Editing Autostereogram (section)

===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.