Editing Motion capture (section)

===Passive markers===
[[Image:MotionCapture.jpg|thumb|A dancer wearing a suit used in an optical motion capture system]]
[[File:Motion capture facial.jpg|thumb|Markers at specific points on an actor's face during facial optical motion capture]]
''Passive optical'' systems use markers coated with a [[retroreflective]] material to reflect light that is generated near the camera's lens. The camera's threshold can be adjusted so only the bright reflective markers will be sampled, ignoring skin and fabric.

The centroid of the marker is estimated as a position within the two-dimensional image that is captured. The grayscale value of each pixel can be used to provide sub-pixel accuracy by finding the centroid of the [[Gaussian]].

An object with markers attached at known positions is used to calibrate the cameras and obtain their positions, and the lens distortion of each camera is measured. If two calibrated cameras see a marker, a three-dimensional fix can be obtained. Typically a system will consist of around 2 to 48 cameras. Systems of over three hundred cameras exist to try to reduce marker swap. Extra cameras are required for full coverage around the capture subject and multiple subjects.

Vendors have constraint software to reduce the problem of marker swapping since all passive markers appear identical. Unlike active marker systems and magnetic systems, passive systems do not require the user to wear wires or electronic equipment.<ref>{{cite journal|title=Motion Capture: Optical Systems|journal=[[Next Generation (magazine)|Next Generation]]|issue=10|publisher=[[Imagine Media]]|date=October 1995|page=53}}</ref> Instead, hundreds of rubber balls are attached with reflective tape, which needs to be replaced periodically. The markers are usually attached directly to the skin (as in biomechanics), or they are [[velcro]]ed to a performer wearing a full-body spandex/lycra [[Mo-cap suit|suit designed specifically for motion capture]]. This type of system can capture large numbers of markers at frame rates usually around 120 to 160 fps although by lowering the resolution and tracking a smaller region of interest they can track as high as 10,000 fps.