Unicode

In computer science, Unicode is an industry standard allowing computers to consistently represent and manipulate text expressed in any of the world's writing systems. Developed in tandem with the Universal Character Set standard and published in book form as The Unicode Standard, Unicode consists of a repertoire of about 100,000 characters, a set of code charts for visual reference, an encoding methodology and set of standard character encodings, an enumeration of character properties such as upper and lower case, a set of reference data computer files, and a number of related items, such as character properties, rules for text normalization, decomposition, collation, rendering and bidirectional display order (for the correct display of text containing both right-to-left scripts, such as Arabic or Hebrew, and left-to-right scripts).

The Unicode Consortium, the non-profit organization that coordinates Unicode's development, has the ambitious goal of eventually replacing existing character encoding schemes with Unicode and its standard Unicode Transformation Format (UTF) schemes, as many of the existing schemes are limited in size and scope and are incompatible with multilingual environments.

Unicode's success at unifying character sets has led to its widespread and predominant use in the internationalization and localization of computer software. The standard has been implemented in many recent technologies, including XML, the Java programming language and modern operating systems.

Origin and development
Unicode has the explicit aim of transcending the limitations of traditional character encodings, such as those defined by the ISO 8859 standard which find wide usage in various countries of the world but remain largely incompatible with each other. Many traditional character encodings share a common problem in that they allow bilingual computer processing (usually using Roman characters and the local language) but not multilingual computer processing (computer processing of arbitrary languages mixed with each other).

Unicode, in intent, encodes the underlying characters — graphemes and grapheme-like units — rather than the variant glyphs (renderings) for such characters. In the case of Chinese characters, this sometimes leads to controversies over distinguishing the underlying character from its variant glyphs (see Han unification).

In text processing, Unicode takes the role of providing a unique code point — a number, not a glyph — for each character. In other words, Unicode represents a character in an abstract way and leaves the visual rendering (size, shape, font or style) to other software, such as a web browser or word processor. This simple aim becomes complicated, however, by concessions made by Unicode's designers in the hope of encouraging a more rapid adoption of Unicode.

The first 256 code points were made identical to the content of ISO 8859-1 so as to make it trivial to convert existing western text. A lot of essentially identical characters were encoded multiple times at different code points to preserve distinctions used by legacy encodings and therefore allow conversion from those encodings to Unicode (and back) without losing any information. For example, the "fullwidth forms" section of code points encompasses a full Latin alphabet that is separate from the main Latin alphabet section. In Chinese, Japanese and Korean (CJK) fonts, these characters are rendered at the same width as CJK ideographs rather than at half the width. For other examples, see Duplicate characters in Unicode.

When writing about a Unicode character, it is normal to write "U+" followed by a hexadecimal number indicating the character's code point. For code points in the BMP, four digits are used; for code points outside the BMP, five or six digits are used, as required. Older versions of the standard used similar notations, but with slightly different rules. For example, Unicode 3.0 used "U-" followed by eight digits, and allowed "U+" to be used only with exactly four digits in order to indicate a code unit, not a code point.

Standard
The Unicode Consortium, based in California, develops the Unicode standard. Any company or individual willing to pay the membership dues may join this organization. Members include virtually all of the main computer software and hardware companies with any interest in text-processing standards, such as Adobe Systems, Apple, HP, IBM, Microsoft, Xerox and many others.

The Consortium first published The Unicode Standard (ISBN 0-321-18578-1) in 1991, and continues to develop standards based on that original work. Unicode is developed in conjunction with the International Organization for Standardization and shares the character repertoire with ISO/IEC 10646: the Universal Character Set. Unicode and ISO/IEC 10646 function equivalently as character encodings, but The Unicode Standard contains much more information for implementers, covering — in depth — topics such as bitwise encoding, collation and rendering. The Unicode Standard enumerates a multitude of character properties, including those needed for supporting bidirectional text. The two standards do use slightly different terminology.

In 2005, the 100,000th character to be entered into the pipeline for standardisation was the MALAYALAM PRASLESHAM. It was encoded based on the contribution by Rachana Akshara Vedi.

Unicode revisions thus far:


 * Unicode 1.0: October 1991
 * Unicode 1.0.1: June 1992
 * Unicode 1.1: June 1993
 * Unicode 2.0: July 1996
 * Unicode 2.1: May 1998
 * Unicode 2.1.2: May 1998
 * Unicode 3.0: September 1999. Covered 16-bit UCS Basic Multilingual Plane from ISO 10646-1:2000.
 * Unicode 3.1: March 2001. Added Supplementary Planes from ISO 10646-2, providing supplementary characters
 * Unicode 3.2: March 2002
 * Unicode 4.0: April 2003
 * Unicode 4.0.1: March 2004
 * Unicode 4.1: March 2005
 * Unicode 5.0: July 2006
 * Unicode 5.1: expected early or mid 2008

Scripts covered
Unicode covers almost all scripts (writing systems) in current use today.

