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Autostereogram

2007 Schools Wikipedia Selection. Related subjects: Health and medicine

   A random dot autostereogram encodes a 3D scene which can be "seen" with
   proper viewing technique. Click on thumbnail to see full-size image.
   Enlarge
   A random dot autostereogram encodes a 3D scene which can be "seen" with
   proper viewing technique. Click on thumbnail to see full-size image.

   An autostereogram is a single-image stereogram (SIS), designed to trick
   the human brain into perceiving a three- dimensional (3D) scene in a
   two-dimensional image. In order to perceive 3D shapes in these
   autostereograms, the brain must overcome the normally automatic
   coordination between focusing and convergence.

   The simplest type of autostereogram consists of horizontally repeating
   patterns and is known as a wallpaper autostereogram. When viewed with
   proper convergence, the repeating patterns appear to float in the air
   above the background. The Magic Eye series of books features another
   type of autostereogram called a random dot autostereogram. In this type
   of autostereogram, every pixel in the image is computed from a pattern
   strip and a depth map. Usually, a hidden 3D scene emerges when the
   image is viewed with proper viewing technique.

   There are two ways an autostereogram can be viewed: wall-eyed and
   cross-eyed. Most autostereograms are designed to be viewed in only one
   way, which is usually wall-eyed. Wall-eyed viewing requires that the
   two eyes adopt a relatively divergent angle, while cross-eyed viewing
   requires a relatively convergent angle.

History

   In 1838, the British scientist Charles Wheatstone published an
   explanation of binocular vision (binocular depth perception) which had
   led him to make stereoscopic drawings and to construct a stereoscope
   based on a combination of mirrors to allow a person to see 3D images
   from two 2D pictures ( stereograms).

   Between 1849 and 1850, David Brewster, a Scottish scientist, improved
   the Wheatstone stereoscope by using lenses instead of mirrors, thus
   reducing the size of the contraption. Brewster noticed that staring at
   repeated patterns in wallpapers could trick the brain into matching
   pairs of them and thus causing the brain to perceive a virtual plane
   behind the walls. This is the basis of wallpaper-style autostereograms
   (also known as single-image stereograms).

   In 1959, Bela Julesz, a vision scientist, psychologist, and MacArthur
   Fellow, discovered the random dot stereogram while working at Bell
   Laboratories on recognizing camouflaged objects from aerial pictures
   taken by spy planes. At the time, many vision scientists still thought
   that depth perception occurred in the eye itself, whereas now it is
   known to be a complex neurological process. Julesz used a computer to
   create a stereo pair of random-dot images which, when viewed under a
   stereoscope, caused the brain to see 3D shapes. This proved that depth
   perception is a neurological process.

   In 1979, Christopher Tyler of Smith-Kettlewell Institute, a student of
   Julesz and a visual psychophysicist, combined the theories behind
   single-image wallpaper stereograms and random-dot stereograms to create
   the first random-dot autostereogram (also known as single-image
   random-dot stereogram) which allowed the brain to see 3D shapes from a
   single 2D image without the aid of optical equipment.

How they work

Simple wallpaper

   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. However, non-repeating patterns such as arrows and words
   appear on the plane where this text lies.
   Enlarge
   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. However, non-repeating patterns such as arrows and words
   appear on the plane where this text lies.

   Stereopsis, or stereo vision, is the visual blending of two similar but
   not identical images into one, with resulting visual perception of
   solidity and depth. In the human brain, stereopsis results from a
   complex set of 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. It
   therefore assesses 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
   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. 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.

   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).
   This autostereogram displays patterns on three different planes by
   repeating the patterns at different spacings.
   Enlarge
   This autostereogram displays patterns on three different planes by
   repeating the patterns at different spacings.

   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 pixels 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.

