From: "busmanus" <[EMAIL PROTECTED]> > I am not sure about the relevance of the Meteg problem, but I do know > about a case, where different relative positions of the same > diacriticals are used for conveying a semantic distinction. In a big > reference work about verse metrics in the world's languages (Erika > Szepes - Istv�n Szerdahelyi: Verstan, published by Gondolat, Budapest, > 1981), when discussing quantitative metrics, a macron above a breve is > used for denoting a neutral syllable of the metrical pattern that is > more frequently filled in by a short syllable than by a long one and > a breve above a macron is used for the reverse, i.e. the difference in > the combinations provides statistical information. > > Actually, these signs are typically (although not inevitably) spacing > characters, but I don't think it makes a significant difference in this > perspective.
When the relative ordering of diacritics becomes significant, but they have the same non-zero combining class, Unicode already has all the features needed to preserve both the logical/semantic and graphical distinction, because this relative order is preserved. However, this relative order does not specify how these diacritics stack on the base letter. In your example with macron and breve, they both share a "above" combining class, and generally most renderers will stack them vertically, with the first above-diacritic centered below the second above-diacritic. (Some fonts or renderers could rather render them side-by-side, with the first diacritic on the starting side for the the current writing direction, and the second diacritic on the ending side; this is another stylistic option, which would preserve visually the semantic distinction, so this does not change the problem, and not a problem of Unicode itself; this case would happen most probably with Semitic scripts, or with Asian texts written vertically). The only problem will happen if the semantic distinction cannot be rendered visually, because the diacritics share the same combining class (so the same logical "position"), but not the same visual position (in some cases, even in the Latin script, some above-diacritics are sometimes rendered on the right side rather than above.) And we have some cases where a below-diacritic like a cedilla is preferably shown above-left, where it could compete with another diacritic. This is probably a pedantic theorical case where the default Unicode combining classes are inappropriate to represent correctly the interaction between diacritics. For these reasons, I really suggest to keep CGJ as a way to encode and force the relative order of diacritics, and forget any other use of CGJ for something else than encoding a logical relative order of distinct logical pairs of diacritics which would otherwise become reordered identically, breaking the semantic of the text. I strongly suggest that CGJ not being used for something else than forcing the relative order of combining characters (and as a consequence, allowing CGJ only between two combining characters, but not just before or after a base character; should these two sequences be acceptable, as they are already valid in Unicode, they will represent distinct semantics for the base character of the combining sequence). As a consequence, CGJ will be inappropriate to encode a logical/semantic difference between umlaut and tr�ma for example (and the special treatment of umlaut versus tr�ma/diaeresis in German, or of the accute accent in Polish, for collation purpose makes CGJ inappropriate for encoding these logical distinctions...) Then, the problem remains: how can we encode logical/semantic distinctions of diacritics which have been unified in Unicode, but are clearly not unified in some languages (German and Polish are such examples...)???? The existing variation selectors VS1..VS256 are not an option here (as they are breaking default grapheme clusters, meaning lots of troubles for text editors or text selection). Isn't it a place where we would really need some combining variation selectors (CVS1..CVS16 at least), to be used in applications or texts that need such distinctions?
