Editing Depth perception (section)

== Monocular cues ==
[[File:Parallax scroll.gif|thumb|Motion parallax]]
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[[Monocular vision|Monocular]] cues provide depth information even when viewing a scene with only one eye.

=== Motion parallax ===
{{main|Parallax}}
When an observer moves, the apparent relative motion of several stationary objects against a background gives hints about their relative distance. If information about the direction and velocity of movement is known, motion parallax can provide absolute depth information.<ref>{{cite journal |author=Ferris SH |year=1972 |title=Motion parallax and absolute distance|journal= Journal of Experimental Psychology |volume=95 |issue=2 |pages=258–263|url=http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/8713/NSMRL_673.pdf?sequence=1|archive-url=https://web.archive.org/web/20190209232805/http://archive.rubicon-foundation.org/xmlui/bitstream/handle/123456789/8713/NSMRL_673.pdf?sequence=1|url-status=usurped|archive-date=February 9, 2019|doi=10.1037/h0033605 |pmid=5071906 }}</ref> This effect can be seen clearly when driving in a car. Nearby things pass quickly, while far-off objects appear stationary. Some animals that lack [[binocular vision]] due to their eyes having little common field-of-view employ motion parallax more explicitly than humans for depth cueing (for example, some types of birds, which bob their heads to achieve motion parallax, and squirrels, which move in lines [[orthogonal]] to an object of interest to do the same<ref>Kral K. (2003). [http://wexler.free.fr/library/files/kral%20(2003)%20behavioural-analytical%20studies%20of%20the%20role%20of%20head%20movements%20in%20depth%20perception%20in%20insects,%20birds%20and%20mammals.pdf "Behavioural-analytical studies of the role of head movements in depth perception in insects, birds and mammals"]. ''Behavioural Processes'' '''64''': 1–12.</ref>).<ref name="parallax" group="note"/>

=== {{visible anchor|Depth from motion}} ===

When an object moves toward the observer, the retinal projection of an object expands over a period of time, which leads to the perception of movement in a line toward the observer. Another name for this phenomenon is ''depth from optical expansion''.<ref>{{cite journal|last=Swanston|first=M.C.|author2=Gogel, W.C.|title=Perceived size and motion in depth from optical expansion|journal=Perception & Psychophysics|year=1986|volume=39|issue=5|pages=309–326|doi=10.3758/BF03202998|pmid=3737362|doi-access=free}}</ref> The dynamic stimulus change enables the observer not only to see the object as moving, but to perceive the distance of the moving object. Thus, in this context, the changing size serves as a distance cue.<ref>{{cite journal|last=Ittelson|first=W.H.|title=Size as a cue to distance: Radial motion|journal=American Journal of Psychology|date=Apr 1951|volume=64|issue=2|pages=188–202|doi=10.2307/1418666|jstor=1418666|pmid=14829626}}</ref> A related phenomenon is the visual system's capacity to calculate time-to-contact (TTC) of an approaching object from the rate of optical expansion{{snd}}a useful ability in contexts ranging from driving a car to playing a [[ball game]]. However, the calculation of TTC is, strictly speaking, a perception of velocity rather than depth.

=== Kinetic depth effect ===
{{main|Kinetic depth effect}}
If a stationary rigid figure (for example, a wire cube) is placed in front of a point source of light so that its shadow falls on a translucent screen, an observer on the other side of the screen will see a two-dimensional pattern of lines. But if the cube rotates, the visual system will extract the necessary information for perception of the third dimension from the movements of the lines, and a cube is seen. This is an example of the ''kinetic depth effect''.<ref>{{cite journal|last=Wallach|first=H.|author2=O'Connell, D.N.|s2cid=11979303|title=The kinetic depth effect|journal=Journal of Experimental Psychology|year=1953|volume=45|pages=205–217|doi=10.1037/h0056880|pmid=13052853|issue=4}}</ref> The effect also occurs when the rotating object is solid (rather than an outline figure), provided that the projected shadow consists of lines which have definite corners or end points, and that these lines change in both length and orientation during the rotation.<ref>{{cite book|last=Kaufman|first=Lloyd|title=Sight and Mind|year=1974|publisher=Oxford University Press|location=New York|pages=139–141}}</ref>