   This picture illustrates how 3D shapes from an autostereogram "emerge"
   from the background plane, when the autostereogram is viewed with
   proper eye divergence.
   Enlarge
   This picture illustrates how 3D shapes from an autostereogram "emerge"
   from the background plane, when the autostereogram is viewed with
   proper eye divergence.
   Depth or z-axis values are proportional to pixel shifts in the
   autostereogram.
   Enlarge
   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.

   The brain can place every tiger icon on its proper depth plane.
   Enlarge
   The brain can place every tiger icon on its proper depth plane.
   This image illustrates how an autostereogram is perceived by a viewer
   Enlarge
   This image illustrates how an autostereogram is perceived by a viewer

Depth maps

   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).
   Patterns in this autostereogram appear at different depth across each
   row.
   Enlarge
   Patterns in this autostereogram appear at different depth across each
   row.

   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 neighbour 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 incoherent 3D patterns when viewed
   cross-eyed. Most Magic Eye pictures are also designed for wall-eyed
   viewing.

   The following wall-eyed autostereogram 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 black, gray and white colors in the background represent a depth
   map showing changes in depth across row.
   Enlarge
   The black, gray and white colors in the background represent a depth
   map showing changes in depth across row.

   Pattern image
   Enlarge
   Pattern image

   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. By convention, the closer the distance is, the brighter the
   colour becomes.

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

Random-dot

                                                          Depth map
                                                            Enlarge
                                                          Depth map

                                                                   Pattern
                                                                   Enlarge
                                                                   Pattern

   A software 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 colour.

   Three raised rectangles appear on different depth plane in this
   autostereogram.
   Enlarge
   Three raised rectangles appear on different depth plane in this
   autostereogram.
   Every pixel in an autostereogram obeys the distance interval specified
   by the depth map.
   Enlarge
   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 gradients in distance. This is
   possible because the grayscale depth map allows individual pixels to be
   placed on one of 2^n 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.
   This random dot autostereogram features a raised shark with fine
   gradient on a flat background.
   Enlarge
   This random dot autostereogram features a raised shark with fine
   gradient on a flat background.

   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 colour
   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. 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.

   The shark figure in this depth map is drawn with a smooth gradient.
   Enlarge
   The shark figure in this depth map is drawn with a smooth gradient.
   The 3D shark in this random-dot autostereogram has a smooth, round
   shape due to the use of depth map with smooth gradient.
   Enlarge
   The 3D shark in this random-dot autostereogram has a smooth, round
   shape due to the use of depth map with smooth gradient.

Animated

   animated autostereogram. 800 × 400 version
   Enlarge
   animated autostereogram. 800 × 400 version

   When a series of autostereograms are shown one after another, in the
   same way moving pictures are shown, the brain perceives an 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. This
   effect is absent from a TFT LCD.

Mechanisms for viewing

   Much advice exists about seeing the intended three-dimensional image in
   an autostereogram. While some people can simply see the 3D image in an
   autostereogram, others must learn to train their eyes to decouple eye
   convergence from lens focusing.

   Not every person can see the 3D 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 vision problem is not corrected in the early childhood, the
   damage becomes permanent and the adult will never be able to see
   autostereograms. It is estimated that some 1% to 5% of the population
   is affected by amblyopia.

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 the Cyclopean image.

   The two eyes converge on the object of attention.
   Enlarge
   The two eyes converge on the object of attention.

   The brain creates a Cyclopean image from the two images received by the
   two eyes.
   Enlarge
   The brain creates a Cyclopean image from the two images received by the
   two eyes.

   The brain gives each point in the Cyclopean image a depth value,
   represented here by a grayscale depth map.
   Enlarge
   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. 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
   perceives depth using binocular vision can be captured by a depth map
   (Cyclopean image) painted based on coordinate shift.
   The eye adjusts its internal lens to get a clear, focused image
   Enlarge
   The eye adjusts its internal lens to get a clear, focused image
   The two eyes converge to point to the same object
   Enlarge
   The two eyes converge to point to the same object

   The eye operates like a photographic camera. It has an adjustable iris
   which can open (or close) to allow more (or less) light to enter the
   eye. As with any camera except pinhole cameras, it needs to 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.
   When a person stares at an object, the two eyeballs rotate sideways to
   point to the object, so that the object appears at the centre 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
   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 a cross-eyed viewing.

