Stereopsis referers to the brain's ability to extract depth information from differences between the images in the two eyes.We have developed algorithms to overcome or reduce problems with stereo displays. Before explaining a brief introduction to stereo vision is needed. Differences between points in these images are called disparities and the brain can use disparities to infer relative depths between points in the environment. There are a number of problems with stereoscopic computer displays because of the limited depth information available in human perception.

Figure 1

The above diagram illustrates a simple stereoscopic display. On the left are the views presented to the two eyes. If a and c are fixated bay the right eye and left eye respectively, the brain fuses c and d and interprets the result as a line behind the screen as shown on ther right. Disparities are the differences between the angles subtended between pairs of features (alpha - beta). There are a number of problems with screen-based stereo displays. First, if disparities are too large the result is seeing double (diplopia). The area in which the images can be fused is called Panum's fusional area and this is remarkably small in the worst case. A second problem is that objects more than 30 meters away have images on the retina that are so similar the brain cannot obtain any useful disparity information. A third problem is called vergence focus conflict - this has to do with the coupling of the focusing mechanism in the eye with the mechanism that makes the eyes converge when we see objects at different distances. Computer graphics systems can provide the correct information for vergence but not focus and this may cause eye strain. Fourthly, frame cancellation occurs for an object shown in front of the screen as shown below. An object clipped by the edge of the screen is interpreted as being occluded by the screen border and because occlusion is the strongest depth cue the depth effect collapses.

Our cyclopean Scale as illustrated below is an algorthm to solve or reduce a number of these problems.

The basic idea is to scale the whole scene about the midpoint between the viewers two eyes (hence cyclopean) until the near point lies just behind the screen. This
• removes the possibility of frame cancellation,
• brings far objects closer where stereo depth becomes available, and
•reduces vergence focus conflict.

Cyclopian Scale is just only first stage in a number of methods we have developed and are continuing to develop to enhance stereoscopic viewing (Nhu Le and Colin Ware).