Motion capture animation is what has made Gollum in the Lord of the Rings and Davy Jones in Pirates of the Caribbean: Dead Man's Chest possible. But it's extremely expensive and requires an indoor environment to capture the movements. You have to place special dots all over the actor's body using reference markers, then have the actor perform in front of meticulously calibrated cameras hooked up to a sophisticated computer system, inside a closed stage with controlled lighting.

Now all that is about to change. Scientists at Disney Research, Pittsburgh and Carnegie Mellon University have shown that motion capture can occur almost anywhere: in natural environments, over large areas and outdoors. The trick is by turning the cameras around: mounting almost two dozen, outward-facing cameras on the actors themselves.

"This could be the future of motion capture," Takaaki Shiratori, a postdoctoral associate at Disney Research, Pittsburgh, says in a news release about the technique. "I think anyone will be able to do motion capture in the not-so-distant future." 

Shiratori presented a paper about the inside-out approach to motion capture, known as "structure from motion" or SfM, Monday at the ACM SIGGRAPH 2011 conference in Vancouver, Canada. 

In this application, SfM is not used primarily to analyze objects in a person's surroundings, but to estimate the pose of the cameras on the person. Researchers mount 20 lightweight cameras on the limbs, and trunk of each subject. Each camera is calibrated with respect to a reference structure. Each person then performed a range-of-motion exercise that allowed the system to automatically build a digital skeleton and estimate positions of cameras with respect to that skeleton.

SfM is used to estimate rough position and orientation of limbs as the actor moves through an environment and to collect sparse 3D information about the environment that can provide context for the captured motion. The rough position and orientation of limbs serves as an initial guess for a refinement step that optimizes the configuration of the body and its location in the environment, resulting in the final motion capture result.

The technique requires a significant amount of computational power; a minute of motion capture now can require an entire day to process. Future work will include efforts to find computational shortcuts, such as performing many of the steps simultaneously through parallel processing.

"Future work will include efforts to find computational shortcuts, such as performing many of the steps simultaneously through parallel processing," the team reports.