Editing Autostereogram (section)

==Mechanisms for viewing==
{{see also|Stereoscopy#Visual requirements}}
Much advice exists about seeing the intended three-dimensional image in an autostereogram. While some people may quickly see the 3D image in an autostereogram with little effort, others must learn to train their eyes to decouple eye convergence from lens focusing.

Not every person can see the 3D [[Optical illusion|illusion]] in autostereograms. Because autostereograms are constructed based on [[stereo vision]], persons with a variety of visual impairments, even those affecting only one eye, are unable to see the three-dimensional images.

People with [[amblyopia]] (also known as lazy eye) are unable to see the three-dimensional images. Children with poor or dysfunctional eyesight during a critical period in childhood may grow up [[stereoblind]], as their brains are not stimulated by stereo images during the critical period. If such a vision problem is not corrected in early childhood, the damage becomes permanent and the adult will never be able to see autostereograms.<ref name="pinker"/>{{efn|It is generally thought that amblyopia is a permanent condition, but NPR reports a case where a patient with amblyopia regains stereo vision ([[Susan R. Barry]]).<ref>{{Cite web |first=Robert |last=Krulwich |date=2006-06-26 |title=Krulwich on Science: Going Binocular: Susan's First Snowfall |website=[[NPR]] |url=http://www.npr.org/templates/story/story.php?storyId=5536894 |archive-url=https://web.archive.org/web/20060714053856/http://www.npr.org/templates/story/story.php?storyId=5536894 |archive-date=2006-07-14}}</ref>}} It is estimated that some 1 percent to 5 percent of the population is affected by amblyopia.<ref>{{cite journal |title=Amblyopia - prevalence, natural history, functional effects and treatment |first1=Ann |last1=Webber |first2=Joanne |last2=Wood |journal=Clinical and Experimental Optometry |date=November 2005 |volume=88 |issue=6 |pages=365–375 |doi=10.1111/j.1444-0938.2005.tb05102.x |pmid=16329744 |s2cid=39141527 |doi-access=free }}</ref>

===3D perception===
[[Depth perception]] results from many monocular and binocular visual clues. For objects relatively close to the eyes, [[binocular vision]] plays an important role in depth perception. Binocular vision allows the brain to create a single [[Cyclopean image]] and to attach a depth level to each point in it.<ref name="julesz"/>

{| align="center" border="0"
|-----
| [[Image:Stereogram Tut Eye Diagram.png|thumb|left|170px|The two eyes converge on the object of attention.]]
| [[Image:Stereogram Tut Eye View Composite.png|thumb|left|170px|The brain creates a Cyclopean image from the two images received by the two eyes.]]
| [[Image:Stereogram Tut Eye View Depthmap.png|thumb|left|170px|The brain gives each point in the Cyclopean image a depth value, represented here by a grayscale depth map.]]
|}
The brain uses coordinate shift (also known as parallax) of matched objects to identify depth of these objects.<ref name="kinsman"/> The depth level of each point in the combined image can be represented by a grayscale pixel on a 2D image, for the benefit of the reader. The closer a point appears to the brain, the brighter it is painted. Thus, the way the brain [[Depth perception|perceives depth]] using [[binocular vision]] can be captured by a depth map (Cyclopean image) painted based on coordinate shift.
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[[Image:Stereogram Tut Eye Focus.png|thumb|left|150px|The eye adjusts its internal lens to get a clear, focused image]]
[[Image:Stereogram Tut Eye Convergence.png|thumb|right|150px|The two eyes converge to point to the same object]]
The eye operates like a photographic camera. It has an adjustable [[iris (anatomy)|iris]] which can open (or close) to allow more (or less) light to enter the eye. As with any camera except [[pinhole camera]]s, it needs to [[Accommodation (eye)|focus]] light rays entering through the iris (aperture in a camera) so that they focus on a single point on the retina in order to produce a sharp image. The eye achieves this goal by adjusting a lens behind the cornea to refract light appropriately.

<div id="wall-eyed-viewing">When a person stares at an object, the two eyeballs rotate sideways to point to the object, so that the object appears at the center of the image formed on each eye's retina. In order to look at a nearby object, the two eyeballs rotate towards each other so that their eyesight can [[convergence (eye)|converge]] on the object. This is referred to as ''cross-eyed viewing''. To see a faraway object, the two eyeballs ''diverge'' to become almost parallel to each other. This is known as ''wall-eyed viewing'', where the convergence angle is much smaller than that in cross-eyed viewing.{{efn|name=strabismus|The terms "cross-eyed" and "wall-eyed" are borrowed from synonyms for various forms of [[strabismus]], a condition where eyes do not point in the same direction when looking at an object. Wall-eyed viewing is informally known as parallel-viewing.}}</div>

