Using Mo-Cap to Track

Crews film a Spy Kids 2 effects scene while the mo-cap systemrecords camera and prop positions.

Director Robert Rodriguez and his effects team on Spy Kids 2: TheIsland of Lost Dreams say they benefited greatly frommotion-capture help in building a couple of effects scenes, but not inthe usual way. Rather than acquiring motion data from separate mo-capsessions and later applying that data to animated characters, theSpy Kids 2 team took a different approach — they attemptedto use mo-cap technology to accurately and cost-effectively trackcamera positions and the positions of actors and props during principalphotography. Their goal: to save time in post as they created animatedshadows connected to live actors, a virtual world behind those actors,and CG creatures interacting with them.

Rodriguez hired LocoMotion, a studio based in Wimberley, Texas, nearAustin, where he shot the movie. LocoMotion set up 15 Vicon 8 opticalmo-cap cameras in a 60'×70' raised oval area above the actual filmset and added optical markers to the 24p HD cameras Rodriguez used tomake the film, as well as cables, cranes, and props standing in for theCG creatures that would be added later. LocoMotion then recorded motiondata from those objects while Rodriguez taped his actors.

“[Filmmakers] wanted to match the shadows in the virtual worldto the live characters, so initially, they asked us to put markers onthe actors and get their motion data that way, in order to createduplicate shadows,” explains Michael McGar, LocoMotion'spresident. “During tests, it quickly became obvious that it wastoo cumbersome and time-consuming to remove the markers from the actorsin post, and the whole idea was to speed things up. So instead, weworked out this idea that we could calculate the position of thecameras and props relative to the actors, and that information couldthen speed up things for the animators since they would not have toplot the positions of the virtual objects by hand, frame byframe.”

Actor Daryl Sabara stands on prop representing a CG monster, whichincludes mo-cap markers, as a Vicon camera hovers above him.

Of course, going into the project, McGar and others wondered howoptical mo-cap technology would perform on a live movie set. In fact,he says, there were a “few issues we had to deal with, butoverall the system worked extremely well.”

Among those “issues,” was the simple question of whetherthe sensitive Vicon cameras would work on an extremely well-lit moviestage, featuring walls of 50k lights.

“A critical issue for us was whether the cameras would workwith those lights nearby since mo-cap systems normally run in dark orwell-controlled lighting situations,” says McGar. “As itturned out, we had to fine-tune the cameras a bit, and then they workedfine. We also had to build special markers to work properly under thoseconditions, and we developed a proprietary algorithm for our dataclean-up software that was crucial because the system is normally setup to detect certain kinds of specific motion — human motion.It's not designed to detect lack of motion. The cameras movedback-and-forth, of course, but there was no rotation going on, likewith the human body. So the software will sometimes try to compensatefor that and create movement that wasn't there. Our algorithmeliminated that problem.”

McGar feels the success of the approach could herald a new revenuestream for motion-capture service bureaus by providing camerapositioning and tracking data services at a modest price, compared tomotion control and other techniques.

For an in-depth interview with Robert Rodriguez about the use of24p HD technology to make Spy Kids 2 and Once Upon a Time inMexico, see the High Def supplement in the August issue ofMillimeter.