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Character Encoding and Glyph Codes

A computer represents characters in numeric form. A character encoding is a mapping that assigns numeric values (character codes) to characters. The mapping varies depending on the character set, which in turn may depend on the language as well as other factors.

Fonts associate each glyph with a double-byte code called its glyph code (which is not the same as a character code). Different fonts may have different glyph codes for the same glyphs, and a single font may have several glyph codes associated with a particular character because several glyphs may represent that character. Because the font and general textual context determine which glyph and which glyph codes represent characters, ATSUI transparently handles the details of mapping character codes to the correct glyph codes. Your application does not have to handle the details of obtaining glyphs from a font.

Different languages may have different requirements in terms of which glyphs they use from a font. A font contains some number of character encodings. Each encoding is an internal conversion table for interpreting a specific character set—that is, a way to map a character code to a glyph code for that font.

The reason a font can have multiple encodings is that the requirements for each writing system that the font supports may be different. A writing system is a method of depicting words visually. It consists of a character set and a set of rules for displaying, ordering, and formatting the glyphs associated with those characters. Writing systems can differ in line direction, the direction in which their glyphs are read, the size of the character set used to represent the script, and contextual variation (that is, whether a glyph changes according to its position relative to other glyphs). Writing systems have specific requirements for text display, text editing, character set, and fonts. A writing system can serve one or several languages. For example, the Roman writing system serves many languages, including French, Italian, and Spanish.

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