Motion Capture and Analysis

So, how do they get those computer characters to look so real?

One technique uses small reflective balls. The balls are attached to key joints and points on a person's body. Infra-red cameras are set up around the athlete or performer and can track the movement of these little balls. If one of the balls is out of view of the cameras, its position can be determined by the other cameras. This is called an "optical" system and is typically limited to an indoor studio setting.

Another technique uses magnetic sensors instead of balls. There are wires leading from these sensors into computer systems that collect the motion information. This is called a "magnetic" system and although the motion capture is not limited to a studio, the wires restrict very complicated motions.

There are also methods that are "electromagnetic". These systems use body suits.

These motions are collected by a computer and then used with animated or "synthetic" characters.

Well, it was pretty clear from the "get-go" that although these systems would do an excellent job at motion capture and analysis, they would not meet our needs. We needed a "non-invasive" method of motion capture and software that might help us analyze the information that was gathered. Clearly tennis equipment and players could not be tagged with these devices during a professional tournament.

We also wanted a visual record of what we were analyzing so that the other researchers on the team not at these tournaments could study the information gathered. So we chose a high speed camera system. The system could collect up to 500 frames/second of footage; we collected most of the footage at 250 frames/second. Your standard home video camera collects 30 frames/second.