Editing Autostereogram (section)

==How they work==

===Simple wallpaper===
[[Image:Stereogram Tut Clean.png|thumb|350px|This is an example of a wallpaper with repeated horizontal patterns. Each pattern is repeated exactly every 140 pixels. The illusion of the pictures lying on a flat surface (a plane) further back is created by the brain. Non-repeating patterns such as arrows and words, on the other hand, appear on the plane where this text lies.]]
[[Stereopsis]], or stereo vision, is the visual blending of two similar but not identical [[real image|image]]s into one, with resulting [[visual perception]] of [[solid]]ity and [[depth perception|depth]].<ref>Cassin, B. and Solomon, S. (1990). ''Dictionary of Eye Terminology'', {{page needed|date=October 2012}}. Gainesville, Florida: Triad Publishing Company. {{ISBN|978-0-937404-33-1}}</ref><ref>{{Cite report |url=http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=940788 |title=Computational reconstruction of the mechanisms of human stereopsis |last=Tyler |first=Christopher W. |last2=Barghout |first2=Lauren |date=1994-03-17 |doi=10.1117/12.171143 |pages=52–68 |last3=Kontsevich |first3=Leonid L.|url-access=subscription }}</ref> In the human brain, stereopsis results from complex mechanisms that form a three-dimensional impression by matching each point (or set of points) in one eye's view with the equivalent point (or set of points) in the other eye's view. Using [[binocular disparity]], the brain derives the points' positions in the otherwise inscrutable ''z''-axis (depth).

When the brain is presented with a repeating pattern like [[wallpaper]], it has difficulty matching the two eyes' views accurately. By looking at a [[Horizontal plane|horizontally]] repeating pattern, but converging the two eyes at a point behind the pattern, it is possible to trick the brain into matching one element of the pattern, as seen by the left eye, with another (similar looking) element, beside the first, as seen by the right eye. With the typical [[#wall-eyed-viewing|wall-eyed viewing]], this gives the illusion of a plane bearing the same pattern but located behind the real wall. The distance at which this plane lies behind the wall depends only on the spacing between identical elements.<ref name="kinsman">Andrew A. Kinsman (1992). ''Random Dot Stereograms'', {{page needed|date=October 2012}}. Rochester: Kinsman Physics. {{ISBN|0-9630142-1-8}}.</ref>

Autostereograms use this dependence of depth on spacing to create three-dimensional images. If, over some area of the picture, the pattern is repeated at smaller distances, that area will appear closer than the background plane. If the distance of repeats is longer over some area, then that area will appear more distant (like a hole in the plane).

[[Image:Stereogram Tut Simple.png|thumb|350px|This autostereogram displays patterns on three different planes by repeating the patterns at different spacings. ([[Image:Stereogram guide parallel.png|10px]])]]
People who have never been able to perceive 3D shapes hidden within an autostereogram find it hard to understand remarks such as, "the 3D image will just pop out of the background, after you stare at the picture long enough", or "the 3D objects will just emerge from the background". It helps to illustrate how 3D images "emerge" from the background from a second viewer's perspective. If the virtual 3D objects reconstructed by the autostereogram viewer's brain were real objects, a second viewer observing the scene from the side would see these objects floating in the air above the background image.

The 3D effects in the example autostereogram are created by repeating the tiger rider icons every 140 [[pixel]]s on the background plane, the shark rider icons every 130 pixels on the second plane, and the tiger icons every 120 pixels on the highest plane. The closer a set of icons are packed horizontally, the higher they are lifted from the background plane. This repeat distance is referred to as the depth or ''z''-axis value of a particular pattern in the autostereogram. The depth value is also known as [[Z-buffer]] value.

