The analog output was then converted to a digital form and fed to the computer animation system. Soon after that, commercial optical tracking systems such as the Op-Eye and SelSpot systems began to be used by the computer graphics community. In the early 1980's, both the MIT Architecture Machine Group and the New York Institute of Technology Computer Graphics Lab experimented with optical tracking of the human body. A combination of special hardware and software pick out the markers in each camera's visual field and, by comparing the images, calculate the three-dimensional position of each marker through time. Early systems could track only a dozen or so markers at a time.
More recent systems can track several dozen at once.
Occlusion problems can be overcome by the use of more cameras, but even so, most current optical systems require manual post-processing to recover trajectories when a marker is lost from view. This will change as systems become more sophisticated. The problem of resolution involves a trade-off of many variables, including camera price, field of view, and space of movement. The more resolution you need, the more the camera costs. The same camera can give you greater movement resolution if focused on a smaller field of view, but this limits the size of motions that are possible.