Actor Jet Li is seen in human form as Emperor Qin only at the very start and end of the film. For the majority of the film, the emperor is a CG character – a burnt, desiccated mummy trapped inside a terracotta shell.

In moments of anger, his terracotta face cracks and pieces fall away to reveal the grotesque mummy underneath. As his curse is to be forever encased in this terracotta shell, the shell quickly reforms with hot clay that glows and smokes as it re-grows over his face.

Not only did the burnt, decayed mummy have to look – and speak – like Jet Li, but the terracotta figure also had to resemble Li, albeit in a stylized form.

“Rob was emphatic that this character, either in mummy or terracotta form, really looked like Jet Li,” says Mårten Larsson, lead effects artist at Digital Domain. Using data generated from a motion capture session with the actor, a digital version of Li was modelled and animated in Autodesk Maya and rendered in Pixar RenderMan.

Multiple layers of highly detailed textures were created in ZBrush. According to Larsson, the main challenge of creating the terracotta shell was in striking a balance between making the terracotta face move with human characteristics while maintaining the look of a solid object.

Cohen wanted the shell, even while cracking and re-sealing, to reveal the subtleties of the facial animation underneath. “We went back and forth on the design of the terracotta a little bit,” explains Larsson.

“Initially we designed a whole system that actually cracked up the character’s whole face and moved it as solid chunks, as if he had terracotta shell on him, but that became really distracting. So in the end, we used a deforming face with cracked textures on it.”

To create the crumbling terracotta shell, a simulation was created on the surface of the character’s face. Digital Domain’s effects team imported the Jet Li character’s geometry from Maya into Houdini.

Here, stress maps were calculated to determine which part of the face was moving during a particular period of time. When an area hit a certain level of stress, it would crack along a seam, creating the appropriate – and appropriately scary – simulation.

Re-sealing the character’s face with molten terracotta was another challenge, Larsson explains, as the effects team needed to convey the extreme heat of molten clay. Heat effects, including smoke and flaking embers, were created by hand in Houdini and rendered with Houdini’s Mantra rendering engine, while Digital Domain’s volumetric renderer Storm was used for smoke effects.

Compositing the mummy of Emperor Qin and his terracotta shell was equally tricky, explains Ted Andre, lead compositor at Digital Domain.

The two objects were rendered separately, but as each object’s light affected the other, the team used two different passes. “These passes had multiple layers, which allowed us to control the overall look and feel on an individual basis,” explains Andre.