Although more than 30 writing systems (alphabets, syllabaries, and others) are included in Unicode, there remain many more still awaiting encoding. Further additions of characters to the already-encoded scripts, as well as symbols, in particular for mathematics and music (in the form of notes and rhythmic symbols), also occur. Michael Everson, Rick McGowan, and Ken Whistler maintain the list of such scripts and their tentative code block assignments on the Unicode Consortium Web site, at Unicode Roadmap. For some scripts on the Roadmap, encoding proposals have been made and are working their way through the approval process. For others, no proposal can be made until the scholarly communities involved can agree on the character repertoire and other details.

Among the scripts awaiting encoding are Egyptian Hieroglyphics, Babylonian and other cuneiforms, Phoenician, and Mayan, together with lesser-known scripts of Asia, Europe, Africa, and the Americas. Many of them are not understood, such as the Rongorongo of Easter Island, Linear A of Crete, and Meroitic of the Upper Nile.

Invented scripts, most of which do not qualify for inclusion in Unicode due to lack of real-world usage, are listed in the ConScript Unicode Registry, along with unofficial but widely-used Private Use Area code assignments. Similarly, many medieval letter variants and ligatures not in Unicode are encoded in the Medieval Unicode Font Initiative. In 1997 Michael Everson made a proposal to encode the characters of the artificial Klingon language in Plane 1 of ISO/IEC 10646-2. The Unicode Consortium rejected this proposal in 2001 as "inappropriate for encoding" — not because of any technical inadequacy, but because users of Klingon normally read, write and exchange data in Latin transliteration. Proposals suggested the inclusion of the elvish scripts Tengwar and Cirth from J. R. R. Tolkien's fictional Middle-earth setting in Plane 1 in 1993. The Consortium withdrew the draft to incorporate changes suggested by Tolkienists, and as of 2005 it remains under consideration. Both Klingon and the Tolkien scripts have assignments in the ConScript Unicode Registry.

Mapping and encodings
Several mechanisms have been specified for implementing Unicode; which one implementers choose depends on available storage space, source code compatibility, and interoperability with other systems.

Unicode defines two mapping methods: the Unicode Transformation Format (UTF) encodings, and the Universal Character Set (UCS) encodings. An encoding maps (possibly a subset of) the range of Unicode code points to sequences of values in some fixed-size range, termed code values. The numbers in the names of the encodings indicate the number of bits in one code value (for UTF encodings) or the number of bytes per code value (for UCS) encodings. UTF-8 and UTF-16 are probably the most commonly used encodings. UCS-2 is an obsolete subset of UTF-16; UCS-4 and UTF-32 are functionally equivalent.

UTF encodings include:


 * UTF-7 — a relatively unpopular 7-bit encoding, often considered obsolete (not part of The Unicode Standard but rather an RFC)
 * UTF-8 — an 8-bit, variable-width encoding, which maximizes compatibility with ASCII.
 * UTF-EBCDIC — an 8-bit variable-width encoding, which maximizes compatibility with EBCDIC. (not part of The Unicode Standard)
 * UTF-16 — a 16-bit, variable-width encoding
 * UTF-32 — a 32-bit, fixed-width encoding

UTF-8 uses one to four bytes per code point and, being compact for Latin scripts and ASCII-compatible, provides the de facto standard encoding for interchange of Unicode text. It is also used by most recent Linux distributions as a direct replacement for legacy encodings in general text handling.

The UCS-2 and UTF-16 encodings specify the Unicode Byte Order Mark (BOM) for use at the beginnings of text files, which may be used for byte ordering detection (or byte endianness detection). Some software developers have adopted it for other encodings, including UTF-8, which does not need an indication of byte order. In this case it attempts to mark the file as containing Unicode text. The BOM, code point U+FEFF has the important property of unambiguity on byte reorder, regardless of the Unicode encoding used; U+FFFE (the result of byte-swapping U+FEFF) does not equate to a legal character, and U+FEFF in other places, other than the beginning of text, conveys the zero-width no-break space (a character with no appearance and no effect other than preventing the formation of ligatures). Also, the units  and   never appear in UTF-8. The same character converted to UTF-8 becomes the byte sequence.

