Passive markers

Passive optical system use markers coated with a retroreflective material to reflect light that is generated near the cameras 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 2 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 3 dimensional fix can be obtained. Typically a system will consist of around 6 to 24 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 markers appear identical. Unlike active marker systems and magnetic systems, passive systems do not require the user to wear wires or electronic equipment. 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 velcroed to a performer wearing a full body spandex/lycra suit designed specifically for motion capture. This type of system can capture large numbers of markers at frame rates as high as 2000fps. The frame rate for a given system is often balanced between resolution and speed: a 4-megapixel system normally runs at 370 hertz, but can reduce the resolution to .3 megapixels and then run at 2000 hertz. Typical systems are $100,000 for 4-megapixel 360-hertz systems, and $50,000 for .3-megapixel 120-hertz systems.