how motion capture works
The essence of motion capture is to enable animated characters to behave like real people. During a motion capture session, actors are dressed in black stretch suits that bear special markers. Usually, these are small, light-weight balls, with a reflective surface. If the actors are using props [for example, weapons] in their scenes, then these props will also bear these markers.
During recording, the specialized motion capture cameras emit an infrared light that is reflected by the markers worn by the actors. The cameras record these reflections, while the computer pinpoints their exact location in 3D space during the entire Length of the performed movement.