This chapter is from the book
This chapter is from the book
This chapter is from the book
The Principle of Unification
The bottom-line philosophy you should draw from the discussions on the character–glyph model and on character positioning is that Unicode encodes semantics, not appearances. In fact, the Unicode standard specifically states that the pictures of the characters in the code charts are for illustrative purposes only—the pictures of the characters are intended to help clarify the meaning of the character code, not to specify the appearance of the character having that code.
The philosophy that Unicode encodes semantics and not appearances also undergirds the principle that Unicode is a plain-text encoding, which we discussed in Chapter 1. The fact that an Arabic letter looks different depending on the letters around it doesn't change what letter it is, and thus it doesn't justify having different codes for the different shapes. The fact that the letters lam and alef combine into a single mark when written doesn't change the fact that a word contains the letters lam and alef in succession. The fact that text from some language might be combined on the same line with text from another language whose writing system runs in the opposite direction doesn't justify storing either language's text in some order other than the order in which the characters are typed or spoken. In all of these cases, Unicode encodes the underlying meaning and leaves it up to the process that draws it to be smart enough to do so properly.
The philosophy of encoding semantics rather than appearance also leads to another important Unicode principle: the principle of unification.
Unlike most character encoding schemes, Unicode aims to be comprehensive. It aims to provide codes for all the characters in all the world's written languages. It also aims to be a superset of all other character encoding schemes (or at least the vast majority). By being a superset, Unicode can be an acceptable substitute for any of those other encodings (technical limitations aside, anyway), and it can serve as a pivot point for processes converting text between any of the other encodings.
Other character encoding standards are Unicode's chief source of characters. The designers of Unicode sought to include all the characters from every computer character encoding standard in reasonably widespread use at the time Unicode was designed. They have continued to incorporate characters from other standards as it has evolved, either as important new standards emerged, or as the scope of Unicode widened to include new languages. The designers of Unicode drew characters from every international and national standard they could get their hands on, as well as code pages from the major computer and software manufacturers, telegraphy codes, various other corporate standards, and even popular fonts, in addition to noncomputer sources. As an example of their thoroughness, Unicode includes code-point values for the glyphs that the old IBM PC code pages would show for certain ASCII control characters. As another example, Unicode assigns values to the glyphs from the popular Zapf Dingbats typeface.
This wealth of sources led to an amazingly extensive repertoire of characters, but also produced redundancy. If every character code from every source encoding retained its identity in Unicode (say, Unicode kept the original code values and just padded them to the same length and prefixed them with some identifier for the source encoding), they would never fit in a 16-bit code space. You would also wind up with numerous alternative representations for things that anyone with a little common sense would consider to be the same thing.
For starters, almost every language has several encoding standards. For example, there might be one national standard for each country where the language is spoken, plus one or more corporate standards devised by computer manufacturers selling into that market. Think about ASCII and EBCDIC in American English, for example. The capital letter A encoded by ASCII (as 0x41) is the same capital letter A that is encoded by EBCDIC (as 0xC1), so it makes little sense to have these two source values map to different codes in Unicode. In that case, Unicode would have two different values for the letter A. Instead, Unicode unifies these two character codes and says that both sources map to the same Unicode value. Thus, the letter A is encoded only once in Unicode (as U+0041), not twice.
In addition to the existence of multiple encoding standards for most languages, most languages share their writing system with at least one other language. The German alphabet is different from the English alphabet—it adds ? and some other letters, for example—but both are really just variations on the Latin alphabet. We need to make sure that the letter ? is encoded, but we don't need to create a different letter k for German—the same letter k we use in English will do just fine.
A truly vast number of languages use the Latin alphabet. Most omit some letters from what English speakers know as the alphabet, and most add some special letters of their own. Just the same, there's considerable overlap between their alphabets. The characters that overlap between languages are encoded only once in Unicode, not once for every language that uses them. For example, both Danish and Norwegian add the letter ? to the Latin alphabet, but the letter ? is encoded only once in Unicode.
Generally, characters are not unified across writing system boundaries. For
instance, the Latin letter B, the Cyrillic letter B, and the Greek letter B are
not unified, even though they look the same and have the same historical
origins. This is partly because their lowercase forms are all different (b, 
,
and 
, respectively), but mostly because the designers of Unicode didn't
want to unify across writing-system
boundaries.5 It made more sense to keep each
writing system distinct.
The basic principle is that, wherever possible, Unicode unifies character codes from its various source encodings, whenever they can be demonstrated beyond reasonable doubt to refer to the same character. One big exception exists: respect for existing practice. It was important to Unicode's designers (and probably a big factor in Unicode's success) for Unicode to be interoperable with the various encoding systems that came before it. In particular, for a subset of "legacy" encodings, Unicode is specifically designed to preserve round-trip compatibility. That is, if you convert from one of the legacy encodings to Unicode and then back to the legacy encoding, you should get the same thing you started with. Many characters that would have been unified in Unicode actually aren't because of the need to preserve round-trip compatibility with a legacy encoding (or sometimes simply to conform to standard practice).
