Editing Depth perception (section)

== Binocular cues ==

[[Binocular vision|Binocular]] cues provide depth information when viewing a scene with both eyes.

=== Stereopsis, or retinal (binocular) disparity, or binocular parallax ===
{{main|Stereopsis}}
[[Animal]]s that have their eyes placed frontally can also use information derived from the different projections of objects onto each [[retina]] to judge depth. By using two images of the same scene obtained from slightly different angles, it is possible to [[triangulation|triangulate]] the distance to an object with a high degree of accuracy. Each eye views a slightly different angle of an object seen by the left and right eyes. This happens because of the horizontal separation parallax of the eyes. If an object is far away, the disparity of that image falling on both retinas will be small. If the object is close or near, the disparity will be large. It is stereopsis that tricks people into thinking they perceive depth when viewing [[Magic Eye]]s, [[autostereogram]]s, [[3-D film|3-D movies]], and [[Stereoscopy|stereoscopic photos]].

=== Convergence ===
{{main|Convergence (eye)}}
Convergence is a binocular oculomotor cue for distance and depth perception. Because of stereopsis, the two eyeballs focus on the same object; in doing so they converge. The convergence will stretch the [[extraocular muscles]]{{snd}}the receptors for this are [[muscle spindle]]s. As happens with the monocular accommodation cue, kinesthetic sensations from these extraocular muscles also help in distance and depth perception. The angle of convergence is smaller when the eye is fixating on objects which are far away. Convergence is effective for distances less than 10 meters.<ref>{{cite book |last1=Okoshi | first1=Takanori. |year=2012 |title=Three-dimensional imaging techniques |pages=387 |publisher=Elsevier |asin=B01D3RGBGS}}</ref>

=== Shadow stereopsis ===
Antonio Medina Puerta demonstrated that retinal images with no parallax disparity but with different shadows were fused stereoscopically, imparting depth perception to the imaged scene. He named the phenomenon "shadow stereopsis". Shadows are therefore an important, stereoscopic cue for depth perception.<ref name="Puerta">{{cite journal |author=Medina Puerta A |year=1989 |title=The power of shadows: shadow stereopsis |journal=J. Opt. Soc. Am. A |volume=6 |pages=309–311 |doi=10.1364/JOSAA.6.000309 |pmid=2926527 |issue=2|bibcode = 1989JOSAA...6..309M }}</ref>

Of these various cues, only convergence, accommodation and familiar size provide absolute distance information. All other cues are relative (as in, 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).