Introduction

We perceive the world as three-dimensional. The inputs to our visual system, however, are only a pair of two-dimensional projections on the two retinal surfaces. As emphasized by Marr and Poggio (1976), it is generally impossible to uniquely determine the three-dimensional world from its two-dimensional retinal projections. How, then, do we usually perceive a well-defined three-dimensional environment? It has long been recognized that, since the world we live in is not random, the visual system has evolved and developed to take advantage of the world's statistical regularities, which are reflected in the retinal images. Some of these image regularities, termed depth cues, are interpreted by the visual system as depth. Numerous depth cues have been discovered. Many of them, such as perspective, shading, texture, motion, and occlusion, are present in the retina of a single eye, and are thus called monocular depth cues. Other cues are called binocular, as they result from comparing the two retinal projections. In the following, we will review our physiologically based models for three binocular depth cues: horizontal disparity (Qian, 1994; Chen and Qian, 2004), vertical disparity (Matthews et al., 2003), and interocular time delay (Qian and Andersen, 1994; Qian and Freeman, 2009). We have also constructed a model for depth perception from monocularly occluded regions (Assee and Qian, 2007), another binocular depth cue, but have omitted it here owing to space limitations.