Depth perception is the mechanism in the human visual system which disambiguates the relative positions of two three-dimensional objects.

The visual system relies on both physiological and psychological cues to make accurate estimates of the depths of objects.

Physiological depth cues are formed by keeping track of the muscular adjustments the visual system makes. It's hard to establish just how much these cues contribute to depth perception, but they are assumed to be weak. They include the following:

Psychological depth cues are based on the analysis of the image on the retina by the visual cortex of the brain. They are usually much stronger than the physiological depth cues.

The following two psychological depth cues are based on parallax, which is the difference in the appearance of an object caused by a difference in viewpoint.

  • Binocular Disparity - The difference in the positions of the same object in images viewed by the left and right eyes. Binocular disparity is the result of binocular parallax, which is the difference between the images viewed by the two eyes.
  • Motion Parallax - A monocular cue that relies on the changes in the images viewed in an eye as the viewer moves.
Physiological and parallax depth cues are effective in perceiving depth of only objects near the viewer. This is because the farther away an object is, the larger the distance to the object is in comparison to the interocular distance or the distance moved by the viewer (in the case of motion parallax). For this reason, accommodation is effective only up to about 2 meters, and convergence up to about 10 meters.

The remaining psychological depth cues are based on the appearance of the object from one viewpoint. They are monocular cues, independent of parallax. They are effective at all distances and are the predominant depth cues for distant objects.

  • Linear Perspective - The sizes of objects change in inverse proprotion to their depth. Objects which are farther appear smaller than objects which are closer. We compare the retinal image size of the object with the known size of object to judge how far it is. Furthermore, objects on the ground which are seen higher in the visual field are perceived to be farther. Similarly, objects in the sky which are seen lower in the visual field are perceived to be farther.
  • Occlusion - If an object overlaps another, we perceive the blocker as being closer.
  • Aerial Perspective - Objects which are far away often appear hazy or bluish. This is due to the atmospheric scattering of light.
  • Texture Gradient - Textures on an object appear coarse if the object is close to the viewer, and fine if the object is far from the viewer.
  • Shading and Shadowing - Known properties of surfaces and mechanisms of reflection are used by the visual system to estimate the orientation and shape of objects. Knowledge of the location of light sources helps us determine location of objects from their shadows.
  • Chromostereopsis - Chromatic Aberration in the eyes results in objects colored with some wavelengths of light to appear closer or farther than objects colored with some other wavelength.
Since the majority of depth cues are monocular, vision loss in one eye does not result in the loss of the ability to perceive depth. In fact, only two depth cues -- convergence and binocular disparity -- require both eyes to operate. Therefore, loss of stereoscopic vision affects depth perception only near the viewer -- and even there, other cues make up for it to a great extent.

Thanks to hobyrne for bug reports.

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