Precomposed Character - Comparing Precomposed and Decomposed Characters

Comparing Precomposed and Decomposed Characters

In the following example, there is a common Swedish surname Åström written in the two alternative methods, the first one with a precomposed Å (U+00C5) and ö (U+00F6), and the second one using a decomposed base letter A (U+0041) with a combining ring above (U+030A) and an o (U+006F) with a combining diaeresis (U+0308). To illustrate the difference, the precomposed characters are here displayed in green and the decomposed base letters in black; depending on your browser, the decomposed combining diacritics may be shown in orange or black.

1. Åström (U+00C5 U+0073 U+0074 U+0072 U+00F6 U+006D)
2. Åström (U+0041 U+030A U+0073 U+0074 U+0072 U+006F U+0308 U+006D)

Except for the different colors, the two solutions are equivalent and should render identically. In practice, however, some Unicode implementations still have difficulties with decomposed characters. In the worst case, combining diacritics may be disregarded or rendered as unrecognized characters after their base letters, as they are not included in all fonts. To overcome the problems, some applications may simply attempt to replace the decomposed characters with the equivalent precomposed characters.

With an incomplete font, however, precomposed characters may also be problematic – especially if they are more exotic, as in the following example (showing the reconstructed Proto-Indo-European word for "dog"):

1. ḱṷṓn (U+1E31 U+1E77 U+1E53 U+006E)
2. ḱṷṓn (U+006B U+0301 U+0075 U+032D U+006F U+0304 U+0301 U+006E)

In some situations, the precomposed green k, u and o with diacritics may render as unrecognized characters, or their typographical appearance may be very different from the final letter n with no diacritic. On the second line, the base letters should at least render correctly even if the combining diacritics could not be recognized.

OpenType has the ccmp "feature tag" to define glyphs that are compositions or decompositions involving combining characters.