When isolated it appears as ﻩ and in the final/initial/medial position in a word it appears as ﻪ/ﻫ/ﻬ respectively. For instance the Arabic letter “heh” has four forms, depending on which sides are flanked by letters. The rules of some languages cause glyphs to change through contextual shaping. It’s not just fonts that cause graphemes to be rendered into varying glyphs. Inserting U+0061 into the asterisk position U+006f U+0302 (*) U+0323 changes the string “ộ” into “ôạ” rather than “ộa”. Programs must also be mindful of the combining characters, like diacritical marks, when inserting or deleting codepoints. Operations such as taking the length of an array of codepoints, or accessing arbitrary array positions are typically not useful for Unicode programs. This is called Normalization Form Canonical Composition (NFC).Ī core concept to remember is that, although codepoints are the building blocks of text, they don’t match up 1-1 with user-perceived characters (graphemes). Another choice is to do the opposite and use the fewest codepoints possible like example E. That is called Normalization Form Canonical Decomposition (NFD). One choice is to decompose a string into as many codepoints as possible like examples A and B (with a weighting factor of which combining marks should come first). A standardized choice of codepoint decomposition for graphemes is called a “normal form.” To meaningfully compare strings codepoint by codepoint for equality, both strings should both be represented in a consistent way. As illustrated above, multiple strings of codepoints may render into the same sequence of graphemes. The numbers (written U+xxxx) for each abstract character and each combining symbol are called “codepoints.” Every Unicode string is expressed as a list of codepoints. For instance ộ can be specified in five ways: Many graphemes can thus be created in more than one way. It assigns numbers to basic letters and combining marks, but also to some of their more common combinations. In reality Unicode takes both approaches. The graphemes can be built from letters and combining marks e.g. ậ = a + ◌̂ + ◌̣. Rather than assigning a distinct number to each, it’s more efficient to assign a number to o and a, and then to each of the combining marks. For instance (o, ô, ọ, ộ) and (a, â, ạ, ậ) follow a pattern. It would be wasteful because there is a combinatorial explosion between letters and diacritical marks. You might imagine that Unicode assigns each grapheme a unique number, but that is not true. They are rendered as “glyphs,” i.e. markings on paper or screen which vary by font, style, or position in a word. Pieces of a single grapheme always stay together in print breaking them apart is either nonsense or changes the meaning of the symbol. It’s the character as a user would understand it. A “grapheme” is a graphical unit that a reader recognizes as a single element of the writing system. Let’s start at the abstraction closest to the user: the grapheme cluster. The representation is further obscured by an additional encoding in memory, on disk, or during network transmission. What a native speaker of a language identifies as a letter or symbol is often stored as multiple values in the internal Unicode representation. Many languages have bindings to the library, so these concepts should be applicable to your language of choice.īefore getting into the example code, it’s important to learn the terminology. We’ll use the C API here for a better view into the internals. IBM (the maintainers of ICU) officially support a C, C++ and Java API. We’ll use the International Components for Unicode (ICU) library, which is mature, portable, and powers the international text processing behind many products and operating systems. This article illustrates text processing ideas with example programs. Realistically this means using a mature third-party library. Human languages are highly varied and internally inconsistent, and any application which treats strings as more than an opaque byte stream must embrace the complexity. The Unicode standard and specifications describe the proper way to divide words and break lines, sort text, format numbers, display text in different directions, split/combine/reorder vowels South Asian languages, and determine when characters may look visually confusable. Unicode also includes characters’ case, directionality, and alphabetic properties. Unicode is more than a numbering scheme for the characters of every language – although that in itself is a useful accomplishment. They contain internationalization features that often aren’t portable or don’t suffice. Most programming languages evolved awkwardly during the transition from ASCII to 16-bit UCS-2 to full Unicode.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |