UTS # Unicode Locale Data Markup Language
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As the characters were being collected from various sources, these were prepared as a group and kept together. Thus, you need to pay close attention to details in order to know what the status of any character is.
As described in Section 5. In such cases, it is assumed that an appropriate rendering technology will be used that can do the glyph processing needed to correctly position the combining marks.
Early Years of Unicode
In many cases, however, legacy standards encoded precomposed base-plus-diacritic combinations as characters rather than composing these combinations dynamically. Unicode has a very large number of precomposed characters, including for Greek and Latin, and over 11, for Korean!
There are also over precomposed characters for Japanese, Cyrillic, Hebrew, Arabic and various Indic scripts. Not all of the precomposed characters are from scripts used in writing human languages. Precomposed characters go against the principle of dynamic composition, but also against the principle that Unicode encodes abstract characters rather than glyphs. In principle, it should be possible for a combining character sequence to be rendered so that the glyphs for the combining marks are correctly positioned in relation to the glyph for the base, or even so that the character sequence is translated into a single precomposed glyph.
In these cases, though, that glyph is directly encoded as a distinct character. There are other cases in which the distinction between characters and glyphs is compromised.
Those cases have some significant differences from the ones we have considered thus far. Before continuing, though, there are some important additional points to be covered in relation to the characters described in Sections 6.
We will return to look at the remaining areas of compromise in Section 6. In each of these cases, Unicode provides alternate representations for a given text element.
For singleton duplicates, what this means is that there are two codepoints that are effectively equivalent and mean the same thing: Characters with exact duplicates Likewise in the case of each precomposed character, there is a dynamically composed sequence that is equivalent and means the same thing as the precomposed character: Precomposed characters and equivalent dynamically composed sequences This type of ambiguity is far from ideal, but was a necessary price of maintaining backward compatibility with source standards and, specifically, the round-trip rule.
In view of this, one of the original design principles of Unicode was to allow for a text element to be represented by two or more different but equivalent character sequences.
In these situations, Unicode formally defines a relationship of canonical equivalence between the two representations. Essentially, this means that the two representations should generally be treated as if they were identical, though this is slightly overstated.
Let me explain in more detail. In precise terms, the Unicode Standard defines a conformance requirement in relation to canonically equivalent pairs that must be observed by software that claims to conform to the Standard: A process shall not assume that the interpretations of two canonical-equivalent character sequences are distinct. In other words, software can treat the two representations as though they were identical, but it is also allowed to distinguish between them; it just cannot always treat them as distinct.
Since the different sequences are supposed to be equal representations of exactly the same thing, it might seem that this requirement is stated somewhat weakly, and that it ought to be appropriate to make a much stronger requirement: Most of the time, it does make sense for software to do that, but there may be certain situations in which it is valid to distinguish them. For example, you may want to inspect a set of data to determine if any precomposed characters are used.
You could not do that if precomposed characters are never distinguished from the corresponding dynamically composed sequence. At this point, I can introduce some convenient terminology that is conventionally used. The formal relationship between a precomposed character and the equivalent decomposed sequence is formally known as canonical decomposition.
These mappings are specified as part of the semantic character properties contained in the online data files that were mentioned in Section 2.
In some cases a text element can be represented as either fully composed or fully decomposed sequences. In many cases, however, there will be more than just these two representations.
In particular, this can occur if there is a precomposed character that corresponds to a partial representation of another text element. Equivalent precomposed, decomposed and partially composed representations Thus, in this example, there are four possible representations that are all canonically equivalent to one another.
We will look at the possibility of having multiple equivalent representations further in Section 9. There is more to be explained regarding the relationship between canonical-equivalent sequences, and we will be looking at further details in Sections 7910 and Before returning to discuss ways in which Unicode design principles have been compromised, there are some specific points worth mentioning regarding certain characters for vowels in Indic scripts.
Indic scripts have combining vowel marks that can be written above, below, to the left or to the right of the syllable-initial consonant. In many Indic scripts, certain vowel sounds are written using a combination of these marks, as illustrated in Figure What is worth noting is that these vowels are not handled in the same way in Unicode for all Indic scripts.
In a number of cases, Unicode includes characters for the precomposed vowel combination in addition to the individual vowel signs. On the other hand, in Thai and Lao, the corresponding vowel sounds can only be represented using the individual component vowel signs. Then again, in Khmer, only precomposed characters can be used.
If you need to encode data, therefore, for a language that uses an Indic script, pay close attention to how that particular script is supported in Unicode. As explained earlier, Unicode assumes that software will support rendering technologies that are capable of the glyph processing needed to handle selection of contextual forms and ligatures.
Such capability was not always available in the past, however. The result is that Unicode includes a number of cases of presentation forms—glyphs that are directly encoded as distinct characters but that are really only rendering variants of other characters or combinations of characters.
There are five blocks in the compatibility area that primarily encode presentation forms. These blocks mostly contain characters that correspond to glyphs for Arabic ligatures and contextual shapes that would be found in different connective relationships with other glyphs initial, medial, final and isolate forms. Unicode includes such characters for Arabic presentation forms.
Characters like this constitute duplicates of other characters. There is a difference between this situation and that described in Sections 6. In the case of the singleton duplicates, the two characters were, for all intents and purposes, identical: Working with our Japanese counterparts was made somewhat more challenging because of the translation issues.
Working with Unicode
In the best of all possible worlds, we would all have spoken a common language. Second best would have been having a technically savvy translator, experienced with software engineering design and concepts. What we actually had was one, lone Apple marketing person, who happened to be bilingual.
Imagine yourself in that situation, having to discuss how to combine Huffman encoding and run-length encoding to compress Japanese input dictionaries. We soon learned the full impact of the phrase "to lose something in translation! We then found out just how useful a white-board can be. Yet one day we hit a stumbling block, and were just not making progress.
We had known that Japanese needed two bytes to encompass the large character set, and we had prototyped how to adapt the system software to use two-byte characters. However, we were having trouble figuring out exactly how things fit together with our counterparts' data formats. Remember [that] we were new to this, so it didn't hit us right away. But all of a sudden, we could see the light go on in both of our faces: We were so, so wrong.
You needed a mixture of single and double bytes to represent even the most common text. Worse yet, some bytes could be both whole single byte-characters, and parts of double-byte characters. We weren't in Kansas anymore! We persevered, and ended up producing a successful product [Apple KanjiTalk]. For the first time, all of the world's characters can be represented in a uniform manner, making it feasible for the vast majority of programs to be globalized: In many ways, the use of Unicode makes programs much more robust and secure.
When systems used a hodge-podge of different charsets for representing characters, there were security and corruption problems that resulted from differences between those charsets, or from the way in which programs converted to and from them.
However, because Unicode contains such a large number of characters, and incorporates the varied writing systems of the world, incorrect usage can expose programs or systems to possible security attacks. This document describes some of the security considerations that programmers, system analysts, standards developers, and users should take into account.
For example, consider visual spoofing, where a similarity in visual appearance fools a user and causes him or her to take unsafe actions.
Understanding Unicode™ - II
They click on the link, and carefully examine the browser's address box to make sure that it is actually going to http: They see that it is, and use their password. However, what they saw was wrong—it is actually going to a spoof site with a fake "citibank.
They use the site without suspecting, and the password ends up compromised. This problem is not new to Unicode: The infamous example here involves "paypaI. Not only was "Paypai. He or she is apparently emailing PayPal customers, saying they have a large payment waiting for them in their account.