Abstract: Stereoscopic display is fundamental to many virtual reality systems. Stereoscopic systems render two perspective views of a scene one for each eye of the user. Ideally the user's visual system combines the stereo image pairs into a single, 3D perceived image. In practice, however, users can have difficulty fusing the stereo image pair into a single 3D image. Researchers have used a number of software methods to reduce fusion problems. We are particularly concerned with the effects of these techniques on stereoscopic HTDs (Head-Tracked Display). In these systems the head is tracked but the display is stationary, attached to a desk, tabletop or wall. This paper comprehensively surveys software fusion techniques. We then geometrically characterize and classify the various techniques and illustrate how they relate to stereoscopic HTD application characteristics.