Computer Graphics World

January 2009 12 ■ ■ ■ ■ CG Character lips, his ears. At the end, they had a collec- tion of thousands of little motions. " is is where we move into emotion capture, not motion capture," Ulbrich says. "We built a library of micro-expressions con- structed from bits and pieces of Brad so we could assemble a performance of what Brad really does. We aren't interpreting his perfor- mance. We have [encoded] the subtleties of how Brad's brain drives his [facial] muscles." To use those bits and pieces, the team ap- plied the library to a facial animation rig that gave the animators control using blendshapes and other techniques. "Some things, like corneal bulge and sticking lips, can't be done with blendshapes, so we used a combination of stuff ," says Barba. Stuff ? He hedges: "We have to tiptoe around the secret sauce." After all this, though, the crew had a facial animation rig with little bits of Pitt—albeit at his current age, not, as they needed, for the 60-, 70-, and 80-year-old faces. For each of those ages, modelers had created heads and faces from scans of wrinkle-faced maquettes. " e maquettes, built by Rich Baker, were the foundation for our work," says Barba. One problem. Baker had based the elderly To create Benjamin Button's skin, the lighting and shading crew combined characteristics from the three maquettes of Button at age 60, 70, and 80, and photographs of Brad Pitt. The maquettes, created by Rick Baker, provided details, such as age spots, that the artists used for texture maps. The silicon models also provided a starting point for shaders that would gave Benjamin's surface its realistic translucen- cy through the scattering of light in the subsurface, sheen through specular lighting on the surface, and shape through such displacement detail as pores and wrinkles. "There's interplay between the characteristics of the sur- face," says Jonathan Litt, lighting supervisor. "The displace- ment affects the subsurface and specular lighting." During the look-development phase, the lighting team captured the surface characteristics of the maquettes using the Light Stage system at the Institute for Creative Technolo- gies' Graphics Lab. This system surrounds a subject—in this case, the maquette—with multiple cameras that fi re simulta- neously. Usually this happens in changing light, but this time, that wasn't important. "We didn't want to invest too much into matching the maquettes exactly," Litt says. "We wanted to use them as a visual reference." Working from a set of photographs taken on the Light Stage with all 164 lights turned on, the crew at Digital Do- main processed the images based on lighting data from the HDR photographs taken on location. Then, compositors projected them onto a basic model in Nuke. The Light Stage images provided the falloff and the intensity change caused by lights at various angles. The HDRs provided the light from the environments the crew needed to match. "Paul [Lambert] applied the lighting from the HDR envi- ronment map in a way that correlated to each of the lighting angles from Light Stage, and basically mixed hundreds of those images until the CG head looked like it was lit by the environment map," Litt explains. "The compositors would run this out in Nuke while calibrating the lighting setup for each sequence. That way, they could quickly see a lit preview head based on the Light Stage photos to know if they were in the right ballpark to calibrate the color for each sequence." Because the maquette was plastic, though, the CG mod- el looked plastic, too. "[Light Stage] was a neat tool for de- velopment," Litt says. "We wrote shaders to emulate human skin based on combining photographs of Pitt and the Light Stage data from the maquettes, but the Light Stage data wasn't used directly in any fi nal frames." Instead, the CG head used procedural shaders driven by texture maps for the skin and head. "We could light the pro- cedural skin by any light, as if it were the real thing," Litt says. "You could plop the head down anywhere." Although Digital Domain's crew wrote specifi c shaders for this show, Litt is quick to defl ect any special recogni- tion. "I want to give full credit to the amazing work that's happened with skin shading in the last fi ve years," he says. "It was a lot of work nailing down the skin shading, though. We pulled from industry-standard approaches to implement our own fl avor in [Nvidia's] Mental Ray and tune how we used our texture maps to drive the shaders. But, this is not a young character with perfectly smooth skin. We had many an iteration tuned to what was happening in wrinkles and creases." –Barbara Robertson Skin Deep

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