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Depth perception is the visual ability to perceive the world in three dimensions. Although any animal capable of moving around its environment must be able to sense the distance of objects in that environment, the term perception is reserved for humans, who are the only beings that can tell each other about their experiences of distances.^[1]

Depth perception arises from a variety of depth cues. These are typically classified into binocular cues that require input from both eyes and monocular cues that require the input from just one eye.^[2] Binocular cues include stereopsis, yielding depth from binocular vision through exploitation of parallax. Monocular cues include size distant objects subtend smaller visual angles than near objects.^[3] A third class of cues requires synthetic integration of binocular and monocular cues.^{[citation needed]}

Of these various cues, only convergence, focus and familiar size provide absolute distance information. All other cues are relative (ie, they can only be used to tell which objects are closer relative to others). Stereopsis is merely relative because a greater or lesser disparity for nearby objects could either mean that those objects differ more or less substantially in relative depth or that the foveated object is nearer or further away (the further away a scene is, the smaller is the retinal disparity indicating the same depth difference).

It would be over-simplification to ignore the mental processes at work as a person sees with two normal eyes. The fact that binocular stereopsis is occurring, enables the brain to infer and perceive certain additional depth in the form of a mental construct. Closing one eye shuts down this stereo construct. Recent work toward improving digital display of stereoscopic images has re-vitalized the field, as practical applications often do. Those working in the field have identified several processes of interpolation, previously ignored or considered irrelevant. These provide a linkage in the mental construct of objects visible to only one eye, while viewing with both eyes in a forward direction. Recent literature has addressed the relationship between the stereo viewing area and the periphery. Recent analysis has demonstrated that objects just outside the angle of double visual coverage, are, in fact, integrated by the mind into the stereo construct by a process of inference. Briefly stated, " all objects, in even moderate focus, within the central viewing field of a single eye, are, an important part of the stereo construct". Their physical position is noted, and SEEN very accurately in the mental stereo visualization process, though visible to only one of the 2 eyes in use.

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