{| border="0"
|-----
| align="center" | [[Image:Stereogram Tut Side View.png|thumb|270px|This picture illustrates how 3D shapes from an autostereogram "emerge" from the background plane, when the autostereogram is viewed with proper eye divergence.]]
| align="center" | [[Image:Stereogram Tut Width.png|thumb|270px|Depth or ''z''-axis values are proportional to pixel shifts in the autostereogram.]]
|}
The brain is capable of almost instantly matching hundreds of patterns repeated at different intervals in order to recreate correct depth information for each pattern. An autostereogram may contain some 50 tigers of varying size, repeated at different intervals against a complex, repeated background. Yet, despite the apparent chaotic arrangement of patterns, the brain is able to place every tiger icon at its proper depth.{{POV statement|date=October 2012|1=The article is insulting towards those of us who can't see the damn things}}

{| border="0"
|-----
| align="center" | [[Image:Stereogram Tut Highlight.png|thumb|260px|The brain can place every tiger icon on its proper depth plane. ([[Image:Stereogram guide parallel.png|10px]])]]
| align="center" | [[Image:Stereogram Tut Highlight Side.png|thumb|270px|This image illustrates how an autostereogram is perceived by a viewer]]{{clear}}
|}

===Depth maps===
{| border="0" align="right"
| colspan="5" | [[Image:Stereogram Tut Horizontal.png|thumb|350px|Depth map example autostereogram: Patterns in this autostereogram appear at different depth across each row.]]
|-----
| align="center" | [[Image:Stereogram Tut Horizontal Depthmap.png|thumb|center|200px|Depth map greyscale example autostereogram: The black, gray and white colors in the background represent a depth map showing changes in depth across row.]]
| align="center" | [[Image:Stereogram Tut Horizontal Pattern.png|thumb|left|50px|Pattern image]]
|}
Autostereograms where patterns in a particular row are repeated horizontally with the same spacing can be read either cross-eyed or wall-eyed. In such autostereograms, both types of reading will produce similar depth interpretation, with the exception that the cross-eyed reading reverses the depth (images that once popped out are now pushed in).

However, icons in a row do not need to be arranged at identical intervals. An autostereogram with varying intervals between icons across a row presents these icons at different depth planes to the viewer.  The depth for each icon is computed from the distance between it and its neighbor at the left. These types of autostereograms are designed to be read in only one way, either cross-eyed or wall-eyed. All autostereograms in this article are encoded for wall-eyed viewing, unless specifically marked otherwise. An autostereogram encoded for wall-eyed viewing will produce inverse patterns when viewed cross-eyed, and vice versa.{{efn|name=Vice versa}}

The wall-eyed depth map example autostereogram to the right encodes 3 planes across the ''x''-axis. The background plane is on the left side of the picture. The highest plane is shown on the right side of the picture. There is a narrow middle plane in the middle of the ''x''-axis. Starting with a background plane where icons are spaced at 140 pixels, one can raise a particular icon by shifting it a certain number of pixels to the left. For instance, the middle plane is created by shifting an icon 10 pixels to the left, effectively creating a spacing consisting of 130 pixels. The brain does not rely on intelligible icons which represent objects or concepts. In this autostereogram, patterns become smaller and smaller down the ''y''-axis, until they look like random dots. The brain is still able to match these random dot patterns.

The distance relationship between any pixel and its counterpart in the equivalent pattern to the left can be expressed in a ''depth map''. A depth map is simply a [[grayscale]] image which represents the distance between a pixel and its left counterpart using a grayscale value between black and white.<ref name="tyler_cadence"/> By convention, the closer the distance is, the brighter the color becomes.

Using this convention, a grayscale [[depth map]] for the example autostereogram can be created with black, gray and white representing shifts of 0 pixels, 10 pixels and 20 pixels, respectively as shown in the greyscale example autostereogram. A depth map is the key to creation of random-dot autostereograms.

===Random-dot===
{{See also|Random dot stereogram}}
{| border="0" align="right"
|-----
| align="center" | [[Image:Stereogram Tut Rectangles Depthmap.png|thumb|200px|Depth map]]
| align="center" | [[Image:Stereogram Tut Rectangles Pattern.png|thumb|35px|Pattern]]
|}
A computer program can take a depth map and an accompanying pattern image to produce an autostereogram. The program tiles the pattern image horizontally to cover an area whose size is identical to the depth map. Conceptually, at every pixel in the output image, the program looks up the grayscale value of the equivalent pixel in the depth map image, and uses this value to determine the amount of horizontal shift required for the pixel.

