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Atypographic approximation is a replacement of an element of thewriting system (usually aglyph) with another glyph or glyphs. The replacement may be a nearlyhomographic character, adigraph, or a character string. An approximation is different from atypographical error in that an approximation is intentional and aims to preserve the visual appearance of the original. The concept of approximation also applies to theWorld Wide Web and other forms of textual information available via digital media, though usually at the level ofcharacters, not glyphs.
Historically, the main cause of typographic approximation was a low quantity of glyphs (such asletterforms andsymbols) available for printing.In the age of World Wide Web anddigital typesetting, especially after the advent ofUnicode and enormous amount ofcomputer fonts, typographic approximations are usually caused either by low ability of humans to distinguish and find needed symbols or by inadequate replacement patterns inword processors,[1] rather than by lack of available characters.
| Normative: | 3 × 2 − 1 |
| Approximated: | 3 x 2 - 1 |
| An ASCII approximation of anarithmetical expression | |
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Ontypewriter, several characters were merged due to limited size of glyph repertoire. Several modern computingcharacters appeared by merger of different symbols, such as the "typewriter"apostrophe,', which can denote a proper apostrophe,’, a singlequotation mark, or theprime symbol.
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Some typewriters havenon-spacing keys for use asdiacritical marks. After the typist pushes, say, acute accent ◌́ the caret does not move. This allows the typist tooverstrike this mark by aspacing letter, say,e and obtainé, an accented letter. Due to geometrical restrictions of amonospaced font, the result could not always be perfect. For example, overstriking was unlikely to be a feasible method to produceuppercase accented letters, such asÉ.
Overstrike was used online printers for the same function. This contributed to standardization of such characters asU+0060 `GRAVE ACCENT.
Overstrike of the same letter was used to simulateboldface letters on line printers.
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| An ASCII approximation (above) may be ugly, but giving some representation of several symbols. Replacements of non-ASCII characters (others than default "*") are highlighted in yellow. | ||||||||
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TheUS-ASCII character set and other variants ofISO/IEC 646 contains 95 graphic characters. It is comparable with a (Latin script) typewriter and insufficient for a qualitytypography. But high availability and robustness of ASCII character encoding prompted computer users to invent ASCII substitutes for various glyphs.
The following ASCII characters are used to approximate certain characters. Note that there are many Latin letters that are homographic to letters of other scripts, however those Latin letters are not listed below.
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There exist various approximation fortypographic alignment. For example,justification may be emulated with inserting ofspaces, and flush-right alignment may be done by padding with spaces.
There are various techniques for approximation oftables (historically used fortext mode displays), such asbox-drawing characters.