The structure of visual spaces

The "visual space" of an optical observer situated at a single, fixed viewpoint is necessarily very ambiguous. Although the structure of the "visual field" (the lateral dimensions, i.e., the "image") is well defined, the "depth" dimension has to be inferred from the image on the basis of "monocular depth cues" such as occlusion, shading, etc. Such cues are in no way "given", but are guesses on the basis of prior knowledge about the generic structure of the world and the laws of optics. Thus such a guess is like a hallucination that is used to tentatively interpret image structures as depth cues. The guesses are successful if they lead to a coherent interpretation. Such "controlled hallucination" (in psychological terminology) is similar to the "analysis by synthesis" of computer vision. Although highly ambiguous, visual spaces do have geometrical structure. The group of ambiguities left open by the cues (e.g., the well known bas-relief ambiguity in the case of shape from shading) may be interpreted as the group of congruences (proper motions) of the space. The general structure of visual spaces for different visual fields is explored in the paper. Applications include improved viewing systems for optical man-machine interfaces.