One way to accomplish this is to make the program scan every line in the output image pixel-by-pixel from left to right. It seeds the first series of pixels in a row from the pattern image. Then it consults the depth map to retrieve appropriate shift values for subsequent pixels. For every pixel, it subtracts the shift from the width of the pattern image to arrive at a repeat interval. It uses this repeat interval to look up the color of the counterpart pixel to the left and uses its color as the new pixel's own color.<ref name="kinsman"/>

{| border="0"
|-----
| align="center" | [[Image:Stereogram Tut Rectangles.png|thumb|250px|Three raised rectangles appear on different depth planes in this autostereogram. ([[Image:Stereogram guide parallel.png|10px]])]]
| align="center" | [[Image:Stereogram Tut Pixel Shift.png|thumb|250px|Every pixel in an autostereogram obeys the distance interval specified by the depth map.]]
|}
Unlike the simple depth planes created by simple wallpaper autostereograms, subtle changes in spacing specified by the depth map can create the illusion of smooth [[gradient]]s in distance. This is possible because the grayscale depth map allows individual pixels to be placed on one of 2<sup>''n''</sup> depth planes, where ''n'' is the number of bits used by each pixel in the depth map. In practice, the total number of depth planes is determined by the number of pixels used for the width of the pattern image. Each grayscale value must be translated into pixel space in order to shift pixels in the final autostereogram. As a result, the number of depth planes must be smaller than the pattern width.

[[Image:Stereogram Tut Random Dot Shark.png|thumb|400px|This random dot autostereogram features a raised shark with fine gradient on a flat background. ([[Image:Stereogram guide parallel.png|10px]])]]
[[File:Stereogram of shark revealed.gif|thumb|400px|The revelation of the shark.]]
The fine-tuned gradient requires a pattern image more complex than standard repeating-pattern wallpaper, so typically a pattern consisting of repeated random dots is used. When the autostereogram is viewed with proper viewing technique, a hidden 3D scene emerges. Autostereograms of this form are known as Random Dot Autostereograms.

Smooth gradients can also be achieved with an intelligible pattern, assuming that the pattern is complex enough and does not have big, horizontal, monotonic patches. A big area painted with monotonic color without change in [[hue]] and [[brightness]] does not lend itself to pixel shifting, as the result of the horizontal shift is identical to the original patch. The following [[depth map]] of a shark with smooth gradient produces a perfectly readable autostereogram, even though the 2D image contains small monotonic areas; the brain is able to recognize these small gaps and fill in the blanks ([[illusory contours]]). While intelligible, repeated patterns are used instead of random dots, this type of autostereogram is still known by many as a Random Dot Autostereogram, because it is created using the same process.
{| border="0"
|-----
| align="center" | [[Image:Stereogram Tut Shark Depthmap.png|left|thumb|250px|The shark figure in this depth map is drawn with a smooth gradient.]]
| align="center" | [[Image:Stereogram Tut Shark.png|thumb|left|250px|The 3D shark in this random-dot autostereogram has a smooth, round shape due to the use of depth map with smooth gradient. ([[Image:Stereogram guide parallel.png|10px]])]]
|}

===Animated===
[[Image:Stereogram Tut Animated Shark Small.gif|thumb|160px|Animated autostereogram. ([[Image:Stereogram guide parallel.png|10px]]) [[:Image:Stereogram Tut Animated Shark.gif|Click here for the 800 × 400 version]] ]]
When a series of autostereograms are shown one after another, in the same way [[film|moving pictures]] are shown, the brain perceives an [[animation|animated]] autostereogram. If all autostereograms in the animation are produced using the same background pattern, it is often possible to see faint outlines of parts of the moving 3D object in the 2D autostereogram image without wall-eyed viewing; the constantly shifting pixels of the moving object can be clearly distinguished from the static background plane. To eliminate this side effect, animated autostereograms often use shifting background in order to disguise the moving parts.

When a regular repeating pattern is viewed on a [[CRT monitor]] as if it were a wallpaper autostereogram, it is usually possible to see depth ripples. This can also be seen in the background to a static, random-dot autostereogram. These are caused by the sideways shifts in the image due to small changes in the deflection sensitivity (linearity) of the line scan, which then become interpreted as depth. This effect is especially apparent at the left hand edge of the screen where the scan speed is still settling after the flyback phase. On a [[TFT LCD]], which functions differently, this does not occur and the effect is not present. Higher quality CRT displays also have better linearity and exhibit less or none of this effect.