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As a result, the number of depth planes must be smaller than the pattern width. 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. 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. Higher quality CRT displays also have better linearity and exhibit less or none of this effect. 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 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.
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. The brain uses coordinate shift also known as parallax of matched objects to identify depth of these objects.
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 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. 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. 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.
The imaginary object also appears bigger than the patterns on the autostereogram because of 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. 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.
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. 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 pupil , 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.
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. Although the lens adjusts reflexively in order to produce clear, focused images, voluntary control over this process is possible.
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. 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. 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.
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.
From Wikipedia, the free encyclopedia. Aligned vergence and accommodation. How one usually views objects. 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 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. Wall-eyed viewing is informally known as parallel-viewing. However, there may be some incoherence due to overlapping an object originally intended to project in front of another object will now project behind it. For example, the black lines in File: See depth of field for relationship between aperture and lens. Visual Cognition , p.
Royal Society of London. Retrieved 6 April — via Google Books. The University of Chicago Press. Interview with Bela Julesz. Seiji Horibuchi and Yuki Inonue. Trajectories, Strategies, and Myths. Journal of Visual Communication and Image Representation, International Society for Optics and Photonics, Clinical and Experimental Optometry. The term stereogram is used as a synonym of stereo pair, autostereogram, and random dot autostereogram throughout the book.
Geometry, Perspective Drawing, and Mechanisms , p. This means that quotes in a heredoc do not need to be escaped, but the escape codes listed above can still be used. Variables are expanded, but the same care must be taken when expressing complex variables inside a heredoc as with string s. It is also possible to use the Heredoc syntax to pass data to function arguments:.
As of PHP 5. Starting with PHP 5. Nowdocs are to single-quoted strings what heredocs are to double-quoted strings. A nowdoc is specified similarly to a heredoc, but no parsing is done inside a nowdoc. The construct is ideal for embedding PHP code or other large blocks of text without the need for escaping.
All the rules for heredoc identifiers also apply to nowdoc identifiers, especially those regarding the appearance of the closing identifier. Nowdoc support was added in PHP 5.
When a string is specified in double quotes or with heredoc, variables are parsed within it. There are two types of syntax: The simple syntax is the most common and convenient. It provides a way to embed a variable, an array value, or an object property in a string with a minimum of effort. The complex syntax can be recognised by the curly braces surrounding the expression.
Enclose the variable name in curly braces to explicitly specify the end of the name. Similarly, an array index or an object property can be parsed. With array indices, the closing square bracket ] marks the end of the index.
The same rules apply to object properties as to simple variables. This isn't called complex because the syntax is complex, but because it allows for the use of complex expressions.
Any scalar variable, array element or object property with a string representation can be included via this syntax. Some examples to make it clear:. It is also possible to access class properties using variables within strings using this syntax. However, the value accessed will be interpreted as the name of a variable in the scope in which the string is defined. Think of a string as an array of characters for this purpose. These specify the offset from the end of the string.
Writing to an out of range offset pads the string with spaces. Non-integer types are converted to integer. Only the first character of an assigned string is used. Formerly, it assigned a NULL byte.
Internally, PHP strings are byte arrays. As a result, accessing or modifying a string using array brackets is not multi-byte safe, and should only be done with strings that are in a single-byte encoding such as ISO Formerly, the empty string was silently converted to an array. Previously an offset like "foo" was silently cast to 0. String s may be concatenated using the '.
See String operators for more information. There are a number of useful functions for string manipulation. Finally, see also the character type functions. A value can be converted to a string using the string cast or the strval function. String conversion is automatically done in the scope of an expression where a string is needed. This happens when using the echo or print functions, or when a variable is compared to a string.
The sections on Types and Type Juggling will make the following clearer. See also the settype function. A boolean TRUE value is converted to the string "1". This allows conversion back and forth between boolean and string values. An integer or float is converted to a string representing the number textually including the exponent part for float s. Floating point numbers can be converted using exponential notation 4.
See the setlocale function. Array s are always converted to the string "Array" ; because of this, echo and print can not by themselves show the contents of an array. See below for tips on viewing the entire contents. Resource s are always converted to string s with the structure "Resource id 1" , where 1 is the resource number assigned to the resource by PHP at runtime.