In UTF-32 and UCS-4, one 32-bit code value serves as a fairly direct representation of any character's code point (although the endianness, which varies across different platforms, affects how the code value actually manifests as an octet sequence). In the other cases, each code point may be represented by a variable number of code values. UTF-32 is widely used as internal representation of text in programs (as opposed to stored or transmitted text), since every Unix operating system which uses the gcc compilers to generate software uses it as the standard "wide character" encoding. Recent versions of the replace.py programming language (beginning with 2.2) may also be configured to use UTF-32 as the representation for unicode strings, effectively disseminating such encoding in high-level coded software.

Punycode, another encoding form, enables the encoding of Unicode strings into the limited character set supported by the ASCII-based Domain Name System. The encoding is used as part of IDNA, which is a system enabling the use of Internationalized Domain Names in all languages that are supported by Unicode.

GB18030 is another encoding form for Unicode, from the Standardization Administration of China. It is the official character set of the People's Republic of China (PRC).

The April Fools' Day RFC of 2005 specified two parody UTF encodings, UTF-9 and UTF-18.

Ready-made versus composite characters
Unicode includes a mechanism for modifying character shape and so greatly extending the supported glyph repertoire. This covers the use of combining diacritical marks. They get inserted after the main character (one can stack several combining diacritics over the same character). Unicode also contains precomposed versions of most letter/diacritic combinations in normal use. These make conversion to and from legacy encodings simpler and allow applications to use Unicode as an internal text format without having to implement combining characters. For example é can be represented in Unicode as (Latin small letter e) followed by U+0301 (combining acute) but it can also be represented as the precomposed character U+00E9 (Latin small letter e with acute). So in many cases, users have many ways of encoding the same character. To deal with this, Unicode provides the mechanism of canonical equivalence.

An example of this arises with Hangul, the Korean alphabet. Unicode provides the mechanism for composing Hangul syllables with their individual subcomponents, known as Hangul Jamo. However, it also provides all 11,172 combinations of precomposed Hangul syllables.

The CJK ideographs currently have codes only for their precomposed form. Still, most of those ideographs comprise simpler elements (often called radicals in English), so in principle Unicode could have decomposed them just as it has happened with Hangul. This would have greatly reduced the number of required code points, while allowing the display of virtually every conceivable ideograph (which might do away with some of the problems caused by the Han unification). A similar idea covers some input methods, such as Cangjie and Wubi. However, attempts to do this for character encoding have stumbled over the fact that ideographs do not actually decompose as simply or as regularly as it seems they should.

A set of radicals was provided in Unicode 3.0 (CJK radicals between U+2E80 and U+2EFF, KangXi radicals in U+2F00 to U+2FDF, and ideographic description characters from U+2FF0 to U+2FFB), but the Unicode standard (ch. 11.1 of Unicode 4.1) warns against using ideographic description sequences as an alternate representation for previously encoded characters:

"This process is different from a formal encoding of an ideograph. There is no canonical description of unencoded ideographs; there is no semantic assigned to described ideographs; there is no equivalence defined for described ideographs. Conceptually, ideograph descriptions are more akin to the English phrase, “an ‘e’ with an acute accent on it,” than to the character sequence &lt;U+006E, U+0301&gt; [sic; 'e' should be U+0065]."

Ligatures
Many scripts, including Arabic and Devanagari, have special orthographic rules which require that certain combinations of letterforms be combined into special ligature forms. The rules governing ligature formation can be quite complex, requiring special script-shaping technologies such as OpenType (by Adobe and Microsoft), Graphite (by SIL International), or AAT (by Apple). Instructions are also embedded in fonts to tell the operating system how to properly output different character sequences. A simple solution to the placement of combining marks or diacritics is assigning the marks a width of zero and placing the glyph itself to the left or right of the left sidebearing (depending on the direction of the script they are intended to be used with). A mark handled this way will appear over whatever character precedes it, but will not adjust its position relative to the width or height of the base glyph; it may be visually awkward and it may overlap some glyphs. Real stacking is impossible, but can be approximated in limited cases (for example, Thai top-combining vowels and tone marks can just be at different heights to start with). Generally this approach is only effective in monospaced fonts but can also be used as a fallback rendering method when more complex methods fail.

As of 2004, most software still cannot reliably handle many features not supported by older font formats, so combining characters generally will not work correctly. For example, (precomposed e with macron and acute above) and  (e followed by the combining macron above and combining acute above) should be rendered identically, both appearing as an e with a macron and acute accent, but in practice, their appearance can vary greatly across software applications. Similarly, underdots, as needed in the romanization of Indic, will often be placed incorrectly. As a workaround, Unicode characters that map to precomposed glyphs can be used for many such characters. The need for such alternatives inherits from the limitations of fonts and rendering technology, not weaknesses of Unicode itself.

