nordin Rahhali, David Hodgins
CG Modeling Supervisor: Francisco Cortina
Animation Supervisor: Kelvin Lee
CG Lighting Supervisor: Hanzhi Tang
Compositing Supervisor: Lou Pecora
CG Modeling Artists: Raul Dominguez, Wayne Kennedy, Daniel Moy
Lead Character Rigger: Richard Grandy
Character Riggers: Marc Wilhite, George Saavedra
CG Animator: Slavik Anishchenko
Lead Compositor: Ted Andre
Compositors: Heather Hoyland, Chia-Chi Hu, Richard Thompson (Technical Developer)
Lead CG Effects Artist: Mårten Larsson
CG Effects Artist: Brian Gazdik
CG Lighting Artists: Diana Sear, Åsa Svedberg
Lead 3D Integration Artist: Geoffrey Baumann
3D Integration Artist: Justin van der Lek
Digital Texture Paint Lead: Stan Seo
Digital Texture Painters: Joseph Lacap, Ting Lo