While the exact structure of this string should not be relied on and is subject to change, it will always be unique for a given resource within the lifetime of a script being executed ie a Web request or CLI process and won't be reused.
As stated above, directly converting an array , object , or resource to a string does not provide any useful information about the value beyond its type. Most PHP values can also be converted to string s for permanent storage. This method is called serialization, and is performed by the serialize function. When a string is evaluated in a numeric context, the resulting value and type are determined as follows.
If the string does not contain any of the characters '. In all other cases it will be evaluated as a float. The value is given by the initial portion of the string.
If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 zero. Valid numeric data is an optional sign, followed by one or more digits optionally containing a decimal point , followed by an optional exponent. The exponent is an 'e' or 'E' followed by one or more digits. For more information on this conversion, see the Unix manual page for strtod 3. To test any of the examples in this section, cut and paste the examples and insert the following line to see what's going on:.
Do not expect to get the code of one character by converting it to integer, as is done in C. The string in PHP is implemented as an array of bytes and an integer indicating the length of the buffer. It has no information about how those bytes translate to characters, leaving that task to the programmer.
Functions that return no textual data — for instance, arbitrary data read from a network socket — will still return strings. Given that PHP does not dictate a specific encoding for strings, one might wonder how string literals are encoded. The answer is that string will be encoded in whatever fashion it is encoded in the script file.
However, this does not apply if Zend Multibyte is enabled; in that case, the script may be written in an arbitrary encoding which is explicity declared or is detected and then converted to a certain internal encoding, which is then the encoding that will be used for the string literals.
Note that there are some constraints on the encoding of the script or on the internal encoding, should Zend Multibyte be enabled — this almost always means that this encoding should be a compatible superset of ASCII, such as UTF-8 or ISO Note, however, that state-dependent encodings where the same byte values can be used in initial and non-initial shift states may be problematic.
Of course, in order to be useful, functions that operate on text may have to make some assumptions about how the string is encoded. Ultimately, this means writing correct programs using Unicode depends on carefully avoiding functions that will not work and that most likely will corrupt the data and using instead the functions that do behave correctly, generally from the intl and mbstring extensions.
However, using functions that can handle Unicode encodings is just the beginning. No matter the functions the language provides, it is essential to know the Unicode specification. For instance, a program that assumes there is only uppercase and lowercase is making a wrong assumption. Edit Report a Bug. Strings A string is series of characters, where a character is the same as a byte. On bit builds and in earlier versions, a string can be as large as up to 2GB bytes maximum Syntax A string literal can be specified in four different ways: Double quoted If the string is enclosed in double-quotes " , PHP will interpret the following escape sequences for special characters: Heredoc A third way to delimit string s is the heredoc syntax: Warning It is very important to note that the line with the closing identifier must contain no other characters, except a semicolon ;.
My name is "MyName". I am printing some Foo. Now, I am printing some Bar2. This should print a capital 'A': Nowdoc Nowdocs are to single-quoted strings what heredocs are to double-quoted strings. This should not print a capital 'A': Variable parsing When a string is specified in double quotes or with heredoc, variables are parsed within it.
He drank some apple juice. He drank some juice made of. He drank some juice made of apples. He drank some orange juice. He drank some purple juice. John Smith drank some apple juice. John Smith then said hello to Jane Smith. John Smith's wife greeted Robert Paulsen. Robert Paulsen greeted the two. The character at index -2 is n. Changing the character at index -3 to o gives strong. Complex curly syntax This isn't called complex because the syntax is complex, but because it allows for the use of complex expressions.
Some examples to make it clear: Warning Writing to an out of range offset pads the string with spaces. Warning Internally, PHP strings are byte arrays. Example 12 Differences between PHP 5. Illegal string offset '1. Useful functions and operators String s may be concatenated using the '. Converting to string A value can be converted to a string using the string cast or the strval function.
NULL is always converted to an empty string. String conversion to numbers When a string is evaluated in a numeric context, the resulting value and type are determined as follows. Details of the String Type The string in PHP is implemented as an array of bytes and an integer indicating the length of the buffer. Some functions assume that the string is encoded in some any single-byte encoding, but they do not need to interpret those bytes as specific characters.
This is case of, for instance, substr , strpos , strlen or strcmp. Another way to think of these functions is that operate on memory buffers, i. Other functions are passed the encoding of the string, possibly they also assume a default if no such information is given.