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Unicode provides a unique number for every character, no matter what the platform, no matter what the program, no matter what the language.
Incorporating Unicode into client-server or multi-tiered applications and websites offers significant cost savings over the use of legacy character sets. Unicode enables a single software product or a single website to be targeted across multiple platforms, languages and countries without re-engineering. It allows data to be transported through many different systems without corruption.
The Unicode Standard specifies the representation of text in modern software products and standards.
Unicode provides a consistent way of encoding multilingual plain text and brings order to a chaotic state of affairs that has made it difficult to exchange text files internationally. Computer users who deal with multilingual text -- business people, linguists, researchers, scientists, and others -- will find that the Unicode Standard greatly simplifies their work. Mathematicians and technicians, who regularly use mathematical symbols and other technical characters, will also find the Unicode Standard valuable.Top
The design of Unicode is based on the simplicity and consistency of ASCII, but goes far beyond ASCII's limited ability to encode only the Latin alphabet. The Unicode Standard provides the capacity to encode all of the characters used for the written languages of the world. To keep character coding simple and efficient, the Unicode Standard assigns each character a unique numeric value and name.
The Unicode Standard and ISO/IEC 10646 support three encoding forms that use a common repertoire of characters. These encoding forms allow for encoding as many as a million characters. This is sufficient for all known character encoding requirements, including full coverage of all historic scripts of the world, as well as common notational systems.
The Unicode Standard does not define glyph images. The standard defines how characters are interpreted, not how glyphs are rendered. The software or hardware-rendering engine of a computer is responsible for the appearance of the characters on the screen. The Unicode Standard does not specify the size, shape, nor style of on-screen characters.Top
UTF-8 support has improved dramatically over the last few years and many people now use UTF-8 on a daily basis in:
and in any other places where byte sequences used to be interpreted in ASCII.Top
In UTF-8 mode, terminal emulators such as xterm or the Linux console driver transform every keystroke into the corresponding UTF-8 sequence and send it to the stdin of the foreground process. Similarly, any output of a process on stdout is sent to the terminal emulator, where it is processed with a UTF-8 decoder and then displayed using a 16-bit font.Top
Full Unicode functionality with all bells and whistles (e.g. high-quality typesetting of the Arabic and Indic scripts) can only be expected from sophisticated multi-lingual word-processing packages. What Linux supports today on a broad base is far simpler and mainly aimed at replacing the old 8- and 16-bit character sets. Linux terminal emulators and command line tools usually only support a Level 1 implementation of ISO 10646-1 (no combining characters), and only scripts such as Latin, Greek, Cyrillic, Armenian, Georgian, CJK, and many scientific symbols are supported that need no further processing support. At this level, UCS support is very comparable to ISO 8859 support and the only significant difference is that we have now thousands of different characters available, that characters can be represented by multibyte sequences, and that ideographic Chinese/Japanese/Korean characters require two terminal character positions (double-width).Top
Level 2 support in the form of combining characters for selected scripts (in particular Thai) and Hangul Jamo is in parts also available (i.e., some fonts, terminal emulators and editors support it via simple overstringing), but precomposed characters should be preferred over combining character sequences where available. More formally, the preferred way of encoding text in Unicode under Linux should be Normalization Form C as defined in Unicode Technical Report #15. Top
Character encoding standards define not only the identity of each character and its numeric value, or code point, but also how this value is represented in bits.
The Unicode Standard defines three encoding forms that allow the same data to be transmitted in a byte, word or double word oriented format (i.e. in 8, 16 or 32-bits per code unit). All three encoding forms encode the same common character repertoire and can be efficiently transformed into one another without loss of data. The Unicode Consortium fully endorses the use of any of these encoding forms as a conformant way of implementing the Unicode Standard.
UTF-8 is popular for HTML and similar protocols. UTF-8 is a way of transforming all Unicode characters into a variable length encoding of bytes. It has the advantages that the Unicode characters corresponding to the familiar ASCII set have the same byte values as ASCII, and that Unicode characters transformed into UTF-8 can be used with much existing software without extensive software rewrites.Top
UTF-16 is popular in many environments that need to balance efficient access to characters with economical use of storage. It is reasonably compact and all the heavily used characters fit into a single 16-bit code unit, while all other characters are accessible via pairs of 16-bit code units.
UTF-32 is popular where memory space is no concern, but fixed width, single code unit access to characters is desired. Each Unicode character is encoded in a single 32-bit code unit when using UTF-32.
All three encoding forms need at most 4 bytes (or 32-bits) of data for each character.Top
Computer text handling involves processing and encoding. Consider, for example, a word processor user typing text at a keyboard. The computer's system software receives a message that the user pressed a key combination for "T", which it encodes as U+0054. The word processor stores the number in memory, and also passes it on to the display software responsible for putting the character on the screen. The display software, which may be a window manager or part of the word processor itself, uses the number as an index to find an image of a "T", which it draws on the monitor screen. The process continues as the user types in more characters.
The Unicode Standard directly addresses only the encoding and semantics of text. It addresses no other action performed on the text. For example, the word processor may check the typist's input as it is being entered, and display misspellings with a wavy underline. Or it may insert line breaks when it counts a certain number of characters entered since the last line break. An important principle of the Unicode Standard is that it does not specify how to carry out these processes as long as the character encoding and decoding is performed properly.
The difference between identifying a code point and rendering it on screen or paper is crucial to understanding the Unicode Standard's role in text processing. The character identified by a Unicode code point is an abstract entity, such as "LATIN CHARACTER CAPITAL A" or "BENGALI DIGIT 5." The mark made on screen or paper -- called a glyph -- is a visual representation of the character.
The Unicode Standard does not define glyph images. The standard defines how characters are interpreted, not how glyphs are rendered. The software or hardware-rendering engine of a computer is responsible for the appearance of the characters on the screen. The Unicode Standard does not specify the size, shape, nor style of on-screen characters.