Second Skin Captures Motion

A new system could make special effects more affordable.

Researchers at MIT have developed a new system that may provide a cheaper and more efficient way to track motion. The system, called Second Skin, could be a cheaper alternative for creating special effects for movies. The researchers say that they hope it will also be used to help people monitor their own motions so that they can practice physical therapy or perfect their tai chi moves.

Coded light: This projector sends out pulses of light several times a second. These light pulses are picked up by photosensors worn by the subject being tracked. Credit: Kristina Grifantini

Multimedia   Watch Second Skin in action.

Traditional tracking systems involve high-speed cameras placed around a specially lit set. The subject being tracked wears special markers that reflect light emitted by the cameras. The cameras capture and record the reflected light several times a second, to track the subject's motion. When the system is used to make movies, software programs and a team of animators convert the data into an animated character. These motion-tracking systems can cost up to hundreds of thousands of dollars. Alternative systems that use magnets, accelerometers, or exoskeletons are, respectively, in need of even more extensive set up and calibration, error prone, or bulky and inflexible.

In contrast to traditional optical tracking systems, Second Skin doesn't rely on cameras at all. Instead, the system uses inexpensive projectors that can be mounted in ceilings or outdoors. Therefore, the system can be used indoors and out without special lighting, and it costs only a few thousand dollars, says Ramesh Raskar, an associate professor at MIT's Media Lab and the main researcher of Second Skin along with graduate student Dennis Miaw.

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"I think it's a breakthrough technology," says Chris Bregler, an associate professor of computer science at New York University, who works on computer vision systems for motion tracking and was not involved in the Second Skin research. "It lets you do motion capture in lots of scenarios where a lot of other people wanted to do motion capture before and couldn't."

Tiny photosensors embedded in regular clothes record movement. The projectors send out patterns of near infrared light--approximately 10,000 different patterns a second. When the patterns hit the tiny photosensors embedded in the subject's clothes, the photosensors capture the coded light and convert it into a binary signal that indicates the position of the sensor. Because the patterns of light will hit the sensors differently, depending on where they are, each sensor receives a unique light pattern. These patterns are recorded about 500 times a second for each sensor. The sensors send the information to a thin, lightweight microcontroller worn by the subject under her clothes, which then transmits the data back to a computer via Bluetooth. The whole system can cost less than $1,000, with each photosensor costing about $2, a vibrating sensor $80, and a projector $50. (Raskar says that at least six projectors are required per system.) "Each photodector is essentially decoding its own indoor location in a similar fashion to GPS," he says.