Standardized subsets
Several subsets of Unicode are standardized: Microsoft Windows since Windows NT 4.0 supports WGL-4 with 652 characters, which is considered to support all contemporary European languages using the Latin, Greek or Cyrillic script. Other standardized subsets of Unicode include the Multilingual European Subsets: MES-1 (Latin scripts only, 335 characters), MES-2 (Latin, Greek and Cyrillic 1062 characters) and MES-3A & MES-3B (two larger subsets, not shown here). Note that MES-2 includes every character in  MES-1, which in turn includes all of WGL-4.

Rendering software which cannot process a Unicode character appropriately most often display it as only an open rectangle, or the Unicode “replacement character” (U+FFFD, �), to indicate the position of the unrecognized character. Some systems have made attempts to provide more information about such characters. The Apple LastResort font will display a substitute glyph indicating the Unicode range of the character and the SIL Unicode fallback font will display a box showing the hexadecimal scalar value of the character.

Operating systems
Unicode has become the dominant scheme for internal processing and sometimes storage (though a lot of text is still stored in legacy encodings) of text. Early adopters tended to use UCS-2 and later moved to UTF-16 (as this was the least disruptive way to add support for non-BMP characters). The best known such system is Windows NT (and its descendants, Windows 2000 and Windows XP), which uses Unicode as the sole internal character encoding. The Java and .NET bytecode environments, Mac OS X, and KDE also use it for internal representation.

UTF-8 (originally developed for Plan 9) has become the main storage encoding on most Unix-like operating systems (though others are also used by some libraries) because it is a relatively easy replacement for traditional extended ASCII character sets.

Multilingual text-rendering engines which use Unicode include Uniscribe for Microsoft Windows, ATSUI for Mac OS X and Pango, a free software engine used by GTK+ (and hence the GNOME desktop).

Input methods
Because keyboard layouts cannot have simple key combinations for all characters, several operating systems provide alternative input methods that allow access to the entire repertoire.

ISO 14755 describes methods for entering Unicode characters from their codepoints; clause 5.1 describes a Basic method whereby a beginning sequence is followed by the hexadecimal representation of the codepoint and ther ending sequence; an example of an ISO 14755-conformant system is GNOME, where the beginning sequence is CTRL+SHIFT+U and the ending sequence is null. In various operating systems, alt codes can be used to input Unicode points; where the code point of the desired character is known, it is possible to create Unicode characters by pressing  , where # represents the hexadecimal code point; for example,  will produce the Unicode character ñ. On some systems, this is limited to the BMP (characters up to U+FFFF).

Some software supports the following input method: first enter the character’s hexadecimal code, then immediately press  . For example, entering f1 and then pressing the combination will produce the character ñ. The code must not be preceeded by any digit or letters a-f as they will be treated as part of the code to be converted.

ISO 14755 also describes a screen-selection entry method; in Microsoft Windows (since Windows 2000), the "Character Map" program (Start/Programs/Accessories/System Tools/Character Map) provides browsing and rich-text editing controls for all Table I characters in the BMP, by selection from a drop-down table, assuming that a Unicode font is selected. Mac OS X (version 10.2 and newer), KDE and GNOME have similar utilities.

E-mail
MIME defines two different mechanisms for encoding non-ASCII characters in e-mail, depending on whether the characters are in e-mail headers such as the "Subject:" or in the text body of the message. In both cases, the original character set is identified as well as a transfer encoding. For e-mail transmission of Unicode the UTF-8 character set and the Base64 transfer encoding are recommended. The details of the two different mechanisms are specified in the MIME standards and are generally hidden from users of e-mail software.

The adoption of Unicode in e-mail has been very slow. Some East-Asian text is still encoded in a local encoding such as Shift-JIS, and some devices, such as cell phones, still cannot handle Unicode data correctly. Support has been improving however.

Web
All W3C recommendations have used Unicode as their document character set since HTML 4.0. Web browsers have supported Unicode, especially UTF-8, for many years. Display problems result primarily from font related issues. In particular Internet Explorer does not render many code points unless it is explicitly told to use a font that contains them.

Although syntax rules may affect the order in which characters are allowed to appear, both HTML 4 and XML (including XHTML) documents, by definition, comprise characters from most of the Unicode code points, with the exception of:


 * most of the C0 and C1 control codes
 * the permanently-unassigned code points D800–DFFF
 * any code point ending in FFFE or FFFF

These characters manifest either directly as bytes according to document's encoding, if the encoding supports them, or users may write them as numeric character references with percent-encoding based on the character's Unicode code point. For example, the references,  ,  ,  ,  ,  ,  ,  , and   (or the same numeric values expressed in hexadecimal, with   as the prefix) display on browsers as Δ, Й, ק, م, ๗, あ, 叶, 葉, and 냻. In HTTP requests, URLs must be percent-encoded.