Stereo-vision based on parallax allows the brain to calculate depths of objects relative to the point of convergence. It is the convergence angle that gives the brain the absolute reference depth value for the point of convergence from which absolute depths of all other objects can be inferred.
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===Simulated 3D perception===
[[Image:Stereogram Tut Eye Trick.png|thumb|270px|Decoupling focus from convergence tricks the brain into seeing 3D images in a 2D autostereogram]]
The eyes normally focus and converge at the same distance in a process known as [[accommodative convergence]]. That is, when looking at a faraway object, the brain automatically flattens the lenses and rotates the two eyeballs for wall-eyed viewing. It is possible to train the brain to decouple these two operations. This decoupling has no useful purpose in everyday life, because it prevents the brain from interpreting objects in a coherent manner. To see a human-made picture such as an autostereogram where patterns are repeated horizontally, however, decoupling of focusing from convergence is crucial.<ref name="pinker"/>

By focusing the lenses on a nearby autostereogram where patterns are repeated and by converging the eyeballs at a distant point behind the autostereogram image, one can trick the brain into seeing 3D images. If the patterns received by the two eyes are similar enough, the brain will consider these two patterns a match and treat them as coming from the same imaginary object. This type of visualization is known as ''wall-eyed viewing'', because the eyeballs adopt a wall-eyed convergence on a distant plane, even though the autostereogram image is actually closer to the eyes.<ref name="kinsman"/> Because the two eyeballs converge on a plane farther away, the perceived location of the imaginary object is behind the autostereogram. The imaginary object also appears bigger than the patterns on the autostereogram because of [[Perspective (graphical)#Foreshortening|foreshortening]].

The following autostereogram shows three rows of repeated patterns. Each pattern is repeated at a different interval to place it on a different depth plane. The two non-repeating lines can be used to verify correct wall-eyed viewing. When the autostereogram is correctly interpreted by the brain using wall-eyed viewing, and one stares at the dolphin in the middle of the visual field, the brain should see two sets of flickering lines, as a result of [[binocular rivalry]].<ref name="julesz"/>

{| align="center"
|-----
| [[Image:Stereogram Tut Eye Trick Stereogram.png|thumb|left|200px|The two black lines in this Autostereogram help viewers establish proper wall-eyed viewing, see right.]]
| [[Image:Stereogram Tut Eye Trick Composite Dolphin.png|thumb|left|300px|When the brain manages to establish proper wall-eyed viewing, it will see two sets of lines.]]
|}
[[Image:Stereogram Tut Eye Object Size.png|thumb|180px|Top-row cubes appear farther away and bigger. {{Stereogram|wall}}]]
While there are six dolphin patterns in the autostereogram, the brain should see seven "apparent" dolphins on the plane of the autostereogram. This is a side effect of the pairing of similar patterns by the brain. There are five pairs of dolphin patterns in this image. This allows the brain to create five apparent dolphins. The leftmost pattern and the rightmost pattern by themselves have no partner, but the brain tries to assimilate these two patterns onto the established depth plane of adjacent dolphins despite binocular rivalry. As a result, there are seven apparent dolphins, with the leftmost and the rightmost ones appearing with a slight flicker, not dissimilar to the two sets of flickering lines observed when one stares at the 4th apparent dolphin.

Because of foreshortening, the difference in convergence needed to see repeated patterns on different planes causes the brain to attribute different sizes to patterns with identical 2D sizes. In the autostereogram of three rows of cubes, while all cubes have the same physical 2D dimensions, the ones on the top row appear bigger, because they are perceived as farther away than the cubes on the second and third rows.

===Viewing techniques===
[[File:Mariposas autoestereoscópicas.JPG|thumb|Butterflies, cross-eyed autostereogram ([[File:Stereogram guide cross-eyed.svg|10px]])]]

If one has two eyes, fairly healthy eyesight, and no neurological conditions which prevent the perception of depth, then one is capable of learning to see the images within autostereograms. "Like learning to ride a bicycle or to swim, some pick it up immediately, while others have a harder time."<ref>Kosslyn and Osherson (1995), p. 64.</ref>

As with a [[Camera|photographic camera]], it is easier to make the eye focus on an object when there is intense ambient light. With intense lighting, the eye can constrict the [[pupil]], yet allow enough light to reach the retina. The more the eye resembles a [[pinhole camera]], the less it depends on [[Accommodation (vertebrate eye)|focusing]] through the [[lens (vertebrate anatomy)|lens]].{{efn|See [[aperture]] on similarity between aperture and pupil. See [[depth of field]] for relationship between aperture and lens.}} In other words, the degree of decoupling between focusing and convergence needed to visualize an autostereogram is reduced. This places less strain on the brain. Therefore, it may be easier for first-time autostereogram viewers to "see" their first 3D images if they attempt this feat with bright lighting.

