Movies - Performance capture

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	Movies - Performance capture

Performance capture is a term popularized by the creators of The Polar Express movie. It differs from standard special effects type motion capture due to the real time and interactive nature of the performance as opposed to capturing data for reference motion for movies, sports analysis and games. In effect it is a digital replacement for the furry or latex rubber costume, allowing the actor to give the performance without wearing the "Barney" suit. Whether the audience is the director, or a live television performance, it is an ensemble Performance, rather than just a list of motions that will be edited together later. It is a combination of motion capture and facial expression capture, using the actions of live actors as input to digital image generators to create more natural and realistic animation. Real time Performance capture allows the director or audience to see the end character interacting with the environment to fix eye contact, timing, reaction and other issues that can effect performances with other characters that may or may not be filmed at the same time.

In performance capture, actors wear the same body suits as in motion capture, but also record the facial movements using either special makeup to enhance the contrast of the face, or reflective markers or LEDs. Markerless systems that track the facial features are being developed but currently suffer from resolution issues that make them difficult to use at camera distances that allow full body motion capture. The actor usually interacts with wireframe models of the objects in the scene. The recorded performance data can be used to animate different actors. In The Polar Express Tom Hanks played five roles - an 8-year old boy, his father, the train's conductor, a hobo, and Santa Claus.

Using Alias Motion Builder software, low polygon count (Low being subjective, a few 10,000 polygons verses millions for film quality characters) can be animated in real time to allow characters to be viewed in real time in digital sets. This unique capability allows Performance capture to create real time animatics. It is expected that the success of animatics in Lucas Films "Star Wars" and other projects will eventually lead to an entire industry segment where all movies will be previsualized before they are greenlighted.

Newer active marker systems such as PhaseSpace modulate the active output of the LED to differentiate each marker, allowing several markers to be on at the same time, while still providing the higher resolution of 3,600 x 3,600 or 12 megapixel resolution while capturing at 120 (128 markers or four persons) to 480 (32 markers or single person) frames per second. The advantage of using active markers is intelligent processing allows higher speed and higher resolution at a lower price. This higher accuracy and resolution requires more processing than older passive technologies, but the additional processing is done at the camera to improve resolution via a subpixel or centroid processing, providing both high resolution and high speed. By using newer processing and technology, these motion capture systems are about 1/3 the cost of passive systems. The key advantage besides higher resolution and data quality is low latency for Performance capture. Additionally the active markers reduce data cleanup times by a factor of ten over older technologies.

Note this active marker system can be used with facial expressions, but requires patience by the actor to wear LEDs on the face, and most facial capture systems require exaggerated expressions. Fingers pose a problem with most optical motion capture systems and require data gloves, or controllers to differentiate hand positions when the hands are often blocked from cameras.

Although performance capture has been used in some earlier films and computer games, The Polar Express was the first movie made solely with the process. This film was directed by Robert Zemeckis, who had a long history of technical innovations in filmmaking (historical composites in Forrest Gump
and the combination of animation and live action in Who Framed Roger Rabbit
) and became a self-professed fan of performance capture (he produced the 2006 thriller Monster House
made using the same technique) because of the creative freedom it gives the director. Zemeckis is currently using performance capture in an adaptation of Beowulf scheduled for 2007 release.

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