=== Perspective ===
{{main|Perspective (visual)}}
The property of parallel lines converging in the distance, at infinity, allows us to reconstruct the relative distance of two parts of an object, or of landscape features. An example would be standing on a straight road, looking down the road, and noticing the road narrows as it goes off in the distance. [[Visual perception]] of perspective in real space, for instance in rooms, in settlements and in nature, is a result of several optical impressions and the interpretation by the [[visual system]]. The [[Visual angle|angle of vision]] is important for the [[apparent size]]. A nearby object is imaged on a larger area on the [[retina]], the same object or an object of the same size further away on a smaller area.<ref>''[https://www.univie.ac.at/mikroskopie/1_grundlagen/optik/Grundlagen%20der%20Optik.pdf Grundlagen der Optik].'' page 24.</ref> The perception of perspective is possible when looking with one eye only, but [[stereoscopic vision]] enhances the impression of the spatial. Regardless of whether the light rays entering the eye come from a three-dimensional space or from a two-dimensional image, they hit the inside of the eye on the retina as a surface. What a person sees, is based on the reconstruction by their visual system, in which one and the same image on the retina can be interpreted both two-dimensionally and three-dimensionally. If a three-dimensional interpretation has been recognised, it receives a preference and determines the perception.<ref>Georg Eisner: ''[http://www.eisner-georg.ch/Andere/Perspektive/Perspektiven.pdf Perspektive und Visuelles System – Wege zur Wahrnehmung des Raumes]'' pp. 102–103</ref>
<gallery>
Perspektivisches Sehen und Interpretation.png|Context-dependent interpretation of the size
08913-Perspective Run.jpg|Shots at different distances
Study in Vanishing Perspective.jpg|The horizon line is at the height of the armrests.
Spatial vision and perspective.jpg|View from a window on the 2nd floor of a house
Mountain panorama in France 3.jpg|Mountain peak near the [[snow line]] and several mountain peaks above the snow line
ISS-40 Sicily and Italy.jpg|[[Earth#Size and shape|Earth curvature]]
</gallery>
In spatial vision, the horizontal line of sight can play a role. In the picture taken from the window of a house, the horizontal line of sight is at the level of the second floor (yellow line). Below this line, the further away objects are, the higher up in the [[visual field]] they appear. Above the horizontal line of sight, objects that are further away appear lower than those that are closer. To represent spatial impressions in [[Perspective (graphical)|graphical perspective]], one can use a [[vanishing point]].<ref>Georg Eisner: ''[http://www.eisner-georg.ch/Andere/Perspektive/Perspektiven.pdf Perspektive und Visuelles System – Wege zur Wahrnehmung des Raumes]'' page 181</ref> When looking at long [[geographical distance]]s, perspective effects also partially result from the angle of vision, but not only by this. In picture 5 of the series, in the background is [[Mont Blanc]], the highest mountain in the Alps. It appears lower than the mountain in front in the center of the picture. Measurements and calculations can be used to determine the proportion of the [[Earth#Size and shape|curvature of Earth]] in the [[subjectivity|subjectively]] perceived proportions.

=== Relative size ===

If two objects are known to be the same size (for example, two trees) but their absolute size is unknown, relative size cues can provide information about the relative depth of the two objects. If one subtends a larger visual angle on the retina than the other, the object which subtends the larger visual angle appears closer.

=== Familiar size ===

Since the visual angle of an object projected onto the retina decreases with distance, this information can be combined with previous knowledge of the object's size to determine the absolute depth of the object. For example, people are generally familiar with the size of an average automobile. This prior knowledge can be combined with information about the angle it subtends on the retina to determine the absolute depth of an automobile in a scene.

