Bi-directional text
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Some writing systems of the world, notably the Arabic (including variants such as Nasta'liq), Persian and Hebrew scripts, are written in a form known as right-to-left (RTL), in which writing begins at the right-hand side of a page and concludes at the left-hand side. This is different from the left-to-right (LTR) direction used by most languages in the world. When LTR text is mixed with RTL in the same paragraph, each type of text is written in its own direction, which is known as bi-directional text. This can get rather complex when multiple levels of quotation are used.
Many computer programs fail to display bi-directional text correctly. For example, the Hebrew name Sarah (שרה) is spelled shin (ש) resh (ר) heh (ה) from right to left. Some Web browsers may display the Hebrew text in this article in the opposite direction.
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[edit] Languages using bi-directional text
There are very few scripts that can be written in either direction.Such was the case with Egyptian hieroglyphics, where the signs had a distinct "head" that faced the beginning of a line and "tail" that faced the end.
Chinese characters can also be written in either direction as well as vertically (top to bottom then right to left), especially in signs (such as plaques, but the orientation of the individual characters is never changed). This can often be seen on tour buses in China, where the company name customarily runs from the front of the vehicle to its rear - that is, from right to left on the right side of the bus, and from left to right on the left side of the bus.
 The right side (text runs from right to left) |  The left side (text runs from left to right) |  The right side of a China Southern Airlines aircraft. The text runs from right to left ( 空 航 方 南 国 中 ). |  The left side shows the text running from left to right ( 中 国 南 方 航 空 ). |
 A photo that shows text on both sides of a China Post vehicle (thanks to the open door) |
[edit] Unicode support
Bidirectional script support is the capability of a computer system to correctly display bi-directional text. The term is often shortened to the jargon term BiDi or bidi.Early computer installations were designed only to support a single writing system, typically for left-to-right scripts based on the Latin alphabet only. Adding new character sets and character encodings enabled a number of other left-to-right scripts to be supported, but did not easily support right-to-left scripts such as Arabic or Hebrew, and mixing the two was not practical. It is possible to simply flip the left-to-right display order to a right-to-left display order, but doing this sacrifices the ability to correctly display left-to-right scripts. With bidirectional script support, it is possible to mix scripts from different scripts on the same page, regardless of writing direction.
In particular, the Unicode standard provides foundations for complete BiDi support, with detailed rules as to how mixtures of left-to-right and right-to-left scripts are to be encoded and displayed.
In Unicode encoding, all non-punctuation characters are stored in writing order. This means that the writing direction of characters is stored within the characters. If this is the case, the character is called "strong". Punctuation characters however, can appear in both LTR and RTL scripts. They are called "weak" characters because they do not contain any directional information. So it is up to the software to decide in which direction these "weak" characters will be placed. Sometimes (in mixed-directions text) this leads to display errors, caused by the bidi-algorithm that runs through the text and identifies LTR and RTL strong characters and assigns a direction to weak characters, according to the algorithm's rules.
In the algorithm, each sequence of concatenated strong characters is called a "run". A weak character that is located between two strong characters with the same orientation will inherit their orientation. A weak character that is located between two strong characters with a different writing direction, will inherit the main context's writing direction (in an LTR document the character will become LTR, in an RTL document, it will become RTL). If a "weak" character is followed by another "weak" character, the algorithm will look at the first neighbouring "strong" character. Sometimes this leads to unintentional display errors. These errors are corrected or prevented with "pseudo-strong" characters. Such Unicode control characters are called marks. The mark (U+200E left-to-right mark (HTML: LRM) or U+200F right-to-left mark (HTML: RLM)) is to be inserted into a location to make an enclosed weak character inherit its writing direction.
For example, to correctly display the U+2122 ™ trade mark sign for an English name brand (LTR) in an Arabic (RTL) passage, an LRM mark is inserted after the trademark symbol if the symbol is not followed by LTR text. If the LRM mark is not added, the weak character ™ will be neighbored by a strong LTR character and a strong RTL character. Hence, in an RTL context, it will be considered to be RTL, and displayed in an incorrect order.
[edit] See also
- Internationalization and localization
- Horizontal and vertical writing in East Asian scripts
- Writing system#Directionality (section on directionality)
- Combining Cyrillic Millions
- Transformation of text
[edit] References
[edit] External links
- Unicode Standards Annex #9 The Bidirectional Algorithm
- W3C guidelines on authoring techniques for bi-directional text - includes examples and good explanations
- GNU FriBidi A free implementation of the Unicode bidirectional algorithm
- ICU International Components for Unicode contains an implementation of the bidirectional algorithm — along with other internationalization services
- UCData: "Pretty Good Bidi Algorithm Library" A small and fast bidirectional reordering algorithm that works pretty good, but not necessarily compliant to the Unicode algorithm
- Bidirectional Scripts in Desktop Software Working group for supporting BiDi in Free Software. Contains several links to readings and implementation regarding BiDi in computer systems.
- Another Wiki about BiDi
- Bidirectional text - Examples and practical advice
- .Net BiDi Implementation
- A freely available rather final version of Israeli standard 5194 - bidirectional text editing
- Work in progress on new version of Bidi editing standard + reference implementation
- Series of articles about pitfalls of BiDi programming
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