For example, the Greek lowercase letter sigma has two forms: σ is used
in the middle of words, and 
is used at the end of words. As with the letters of
the Arabic alphabet, this example involves two different glyphs for the same
letter. Unicode would normally just have a single code point for the lowercase
sigma,
If a letter does double duty as a symbol, this generally isn't
sufficient grounds for different character codes either. The Greek letter pi
(
), for example, is still the Greek letter pi even when it's being used as
the symbol of the ratio between a circle's diameter and its circumference,
so it's still represented with the same character code. Some exceptions
exist, however: The Hebrew letter aleph (
) is used in mathematics to represent
the transfinite numbers, and this use is given a separate character code. The
rationale here is that aleph-as-a-mathematical-symbol is a left-to-right
character like all the other numerals and mathematical symbols, whereas
aleph-as-a-letter is a right-to-left character. The letter is used in physics
as the symbol of the angstrom unit. -as-the-angstrom is given its own character
code because some of the variable-length Japanese encodings did.
The business of deciding which characters can be unified can be complicated. Looking different is definitely not sufficient grounds by itself. For instance, the Arabic and Urdu alphabets have a very different look, but the Urdu alphabet is really just a particular calligraphic or typographical variation of the Arabic alphabet. The same set of character codes in Unicode, therefore, is used to represent both Arabic and Urdu. The same thing happens with Greek and Coptic,6 modern and old Cyrillic (the original Cyrillic alphabet had different letter shapes and some letters that have since disappeared), and Russian and Serbian Cyrillic (in italicized fonts, some letters have a different shape in Serbian from their Russian shape to avoid confusion with italicized Latin letters).
By far the biggest, most complicated, and most controversial instance of character unification in Unicode involves the Han ideographs. The characters originally developed to write the various Chinese languages, often called "Han characters" after the Han Dynasty, were also adopted by various other peoples in East Asia to write their languages. Indeed, the Han characters are still used (in combination with other characters) to write Japanese (who call them kanji) and Korean (who call them hanja).
Over the centuries, many of the Han characters have developed different forms in the different places where they're used. Even within the same written language—Chinese—different forms exist: In the early 1960s, the Mao regime in the People's Republic of China standardized or simplified versions of many of the more complicated characters, but the traditional forms are still used in Taiwan and Hong Kong.
Thus the same ideograph can have four different forms: one each for Traditional Chinese, Simplified Chinese, Japanese, and Korean (and when Vietnamese is written with Chinese characters, you might have a fifth form). Worse yet, it's very often not clear what really counts as the "same ideograph" between these languages. Considerable linguistic research went into coming up with a unified set of ideographs for Unicode that can be used for both forms of written Chinese, Japanese, Korean, and Vietnamese.7 In fact, without this effort, it would have been impossible to fit Unicode into a 16-bit code space.
In all of these situations where multiple glyphs are given the same character code, it either means the difference in glyph is simply the artistic choice of a type designer (for example, whether the dollar sign has one vertical stroke or two), or it's language dependent and a user is expected to use an appropriate font for his or her language (or a mechanism outside Unicode's scope, such as automatic language detection or some kind of tagging scheme, would be used to determine the language and select an appropriate font).
The opposite situation—different character codes being represented by the same glyph—can also happen. One notable example is the apostrophe (’). There is one character code for this glyph when it's used as a punctuation mark and another when it's used as a letter (it's used in some languages to represent a glottal stop, such as in “Hawai’i”).
Alternate-Glyph Selection
One interesting blurring of the line that can happen from time to time is the situation where a character with multiple glyphs needs to be drawn with a particular glyph in a certain situation, the glyph to use can't be algorithmically derived, and the particular choice of glyph needs to be preserved even in plain text. Unicode has taken different approaches to solving this problem in different situations. Much of the time, the alternate glyphs are simply given different code points. For example, five Hebrew letters have different shapes when they appear at the end of a word from the shapes they normally have. In foreign words, these letters keep their normal shapes even when they appear at the end of a word. Unicode gives different code point values to the regular and "final" versions of the letters. Examples like this can be found throughout Unicode.
Two special characters, U+200C ZERO WIDTH NON-JOINER (ZWNJ for short) and U+200D ZERO WIDTH JOINER (ZWJ for short), can be used as hints of which glyph shape is preferred in a particular situation. ZWNJ prevents formation of a cursive connection or ligature in situations where one would normally happen, and ZWJ produces a ligature or cursive connection where one would otherwise not occur. These two characters can be used to override the default choice of glyphs.
The Unicode Mongolian block takes yet another approach. Many characters in the Mongolian block have or cause special shaping behavior to happen, but sometimes the proper shape for a particular letter in a particular word can't be determined algorithmically (except with an especially sophisticated algorithm that recognizes certain words). The Mongolian block includes three "variation selectors," characters that have no appearance of their own, but change the shape of the character that precedes them in some well-defined way.
Beginning in Unicode 3.2, the variation-selector approach has been extended to all of Unicode. Unicode 3.2 introduces 16 general-purpose variation selectors, which work the same way as the Mongolian variation selectors: They have no visual presentation of their own, but act as "hints" to the rendering process that the preceding character should be drawn with a particular glyph shape. The list of allowable combinations of regular characters and variation selectors is given in a file called StandardizedVariants.html in the Unicode Character Database.
For more information on the joiner, non-joiner, and variation selectors, see Chapter 12.