   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.

Simulated 3D perception

   Decoupling focus from convergence tricks the brain into seeing 3D
   images in a 2D autostereogram
   Enlarge
   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 man-made picture such as an
   autostereogram where patterns are repeated horizontally, however,
   decoupling of focusing from convergence is crucial.

   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. 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 foreshortening.

   The following autostereogram shows 3 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.

   The two black lines in this Autostereogram help viewers establish
   proper wall-eyed viewing, see right.
   Enlarge
   The two black lines in this Autostereogram help viewers establish
   proper wall-eyed viewing, see right.

   When the brain manages to establish proper wall-eyed viewing, it will
   see two sets of lines.
   Enlarge
   When the brain manages to establish proper wall-eyed viewing, it will
   see two sets of lines.

   While there are 6 dolphin patterns in the autostereogram, the brain
   should see 7 "apparent" dolphins on the plane of the autostereogram.
   This is a side effect of the pairing of similar patterns by the brain.
   There are 5 pairs of dolphin patterns in this image. This allows the
   brain to create 5 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 7
   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.
   Top-row cubes appear bigger.
   Enlarge
   Top-row cubes appear bigger.

   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 3 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

   As with a 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 iris, yet allow enough light to reach the retina.
   The more the eye resembles a pinhole camera, the less it depends on
   focusing through the 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 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 lens adjusts reflexively in order to
   produce clear, focused images, voluntary control over this process is
   possible. The viewer alternates instead between converging and
   diverging the two eyes, in the process seeing "double images" typically
   seen when one is 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.
   The bottom part of this random dot autostereogram is free of 3D images.
   It is easier to trick the brain into matching pairs of patterns in this
   area.
   Enlarge
   The bottom part of this random dot autostereogram is free of 3D images.
   It is easier to trick the brain into matching pairs of patterns in this
   area.

   When one moves one's attention from a depth plane to another (for
   instance, from the top row to the second row in the cube
   autostereogram), 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 match the
   current convergence degree of the two eyeballs.

   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 stare at her reflection on the shiny
   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.

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

Terminology

     * Stereogram was originally used to describe a pair of 2D images used
       in stereoscope to present a 3D image to viewers. The term is now
       often used interchangeably with autostereogram or random dot
       autostereogram. But Dr. Tyler, inventor of the autostereogram,
       consistently refers to single-image stereograms as autostereograms
       to distinguish them from other forms of stereograms.

     * Random Dot Stereogram (RDS) originally described a pair of 2D
       images showing random dots which, when viewed with a stereoscope,
       produced a 3D image. The term is now often used interchangeably
       with random dot autostereogram.

     * Single Image Stereogram (SIS) is a synonym of autostereogram. SIS
       differs from most stereograms in its use of a single 2D image
       instead of a stereo pair. When the single 2D image is viewed with
       proper eye convergence, it causes the brain to fuse different
       patterns perceived by the two eyes into a virtual 3D image without
       the aid of any optical equipment.

     * Wallpaper autostereogram is a 2D image where patterns are repeated
       at various intervals to raise or lower each pattern's perceived 3D
       location in relation to a virtual background plane.

     * Random-dot autostereogram is also known as Single Image Random Dot
       Stereogram (SIRDS). This term also refers to autostereograms where
       intelligible patterns instead of random dots are used.

     * Single Image Random Text Stereogram (SIRTS) is an alternative to
       SIRDS using random normally ASCII text instead of dots to produce a
       3D form of ASCII art.

   Retrieved from " http://en.wikipedia.org/wiki/Autostereogram"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
