Freetrack uses a webcam and markers to detect the player's head movement in order to reproduce those movement in a game. The principle is similar to the famous TrackIRand to the other software implementation as Freelookand Cam2Pan.
Camera with a passe IR filter
4 IR leds, 4 resistances, a pack battery pack
Note: The system may also work with red or white leds and without the pass IR filter. In that case, it will be more sensitive to external disturbance that may corrupt the motion capture process.