[[Vergence]]<!--"vergence" is a real word spelled correctly--> control is important in being able to see 3D images. Thus it may help to concentrate on converging/diverging the two eyes to shift images that reach the two eyes, instead of trying to see a clear, focused image. Although the [[accommodation reflex|lens adjusts reflexively]] in order to produce clear, focused images, voluntary control over this process is possible.<ref>{{cite journal |first1=Leon N. Jr |last1=McLin |first2=Clifton M. |last2=Schor |title=Voluntary effort as a stimulus to accommodation and vergence |journal=Investigative Ophthalmology & Visual Science |date=1988 |volume=29 |number=11 |pages=1739–1746 |pmid=3182206}}</ref> The viewer alternates instead between converging and diverging the two eyes, in the process seeing "double images" typically seen when one is [[drunkenness|drunk]] or otherwise intoxicated. Eventually the brain will successfully match a pair of patterns reported by the two eyes and lock onto this particular degree of convergence. The brain will also adjust eye lenses to get a clear image of the matched pair. Once this is done, the images around the matched patterns quickly become clear as the brain matches additional patterns using roughly the same degree of convergence.

[[Image:Chess Single Image Stereogram by 3Dimka.jpg|thumb|400px|A type of wallpaper autostereogram featuring 3D objects instead of flat patterns ([[Image:Stereogram guide parallel.png|10px]])]]
[[Image:Stereogram Tut Shark Bottom Clear.png|thumb|400px|The bottom part of this autostereogram is free of 3D images. It is easier to trick the brain into matching pairs of patterns in this area. ([[Image:Stereogram guide parallel.png|10px]])]]
When one moves one's attention from one depth plane to another (for instance, from the top row of the chessboard to the bottom row), the two eyes need to adjust their convergence to match the new repeating interval of patterns. If the level of change in convergence is too high during this shift, sometimes the brain can lose the hard-earned decoupling between focusing and convergence. For a first-time viewer, therefore, it may be easier to see the autostereogram, if the two eyes rehearse the convergence exercise on an autostereogram where the depth of patterns across a particular row remains constant.

In a random dot autostereogram, the 3D image is usually shown in the middle of the autostereogram against a background depth plane (see the shark autostereogram). It may help to establish proper convergence first by staring at either the top or the bottom of the autostereogram, where patterns are usually repeated at a constant interval. Once the brain locks onto the background depth plane, it has a reference convergence degree from which it can then match patterns at different depth levels in the middle of the image.

The majority of autostereograms, including those in this article, are designed for divergent (wall-eyed) viewing. One way to help the brain concentrate on divergence instead of focusing is to hold the picture in front of the face, with the nose touching the picture. With the picture so close to their eyes, most people cannot focus on the picture. The brain may give up trying to move eye muscles in order to get a clear picture. If one slowly pulls back the picture away from the face, while refraining from focusing or rotating eyes, at some point the brain will lock onto a pair of patterns when the distance between them matches the current convergence degree of the two eyeballs.<ref name="beyond3d"/>

Another way is to stare at an object behind the picture in an attempt to establish proper divergence, while keeping part of the eyesight fixed on the picture to convince the brain to focus on the picture. A modified method has the viewer focus on their reflection on a reflective surface of the picture, which the brain perceives as being located twice as far away as the picture itself. This may help persuade the brain to adopt the required divergence while focusing on the nearby picture.
[[File:Sphere Cube Triangle 3D Stereogram Illusion.png|alt=Sphere Cube Triangle 3D Stereogram Illusion|thumb|400x400px|Mapped Textured Stereogram (MTS) where a textural image is mapped onto a depth-map rather than mapping a random pattern.]]
Those who wear eyeglasses with so-called "progressive" lenses, in which the focal length gradually changes so as to ease the viewing of close objects using the lower part of the lens, may find that viewing a stereogram is easier if the glasses are pushed up a little so that the stereogram is viewed through a part of the lens optimized for images that are closer than the actual distance to the stereogram.  When the eyes are made divergent by looking (or pretending to look) at a far-away object, the overcorrection from viewing the stereogram through the "wrong" part of the lens can bring the stereogram into focus without needing to overcome the tendency to focus on the far-away object while attempting to focus on the stereogram. 

For crossed-eyed autostereograms, a different approach needs to be taken. The viewer may hold one finger between their eyes and move it slowly towards the picture, maintaining focus on the finger at all times, until they are correctly focused on the spot that will allow them to view the illusion.

[[Stereoblindness]], however, is not known to permit the usages of any of these techniques, especially for persons in whom it may be, or is, permanent.