Computer Graphics World

January 2009 15 CG Character ■ ■ ■ ■ cameras, the system triangulates that pat- tern into 3D space. We could see the 3D lo- cator moving around in the environment." At the end of that process, a tracking art- ist had a point cloud moving in 3D space in the virtual set. " at was the foundation from which everything started building," Maldonado says. "We attached the CG head to the point cloud." Benjamin Button's hat complicated the problem of setting the CG head on the actor's shoulders, especially because the CG head wasn't exactly the same size as the actor's head. "When you have a collar, you have a little wiggle room, but the hat has a specifi c line," says Roble. "If there's any motion be- tween the digital head and the hat, you see it. You see the hat moving on top of the head, or the head moving under the hat." To help the 21-person tracking team, Roble tweaked the studio's custom soft- ware, Track. "I made it faster," he says, "defi nitely faster. And, more accurate." Once they had tracked the head onto the clavicle and sometimes under the hat, the tracking artists could look at it from diff erent camera views to assure that the CG object was in the correct place. ey also did track- ing checks at 2 resolution on large screens. "We used the alpha channels from the roto team to do a quick composite through Maya or Nuke to help verify that the track- ing was good," says Maldonado. When they were satisfi ed, the head moved on to animation and lighting. Tiny Tweaks "[Fincher] put a lot of eff ort into making performance selects that timed correctly with the body," Preeg says, "but in many cases, we had to change Brad's perfor- mance." In general, the animators tried to match the 3D heads to the motion of the small actors' heads. Often Pitt had matched that motion during his performance cap- ture, but sometimes his head moved too much—for example, when the small actor was walking. Or, his head tilt wouldn't cor- rectly match the eye lines. Other times, the animators needed to change a small actor's A Blur and a Final Touch At the end of the pipeline, Benjamin Button arrived at compositing in 60 layers, which the compositors could tweak in Digital Domain's Nuke. They had indi- vidual layers for lips, teeth, eye matter, hair, six different skin passes, layers for specular and diffuse lighting, and much more. Because the layers arrived without motion blur, the compositors added it us- ing Vector Blur Z, a motion-blur technique developed by creative director of software Doug Roble and implemented in Nuke. The technique helped reduce rendering time. "Motion blur is interesting because parts of a scene are blurred and parts aren't," Roble says. "It depends on where the geometry is in the scene. It's great if you can calculate it in rendering, but it's expensive because the renderer has to essentially render a whole bunch of sub-frames." To add motion blur in Nuke, Roble had Nvidia's Mental Ray output depth and velocity, as well as color, for each pixel. "The renderer doesn't compute the streak, but it tells us how it's moving, if it's moving," Roble says. "So with that information, we can streak the pixel in Nuke just as the renderer would, but more quickly. And, the Z-depth tells us when part of Benjamin's face goes behind another part so we know not to blur there." Vector Blur Z has made a huge difference at Digital Domain, cutting the time to compute motion blur on a previous fi lm from 16 hours to four on some shots. There's no comparison data for Benjamin Button because the crew always used Nuke on the project. "If we had had to motion-blur Benjamin with the renderer, the times would have gone through the roof," says compositing supervisor Paul Lambert. "Tweaking the amount of motion blur added a slight bit of work in compositing, but it was part of our template, so we just took it in stride." Of all the shots in Benjamin Button, one of the most diffi cult for the composi- tors was of Benjamin getting drunk in a bar. For that, in addition to the usual ad- justments to fi t the somewhat drunken digital head into the scene, they painted Benjamin's face fl ushed using tools in Nuke that would stick the paint to the face. The shot took 150 iterations before they fi nished. Usually, though, the work fl owed remarkably smoothly. "The lighting renders were very close to the plate, so we rarely had to do over- all color corrections," Lambert says. "Daylight took more work than dark, moody lighting because we had to increase the amount of subsurface scattering at cer- tain angles. And, we did have to change things like skin blemishes. But, we knew what to tweak based on previous shots and the environment Benjamin was in." Lambert worked for 18 months on the show and yet, like other members of the crew, claims he never found the work tedious. "Because of the pipeline, it was a really good experience," he says. "It was awesome to see the character come to life." –Barbara Robertson was a really good experience," he says. "It was awesome to see the character much—for example, when the small actor was walking. Or, his head tilt wouldn't cor- January 2009 much—for example, when the small actor was walking. Or, his head tilt wouldn't cor- much—for example, when the small actor was walking. Or, his head tilt wouldn't cor- much—for example, when the small actor was walking. Or, his head tilt wouldn't cor- come to life." –Barbara Robertson

• Cover