Unicode compatibility characters: Difference between revisions

Revision as of 16:56, 30 September 2013

Revision as of 18:32, 16 December 2013

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* '''Precomposed Roman numerals'''. For example, Roman numeral twelve (‘Ⅻ’: U+216B) can be decomposed into a Roman numeral ten (‘Ⅹ’: U+2169) and two Roman numeral ones (‘Ⅰ’: U+2160).

* '''Precomposed [[vulgar fraction|fractions]]'''. These decomposition have the keyword &lt;fraction&gt;. A fully conforming text handler should<ref>{{cite web|author=The Unicode Consortium|authorlink=Unicode Consortium|year=2010|title=The Unicode Standard, Version 6.0.0|publisher=Addison-Wesley Professional|isbn=978-0321480910|pages=p212|url=http://www.unicode.org/versions/Unicode6.0.0/ch06.pdf#G12861}}</ref> display the vulgar fraction ¼ (U+00BC) identically to the composed fraction 1⁄4 (numeral 1 with fraction slash U+2044 and numeral 4).

* '''Contextual glyphs or forms '''. These arise primarily in the Arabic script. Using fonts with glyph substitution capabilities such as [[OpenType]] and [[Apple Advanced Typography|TrueTypeGX]], Unicode conforming software can substitute the proper glyphs for the same character depending on whether that character appears at the beginning, end, middle of a word or in isolation. Such glyph substitution is also necessary for vertical (top to bottom) text layout for some East Asian languages. In this case glyphs must be substituted or synthesized for wide, narrow, small and square glyph forms. Non-conforming software or software using other character sets instead use multiple separate character for the same letter depending on its position: further complicating text processing.

The UCS, Unicode character properties and the Unicode algorithms provide software implementations with everything needed to properly display these characters from their decomposition equivalents. Therefore these decomposable compatibility characters become redundant and unnecessary. Their existence in the character set requires extra text processing to ensure text is properly compared and collated (see [[Unicode normalization]]). Moreover, these compatibility characters provide no additional or distinct semantics. Nor do these characters provide any visually distinct rendering provided the text layout and fonts are Unicode conforming. Also, none of these characters are required for round-trip convertibility to other character sets, since the transliteration can easily map decomposed characters to precomposed counterparts in another character set. Similarly, contextual forms, such as a final Arabic letter can be mapped based on its position within a word to the appropriate legacy character set form character.