|Authors||Asmus Freytag (firstname.lastname@example.org)|
This report presents the specifications of a informative property for Unicode characters that is useful when interoperating with East Asian Legacy character sets.
This document has been reviewed by Unicode members and other interested parties, and has been approved by the Unicode Technical Committee as a Unicode Standard Annex. It is a stable document and may be used as reference material or cited as a normative reference from another document.
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In mixed-width, East Asian, legacy encodings there is a concept of an inherent width of a character. For a fixed pitch font, this width translates to a display width of either one half or a whole unit width. A common name for this unit width is "Em". It is customarily the height of the letter 'M', but since in East Asian fonts the standard character cell is square, it is the same as the unit width.
Note: the character width for a fixed pitch Latin font like Courier is generally 3/5 of an em.
Layout and line breaking (to cite only two examples) in an East Asian context show systematic variations depending on the value of the East-Asian Width property (even for non-fixed pitch fonts). Further, the same information is useful in creating correct transcoding tables for East Asian character sets.
The East Asian Width property provides a useful concept for implementations that
This Unicode Technical Report does not provide rules or specifications of how this property might be used in font design or line layout, since, while a useful property for this purpose, it is only one of several character properties that would need to be considered.
By convention, 1/2 Em wide characters of East Asian legacy encodings are called "half-width" (or hankaku characters in Japanese), the others are called correspondingly "full-width" (or zenkaku) characters. Legacy encodings often use a single byte for the half-width characters and two bytes for the full-width characters. In the Unicode Standard, no such distinction is made, but understanding the distinction is often necessary when interchanging data with legacy systems, especially when fixed size buffers are involved.
Some character blocks in the compatibility zone contain characters that are explicitly marked "half-width" and "full-width" in their character name but for all other characters the width property must be implicitly derived. Some characters behave differently in East Asian context than in non-East Asian content. Their default width property is considered ambiguous and needs to be resolved into an actual width property based on context.
This technical report assigns to each Unicode character one of the six values Ambiguous, Full Width, Half Width, Narrow, Wide, or Not East Asian Neutral (defined below) as its default width property. For any given operation, these six default property values resolve into only two property values narrow and wide, depending on context.
ED1. East Asian Width - in the context of interoperating with East Asian legacy character encodings and implementing East Asian typography, the East Asian Width is a categorization of character. It can take on two abstract values, narrow and wide. In legacy implementations, there is often a corresponding difference in encoding length (one or two bytes) as well as a difference in displayed width. However, the actual display width of a glyph is given by the font and may be further adjusted by layout. An important class of fixed width legacy fonts contains glyphs of just two widths with the wider glyphs twice as wide as the narrower glyphs.
Note: For convenience, the classification further distinguishes among explicitly or implicitly wide and narrow characters.
ED2. East Asian Full-width (F) - all characters that are defined as FULL WIDTH in the Unicode Standard and therefore are compatibility equivalents of characters elsewhere in the Unicode Standard that are implicitly narrow but unmarked.
ED3. East Asian Half-width (H) - all characters that are explicitly defined as HALF WIDTH in the Unicode Standard and therefore are compatibility characters of characters elsewhere in the Unicode Standard that are implicitly wide but unmarked.
ED4. East Asian Wide (W) - all other characters that are always wide. These characters ocur only in the context of East Asian typography where they are wide characters (such as the Unified Han Ideographs or Squared Katakana Symbols). This category includes characters that have explicit half-width counterparts.
ED5. East Asian Narrow (Na) - all other characters that are always narrow and have explicit full-width counterparts. These characters are implicitly narrow in East Asian typography and legacy character sets since they have explicit full-width counterparts. All of ASCII is an example of East Asian Narrow characters.
It is useful to distinguish characters explicitly defined as half-width from other characters that have a full-width equivalent. In particular, half-width punctuation behaves in some important ways like ideographic punctuation.
ED6. East Asian Ambiguous (A) - all characters that can be sometimes wide and sometimes narrow. Ambiguous characters require additional information not contained in the character code to further resolve their width.
Ambiguous characters occur in East Asian legacy character sets as wide characters, but as narrow (i.e. normal-width) characters in non-East Asian usage (Examples are the Greek and Cyrillic alphabet found in East Asian character sets, but also some of the mathematical symbols). Private Use characters are considered ambiguous, since additional information is required to know whether they should be treated as wide or narrow.
Figure 1: Venn diagram showing the set relations for five of the six categories.
Because East Asian legacy character sets do not always include complete case pairs of Latin characters, two members of a case pair may have different East Asian Width properties:
Ambiguous: 01D4 LATIN SMALL LETTER U WITH CARON NEA Neutral: 01D3 LATIN CAPITAL LETTER U WITH CARON
ED7. Not East Asian (Neutral) - all other characters. Neutral characters do not occur in legacy East Asian character sets. By extension, they also do not occur in East Asian typography. For example, there is no traditional Japanese way of typesetting Devanagari.
Strictly speaking, it makes no sense to talk of narrow and wide for neutral characters, but since for all practical purposes they behave like Na, they are treated as narrow characters (the same as Na) under the recommendations below.
