This section details the requirements of any system that deals with stereo vision, usually limitations imposed by the human visual system.
The first requirement for stereovisual images is that both images in the stereo images be ``in focus'', that is, focused on the same plane in space, the plane of the subject of the image. If they are different, then the image received by one eye will be blurred and out of focus in the same way as a regular photograph can be out of focus. The end result is that the brain becomes confused when trying to combine the crisp image with the blurred one, and cannot properly view the stereo image.
The cameras should be angled inwards so that they are directed towards the subject. This means that as the distance to the subject changes, so must the angle of inclination of the cameras. Subjects at an infinite distance (that is, in the background) would require the cameras to be parallel to each other. Also, subject material closer than the focal length of humans would almost certainly not be properly comprehensible2, though it would be possible to record. However, the results of these kinds of experiments would be interesting, as a major restriction with humans is the limits of the eye muscles and the way the brain insists on trying to keep the eyes focused on the subject.
When displaying a stereo image the human's eyes are focused on the screen, but the stereo image being displayed may be of a subject at a completely different distance from the viewer. This mismatch is a problem with all stereo displays of this kind, as well as many virtual reality environments, such as first person video games.
All of these problems are physical problems which lie outside the scope of this system3. For example, determining the distance to a subject (required to focus the cameras) requires either sophisticated cameras, additional sensory hardware or highly sophisticated artificial intelligence software. However, a system without such features would be substantially limited in the subjects and images that it could view, especially for dynamic images.