=== Absolute size ===

Even if the actual size of the object is unknown and there is only one object visible, a smaller object seems farther away than a large object that is presented at the same location.<ref>Sousa, R., Brenner, E., & Smeets, J.B.J. (2011). "Judging an unfamiliar object's distance from its retinal image size". [https://dx.doi.org/10.1167/11.9.10 ''Journal of Vision'', 11(9), 10, 1–6].
Sousa, R., Smeets, J.B.J., & Brenner, E. (2012). "Does size matter?" [https://dx.doi.org/10.1068/p7324 ''Perception'', 41(12), 1532–1534].
</ref>

=== Aerial perspective ===
{{main|Aerial perspective}}
Due to light scattering by the atmosphere, objects that are a great distance away have lower luminance [[Contrast (vision)|contrast]] and lower [[color saturation]]. Due to this, images seem hazy the farther they are away from a person's point of view. In [[computer graphics]], this is often called "[[distance fog]]". The foreground has high contrast; the background has low contrast. Objects differing only in their contrast with a background appear to be at different depths.<ref>{{cite journal |vauthors=O'Shea RP, Blackburn SG, Ono H |year=1994 |title=Contrast as a depth cue |journal=Vision Research |volume=34 |pages=1595–1604 |doi=10.1016/0042-6989(94)90116-3 |pmid=7941367 |issue=12|s2cid=149436 }}</ref> The color of distant objects is also shifted toward the blue end of the [[spectrum]] (for example, distant mountains). Some painters, notably [[Cézanne]], employ "warm" pigments (red, yellow and orange) to bring features forward towards the viewer, and "cool" ones (blue, violet, and blue-green) to indicate the part of a form that curves away from the [[picture plane]].

=== Accommodation ===
{{main|Accommodation (eye)}}
Accommodation is an oculomotor cue for depth perception. When humans try to focus on distant objects, the [[ciliary muscles]] relax, allowing the eye lens to become thinner, which increases the [[focal length]]. Depth perception of distant objects is made possible by other methods besides accommodation. The [[Proprioception|kinesthetic sensations]] of the contracting and relaxing ciliary muscles (intraocular muscles) are sent to the visual cortex where they are used for interpreting distance and depth. Accommodation is only effective for distances less than 2 meters.

{{Anchor|Interposition2020-11-04}}

=== Occultation ===
{{main|Occultation}}
Occultation (also referred to as ''interposition'') happens when near surfaces overlap far surfaces.<ref>{{cite web | url=http://www.psychol.ucl.ac.uk/alan.johnston/Depth.html | title=Depth Perception | publisher=UCL Division of Psychology and Language Sciences | access-date=22 September 2013 | author=Johnston, Alan | archive-url=https://web.archive.org/web/20130927185827/http://www.psychol.ucl.ac.uk/alan.johnston/Depth.html | archive-date=27 September 2013 | url-status=dead }}</ref> If one object partially blocks the view of another object, humans perceive it as closer. However, this information only allows the observer to make a "ranking" of relative nearness. The presence of monocular [[ambient occlusion]]s consist of the object's texture and geometry. These phenomena are able to reduce depth perception latency both in natural and artificial stimuli.<ref>{{cite journal |vauthors=Gillam B, Borsting E |year=1988 |title=The role of monocular regions in stereoscopic displays |journal=Perception |volume=17 |pages=603–608 |doi=10.1068/p170603 |pmid=3249668 |issue=5|s2cid=42118792 }}</ref><ref>{{cite book |last1=Schacter |first1=Daniel L. |first2=Daniel T. |last2=Gilbert |first3=Daniel M. |last3=Wegner |chapter=Sensation and Perception |title=Psychology |chapter-url=https://archive.org/details/psychology0000scha |chapter-url-access=registration |edition=2nd |location=New York |publisher=Worth, Inc. |year=2011 |pages=[https://archive.org/details/psychology0000scha/page/136 136–137]|isbn=9781429237192 }}</ref>

=== Curvilinear perspective ===
{{main|Curvilinear perspective}}
At the outer extremes of the [[visual field]], parallel lines become curved, as in a photo taken through a [[fisheye lens]]. This effect, although it is usually eliminated from both art and photos by the cropping or framing of a picture, greatly enhances the viewer's sense of being positioned within a real, three-dimensional space. (Classical perspective has no use for this so-called "distortion", although in fact the "distortions" strictly obey optical laws and provide perfectly valid visual information, just as classical perspective does for the part of the field of vision that falls within its frame.)

