Motion Capture Primer

Motion capture is the process of recording a live motion event and translating it into usable mathematical terms by tracking a number of key points in space over time and combining them to obtain a single three-dimensional (3D) representation of the performance. The captured subject could be anything that exists in the real world and has motion and the key points are the areas that best represent the motion of the subject's different moving parts. These points should be pivot points or connections between rigid parts of the subject. Human motion capture systems can be classified as outside-in, inside-out, and inside-in systems. An outside-in system uses external sensors to collect data from sources placed on the body, an inside-out systems have sensors placed on the body that collect external sources, and inside-in systems have their sources and sensors placed on the body. The principal technologies used today that represent these categories are optical, electromagnetic, and inertial human tracking systems. A typical optical motion capture system is based on a single computer that controls the input of several digital CCD (chargecoupled device) cameras. Electromagnetic motion capture systems are part of the six degrees of freedom electromagnetic measurement systems' family and consist of an array of receivers that measure their spatial relationship to a nearby transmitter. These receivers or sensors are placed on the body and are connected to an electronic control unit, in most cases by individual cables. These systems are called magnetic trackers and they emerged from the technology used in military aircraft for helmet-mounted displays (HMDs).