Bi-directional text is text containing text in both text directionalities, both right-to-left (RTL) and left-to-right (LTR). It generally involves text containing different types of alphabets, but may also refer to boustrophedon, which is changing text directionality in each row.
Some writing systems of the world, notably the Arabic 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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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. Right-to-left scripts were introduced through encodings like ISO/IEC 8859-6 and ISO/IEC 8859-8, storing the letters (usually) in writing and reading order. 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.
Possible BiDi-types of a character, to be used by the BiDi algorithm, are:
| Bidirectional character type (Unicode character property Bidi_Class)[1] | |||||
|---|---|---|---|---|---|
| Type[2] | Description | Strong/Weak effect | General scope | Bidi_Control character[3] | |
| L | Left-to-Right | Strong | Most alphabetic and syllabic characters, Han ideographs, non-European or non-Arabic digits, LRM character, ... | U+200E left-to-right mark (LRM) | |
| LRE | Left-to-Right Embedding | Strong | LRE character only | U+202A left-to-right embedding (LRE) | |
| LRO | Left-to-Right Override | Strong | LRO character only | U+202D left-to-right override (LRO) | |
| R | Right-to-Left | Strong | Hebrew alphabet and related punctuation, RLM character | U+200F right-to-left mark (RLM) | |
| AL | Right-to-Left Arabic | Strong | Arabic, Thaana and Syriac alphabets, and most punctuation specific to those scripts | ||
| RLE | Right-to-Left Embedding | Strong | RLE character only | U+202B right-to-left embedding (RLE) | |
| RLO | Right-to-Left Override | Strong | RLO character only | U+202E right-to-left override (RLO) | |
| Pop Directional Format | Weak | PDF character only | U+202C pop directional formatting (PDF) | ||
| EN | European Number | Weak | European digits, Eastern Arabic-Indic digits, ... | ||
| ES | European Separator | Weak | plus sign, minus sign, ... | ||
| ET | European Number Terminator | Weak | degree sign, currency symbols, ... | ||
| AN | Arabic Number | Weak | Arabic-Indic digits, Arabic decimal and thousands separators, ... | ||
| CS | Common Number Separator | Weak | colon, comma, full stop, no-break space, ... | ||
| NSM | Nonspacing Mark | Weak | Characters in General Categories Mark, nonspacing and Mark, enclosing (Mn, Me) | ||
| BN | Boundary Neutral | Weak | Default ignorables, non-characters, control characters other than those explicitly given other types | ||
| B | Paragraph Separator | Neutral | paragraph separator, appropriate Newline Functions, higher-level protocol paragraph determination | ||
| S | Segment Separator | Neutral | Tab | ||
| WS | Whitespace | Neutral | space, figure space, line separator, form feed, General Punctuation block spaces | This set is smaller than Unicode whitespace list | |
| ON | Other Neutrals | Neutral | All other characters, including object replacement character | ||
Notes
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There are very few scripts that can be written in either direction.
Writing a boustrophedon requires every second line to use mirrored glyphs.
Egyptian hieroglyphs can be written bi-directional too, 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.
Another variety of writing style, called boustrophedon, was used in some ancient Greek inscriptions, Tuareg, and Hungarian runes. This method of writing alternates direction, and usually reverses the individual characters, on each successive line.