fresnel bi prism

Bi-directional text

Bi-directional text is used as some writing systems of the world, notably the Arabic (including variants such as Nasta'liq) 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 should be 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) should be 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.

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, especially in signs (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.

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 languages. 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. To correct or prevent these errors, you can use "pseudo-strong" characters. These Unicode control characters are called "marks". The mark (U+200E LTR or U+200F RTL) is to be inserted into a location to make an enclosed weak character inherit its writing direction.

For example, to have the trademark symbol ™ (TM; U+8482) for an English name brand (LTR) in an Arabic (RTL) passage display correctly, you need to add an LTR mark after the trademark symbol if the symbol is not followed by LTR text. This is because if you do not add the LTR mark, the weak character ™ will be neighboured 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.

Combining Cyrillic Millions

The glyph    ҉, or "Combining Cyrillic Millions", is a Cyrillic numeral representing a modifier of one million. It has gained popularity as an Internet meme; it is supposedly a character that is responsible for creating "backwards text" that applies to every character succeeding the symbol on the same line. However, the symbol itself has no special properties. Rather, it is simply the Combining Cyrillic Millions symbol surrounded by multiple style modifiers, including ones which override the left-to-right flow of text.

When the symbol is copied and pasted, the style formatting is quietly copied along with it. This has caused confusion, leading many users of the symbol to believe the backwards text is a property of the symbol itself rather than the Unicode that accompanies it.

The effect of the current iteration of the symbol can be seen as follows (although any symbol could actually be substituted). ‪‫‬‭

‬‬‫‬‭‮‪‫‬‭‮҉Backwards Text.‬

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