Fonts
Free and retail fonts based on Unicode are commonly available, since TrueType and OpenType support Unicode. These font formats map Unicode code points to glyphs.

Thousands of fonts exist on the market, but fewer than a dozen fonts — sometimes described as "pan-Unicode" fonts — attempt to support the majority of Unicode's character repertoire. Instead, Unicode-based fonts typically focus on supporting only basic ASCII and particular scripts or sets of characters or symbols. Several reasons justify this approach: applications and documents rarely need to render characters from more than one or two writing systems; fonts tend to demand resources in computing environments; and operating systems and applications show increasing intelligence in regard to obtaining glyph information from separate font files as needed, i.e. font substitution. Furthermore, designing a consistent set of rendering instructions for tens of thousands of glyphs constitutes a monumental task; such a venture passes the point of diminishing returns for most typefaces.

Philosophical and completeness criticisms
Han unification (the identification of forms in the three East Asian languages which one can treat as stylistic variations of the same historical character) has become one of the most controversial aspects of Unicode, despite the presence of a majority of experts from all three regions in the Ideographic Rapporteur Group (IRG), which advises the Consortium and ISO on additions to the repertoire and on Han unification.

Unicode has been criticized for failing to allow for older and alternative forms of kanji which, critics argue, complicates the processing of ancient Japanese and uncommon Japanese names, although it follows the recommendations of Japanese language scholars and of the Japanese government and contains all of the same characters as previous widely used encoding standards. There have been several attempts to create an alternative encodings that preserve the minor, stylistic differences between Chinese, Japanese, and Korean characters in opposition to Unicode's policy of Han unification. Among them are TRON (although it is not widely adopted in Japan, there are some users who need to handle historical Japanese text and favor it), and UTF-2000.

Many older forms were not included in early versions of the Unicode standard, but Unicode 4.0 contains more than 70,000 Han characters and work continues on adding characters from the early literature of China, Korea, and Japan. Some argue, however, that this is not satisfactory, pointing out as an example the need to create new characters, representing words in various Chinese dialects, more of which may be invented in the future.

Despite these problems, the official encoding of China, GB-18030, supports the full range of characters in Unicode.

Mapping to legacy character sets
Injective mappings must be provided between characters in existing legacy character sets and characters in Unicode to facilitate conversion to Unicode and allow interoperability with legacy software. Lack of consistency in various mappings between earlier Japanese encodings such as Shift-JIS or EUC-JP and Unicode led to round-trip format conversion mismatches, particularly the mapping of the character JIS X 201 '～' (1-33, WAVE DASH), heavily used in legacy database data, to either '～' U+FF5E FULLWIDTH TILDE (in Microsoft Windows) or '〜' U+301C WAVE DASH (other vendors).

Some Japanese computer programmers objected to Unicode because it requires them to separate the use of '\' U+005C REVERSE SOLIDUS (backslash) and '¥' U+00A5 YEN SIGN, which was mapped to 0x5C in JIS X 0201, and there is a lot of legacy code with this usage. (This encoding also replaces tilde '~' 0x7E with overline '¯', now 0xAF.) The separation of these characters exists in ISO 8859-1, from long before Unicode.

Indic scripts
Thai language support has been criticized for its illogical ordering of Thai characters. The vowels เ, แ, โ, ใ, ไ that are written to the left of the preceding consonant are in visual order instead of logical order, unlike the Unicode representations of other Indic scripts. This complication is due to Unicode inheriting the Thai Industrial Standard 620, which worked in the same way. This ordering problem complicates the Unicode collation process slightly, requiring table lookups to reorder Thai characters for collation.

Indic scripts of India itself such as Hindi, Tamil and Telugu are each allocated only 128 code points, matching the ISCII standard. The correct rendering of Unicode Indic text requires transforming the stored logical order characters into visual order and the forming of ligatures out of components. Some local scholars argued in favor of assignments of Unicode codepoints to these ligatures, going against the practice for other writing systems, though Unicode contains some Arabic and other ligatures for back compatibility purposes only. Encoding of any new ligatures in Unicode will not happen, in part because the set of ligatures is font-dependent, and Unicode is an encoding independent of font variations. The same kind of issue arose for Tibetan script (the Chinese National Standard organization failed to achieve a similar change).