In a broad sense, wide characters include W, F, and A (when in EA context), while narrow characters include N, Na, H, and A (when not in EA context).
Figure 2: Examples for each character class and their resolved widths
When converting a DBCS mixed-width encoding to and from Unicode, the full-width characters in such a mixed-width encoding are mapped to the full-width compatibility characters in the FFxx block, whereas the corresponding half-width characters are mapped to ordinary Unicode characters (e.g. ASCII in U+0021..U+007E, plus a few other scattered characters).
In the context of interoperability with DBCS character encodings, that restricted set of Unicode characters in the General Scripts area can be construed as half-width, rather than full-width. (This applies only to the restricted set of characters which can be paired with the full-width compatibility characters.)
In the context of interoperability with DBCS character encodings, all other Unicode characters which are not explicitly marked as half-width can be construed as full-width.
In any other context, Unicode characters not explicitly marked as being either full-width or half-width compatibility forms are neither half-width or full-width.
Seen in this light, the "half-width" and "full-width" properties are not unitary character properties in the same sense as "space" or "combining" or "alphabetic". They are, instead, relational properties of a pair of characters, one of which is explicitly encoded as a half-width or full-width form for compatibility in mapping to DBCS mixed-width character encodings.
What is "full-width" by default today could in theory become "half-width" tomorrow by the introduction of another character on the SBCS part of a mixed-width code page somewhere, requiring the introduction of another full-width compatibility character to complete the mapping. However, since the single byte part of mixed-width character sets is limited, there are not going to be many candidates and neither UTC and WG2 have any intention to add additional compatibility characters for this purpose.
Ambiguous width characters are all those characters that can occur as full-width characters in any of a number of East Asian legacy character encodings. They have a 'resolved' width of either narrow or wide depending on the context of their use. If they are not used in context of the specific legacy encoding they belong to, their width resolves to narrow. Otherwise it resolves to full-width or half-width. The term context as used here includes extra information such as explicit markup, knowledge of the source codepage, font information, or language identification. For example:
East Asian Width is an informative character property and implies no conformance requirements.
When mapping Unicode to legacy character encodings
When processing or displaying data
The classifications presented here are based on the most widely used mixed-width legacy character sets in use in East Asia as of this writing. In particular, the assignment of the neutral or ambiguous categories depend on the contents of these character sets. For example, an implementation that knows a-priori that it only needs to interchange data with the Japanese Shift-JIS character set, but not other East Asian character sets, could reduce the number of characters in the ambiguous classification to those actually encoded in Shift-JIS. Or such a reduction could be done implicitly at runtime in the context of interoperating with Shift-JIS fonts or data sources. Conversely, if additional character sets are created and widely adopted for legacy purposes, more characters would need to be classified as ambiguous.
All unassigned characters are by default classified as non-East Asian neutral, except for the range U-00020000 to U-0002FFFD, since all code positions from U-00020000 to U-0002FFFD are intended for CJK ideographs (W). All Private use characters are by default classified as ambiguous, since their definition depends on context. This includes surrogate pairs.
Combining marks have been classified and are given a property assignment based on their typical applicability. For example combining marks typically applied to characters of class N, Na or W are classified as A. Combining marks for purely non-East Asian scripts are marked as N, and non-spacing marks used only with wide characters are given a W. Even more so than for other characters, the East Asian width property for combining marks is not the same as their display width.
In particular, non-spacing marks do not possess actual advance width. Therefore, even when displaying combining marks, the East Asian Width property cannot be related to the advance width of these characters. However, it can be useful in determining the the encoding length in a legacy encoding, or the choice of font for the range of characters including that non-spacing mark. The width of the glyph image of a non-spacing mark should always be chosen as the appropriate one for the width of the base character.
The East Asian Width classification of characters of the Unicode Standard, Version 3.0 is available as the file EastAsianWidth.txt in the Unicode Character Database. This is a tab-delimited three column plain text file, with code position, East Asian Width designator and character name (for reference purpose only). The abbreviated way of listing the Ideographic, Hangul, Surrogate and Private use ranges is the same as in UnicodeData.txt
The sets of East Asian Narrow, East Asian Fullwidth and East Asian Halfwidth are fixed for all practical purposes. New characters for most scripts will be East Asian Neutral characters, unless the script is an East Asian script using wide characters, and then the new characters will be classified as East Asian Wide. One important exception consists of those new symbol characters which are, or are expected to be used both as wide characters in East Asian usage and narrow characters in non-East Asian usage. These would need to be classified as East Asian Ambiguous.
Michel Suignard provided extensive input into the analysis and source material for the detail assignments of these properties. Mark Davis and Ken Whistler performed consistency checks on the data files.
Version 6.0: Restated the definitions so that the wording more clearly reflects the intent. No changes to the assignments of properties to any character were made. Added a section on classifying characters that are to be added to the standard in the future. Also added figure 2.
Version 5.0: Changed the spelling of the title and made minor clarifying changes to the definitions and the description of ambiguous characters and combining marks. As result of the Unicode 3.0 beta process, changed some CJK punctuation characters from W to A since they are also used in Western mathematical notation. Removed some historic information and made other edits to prepare TR for publication as part of Unicode 3.0.
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