Home > Articles > Programming

  • Print
  • + Share This
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.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020