Techniques

The main objective with motion capture is to obtain in a 3D space the movement of a person or of an object captured. There are several methods used in order to achieve this goal. The three main existing techniques are :

Mechanical Mocap

Initially this technique involved the subject wearing a real “exoskeleton”.The principle of mechanical Mocap is to register the angle(s) made at each articulation. Angular sensors are placed on each articulation in order to register the value of each rotation. By using this vital information it becomes possible to recalculate the movement of an actor in real time.
New prototypes based on gyroscopic markers help to prevent localisation problems in the open space, however they are still not able for the moment to offer a reliable result regarding the interaction between actors and objects. Mechanical motion capture has nevertheless greatly improved becoming more flexible and light throughout the years.

Magnetic Mocap

An electromagnetic field is created from a precise point in space which will become interpreted as the original reference point of the acquired 3D. “Markers” (made from a ball of electric fibre) are placed on the actors/objects to be captured, and interfere with the electric field created. This interference can provide us with the correct positioning of the different markers with regard to the time as an reference ”aerial” , the original starting point of the magnetic field.

Optical Mocap

Optical Mocap is a network of 3 to 32 (or up to more than 200 for American movies) cameras that are installed, calibrated and form a perimeter of capture. The markers made from a highly reflective material, are placed on the subject, while the cameras themselves project an infra-red light onto the markers that make them light up. The 2D images that are obtained on each camera allow the calculation of the 3D position (x,y,z) of each marker (triangulation). The result of the movements studied is a precise and robust image of the movements studied.

New Technologies

Everyday new alternatives in this technology are being developed. The “markerless” techniques allow a capture to take place simply using videos of the movement. Although this technique allows the production costs to be reduced and an easier manipulation of the equipment, the technique greatly lacks the precision obtained when using optical motion capture with markers.