=== Texture gradient ===
{{main|Texture gradient}}
Fine details on nearby objects can be seen clearly, whereas such details are not visible on faraway objects. Texture gradients are the grains of an item. For example, on a long gravel road, the gravel near the observer can be clearly seen of shape, size and colour. In the distance, the road's texture cannot be clearly differentiated.

=== Lighting and shading ===
{{main|Lighting|Shading}}
The way that light falls on an object and reflects off its surfaces, and the shadows that are cast by objects provide an effective cue for the brain to determine the shape of objects and their position in space.<ref>{{cite book |last=Lipton |first=L. |year=1982 |url=http://www.stereoscopic.org/library/index.html |title=Foundations of the Stereoscopic Cinema – A Study in Depth |location=New York |publisher=Van Nostrand Reinhold |page=56}}</ref>

=== Defocus blur ===
{{main|Depth of field}}
Selective image blurring is very commonly used in photography and video to establish the impression of depth. This can act as a monocular cue even when all other cues are removed. It may contribute to depth perception in natural retinal images, because the depth of focus of the [[human eye]] is limited. In addition, there are several depth estimation algorithms based on defocus and blurring.<ref>{{cite journal |author=Mather G |title=Image Blur as a Pictorial Depth Cue |journal=Proceedings: Biological Sciences |volume=263 |date=22 February 1996 |pages=169–172 |issue=1367 |bibcode=1996RSPSB.263..169M |doi=10.1098/rspb.1996.0027|pmid=8728981 |s2cid=30513172 }}</ref> Some jumping spiders are known to use image defocus to judge depth.<ref>{{cite journal |author=Takashi Nagata |title=Depth Perception from image defocus in a jumping spider |journal=Science |volume=335 |date=27 January 2012 |pages=469–471 |issue=6067 |doi=10.1126/science.1211667 |pmid=22282813 |last2=Koyanagi |first2=M |last3=Tsukamoto |first3=H |last4=Saeki |first4=S |last5=Isono |first5=K |last6=Shichida |first6=Y |last7=Tokunaga |first7=F |last8=Kinoshita |first8=M |last9=Arikawa |first9=K|bibcode = 2012Sci...335..469N |display-authors=9 |last10=Terakita |first10=Akihisa |s2cid=8039638 |url=https://ir.soken.ac.jp/?action=repository_uri&item_id=4203 }}</ref>

=== Elevation ===
When an object is visible relative to the horizon, humans tend to perceive objects which are closer to the horizon as being farther away from them, and objects which are farther from the horizon as being closer to them.<ref>{{cite book |first1=Neil R. |last1=Carlson |first2=Harold L. |last2=Miller Jr. |first3=Donald S. |last3=Heth |first4=John W. |last4=Donahoe |first5=G. Neil |last5=Martin |year=2010 |title=Psychology: The Science of Behavior |page=187 |edition=7th |isbn=978-0-205-76223-1 |publisher=Pearson}}</ref> In addition, if an object moves from a position close to the horizon to a position higher or lower than the horizon, it will appear to move closer to the viewer.

=== Ocular parallax ===
Ocular parallax is a perceptual effect where the rotation of the eye causes perspective-dependent image shifts. This happens because the optical center and the rotation center of the eye are not the same.<ref name="Konrad Angelopoulos Wetzstein 2020 pp. 1–12">{{cite journal | last1=Konrad | first1=Robert | last2=Angelopoulos | first2=Anastasios | last3=Wetzstein | first3=Gordon | title=Gaze-Contingent Ocular Parallax Rendering for Virtual Reality | journal=ACM Transactions on Graphics | volume=39 | issue=2 | date=2020-04-30 | issn=0730-0301 | doi=10.1145/3361330 | pages=1–12| arxiv=1906.09740 }}</ref> Ocular parallax does not require head movement. It is separate and distinct from motion parallax.