Vision serves many roles. One of the most important roles is the ability to quickly and accurately perceive the relative positions of objects as we move among them. Perhaps it is for this reason that the visual system employs a variety of different methods to perceive the relative depth of objects in a scene, often making use of parallax (the change in the apparent relative orientations of objects when viewed from different positions). Indeed, the perception of depth, that is, the ability to judge spatial relationships in three dimensions (3D), is an interesting neural processing problem because the visual system must recover the 3D from a flat, two-dimensional retinal image.