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Depth Perception

Depth perception is the ability of observers to discriminate the distances of objects, particularly their relative distances, and identify the three-dimensional shape of surfaces. This entry concentrates on sources of visual information contributing to depth perception and separates these into two main types: those that apply when the viewer stands still with one eye closed (pictorial cues) and those that come into play when the viewer moves or opens the other eye.

It is hard to imagine perceiving the world without a perception of depth. Animals move around continually, receiving changing images as they do. If they were unable to relate these images to a stable scene they could not operate successfully in a 3D world. This may explain why clinical disorders of depth perception per se are so rare (as opposed to malfunctions of systems that contribute to depth perception, such as stereoscopic vision).

Generally, in nature, depth vision is a consequence of moving through the world. Most animals have eyes positioned on the side of their head so that they can look out for predators; some have a 360° field of view. They are able to recover depth information about the scene by moving. Binocular stereopsis (seeing depth with two eyes) is not necessary for depth perception and acute stereoscopic vision is almost uniquely the preserve of carnivores. It allows them to remain perfectly still in the grass and yet gain the advantages of depth vision.

Pictorial Cues

Of course, it is possible to perceive depth and the 3D structure of a scene without either moving or using binocular stereopsis. When people look at a photograph, which is taken from a single vantage point, they perceive depth in the scene. There are rare examples when this perception can be thoroughly misleading. For example, Figure 1 shows the Ames room, which appears to be a normal shaped room even when we know, from seeing the people in different parts of the room, that this cannot be the case. Assumptions about floors being perpendicular to walls and windows or tiles being rectangular are so strong that they affect our perception of depth and size. The Ames room makes clear that we cannot deduce the depth structure of a scene in a photograph unless we make assumptions about the scene. Remarkably often, those assumptions prove to be correct but occasionally they can fail.

Perspective

One pictorial cue is perspective. Parallel lines in the world project to straight lines in the image that eventually come together at a vanishing point, as shown in Figure 1, for example. This is one of the reasons that the viewer is so convinced by the Ames room: It is highly unlikely that a set of lines in the image would all converge to a point unless the lines in the world were in fact parallel (but, of course, in the Ames room they are not). Over many centuries, artists have discovered how to use perspective in paintings. First, they learned to paint lines that recede toward a single vanishing point (in the 15th century, e.g, Masolino), then much later, they painted lines that recede toward two points (in the 18th century, e.g., Canaletto's pictures of buildings viewed from one corner), and finally, they drew lines that recede toward three vanishing points (in the 20th century, e.g., M. C. Escher's pictures). For these perspective cues to be informative, the visual system must assume that converging lines in the image correspond to parallel lines in the real world. This is true sufficiently often for perspective to be a powerful cue to depth.

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