The visual perception of 3D shape
Trends in Cognitive Sciences
A fundamental problem for the visual perception of 3D shape is that patterns of optical stimulation are inherently ambiguous. Recent mathematical analyses have shown, however, that these ambiguities can be highly constrained, so that many aspects of 3D structure are uniquely specified even though others might be underdetermined. Empirical results with human observers reveal a similar pattern of performance. Judgments about 3D shape are often systematically distorted relative to the actual
... ure of an observed scene, but these distortions are typically constrained to a limited class of transformations. These findings suggest that the perceptual representation of 3D shape involves a relatively abstract data structure that is based primarily on qualitative properties that can be reliably determined from visual information. One of the most remarkable phenomena in the study of human vision is the ability of observers to perceive the 3D shapes of objects from patterns of light that project onto the retina. Indeed, were it not for our own perceptual experiences, it would be tempting to conclude that the visual perception of 3D shape is a mathematical impossibility, because the properties of optical stimulation appear to have so little in common with the properties of real objects encountered in nature. Whereas real objects exist in 3-dimensional space and are composed of tangible substances such as earth, metal or flesh, an optical image of an object is confined to a 2-dimensional projection surface and consists of nothing more than patterns of light. Nevertheless, for many animals, including humans, these seemingly uninterpretable patterns of light are the primary source of sensory information about the arrangement of objects and surfaces in the surrounding environment. Scientists and philosophers have speculated about the nature of 3D shape perception for over two millennia, yet it remains an active area of research involving many different disciplines, including psychology, neuroscience, computer science, physics and mathematics. The present article reviews the current state of the field from the perspective of human vision: it will first summarize how patterns of light at a point of observation are mathematically related to the structure of the physical environment; it will then consider some recent psychophysical findings on the nature of 3D shape perception; it will evaluate some possible data structures by which 3D shapes might be perceptually represented; and it will summarize recent research on the neural processing of 3D shape.