2016-11-09 0:42 GMT+01:00 Richard Wordingham < [email protected]>:
> On Wed, 9 Nov 2016 00:00:01 +0100 > Philippe Verdy <[email protected]> wrote: > > > 2016-11-08 9:30 GMT+01:00 Richard Wordingham < > > [email protected]>: > > > > > TUS Section 2.11 says, "If the combining characters can interact > > > typographically—for example, U+0304 combining macron and U+0308 > > > combining diaeresis — then the order of graphic display is > > > determined by the order of coded characters (see Table 2-5). > > > By default, the diacritics or other combining characters are > > > positioned from the base character’s glyph outward". > > > The interpretation of "If the combining characters can interact > > typographically" should be better read as "If the combining > > characters have the same non-zero combining class or any one of them > > has a zero combining class". > > The combining marks in question both have canonical combining class 0. > > > But now normalization is everywhere and causes the pairs using the > > condition above to be freely reordered (or decomposed and recomposed, > > meaning that the encoding order is NOT significant at all). > > I believe a renderer is permitted to treat canonically equivalent > sequence differently so long as it does not believe it should treat > them differently. However, that is irrelevant to this case. > This is DIRECTLY relevant to the sentence in TUS you quoted, which is all about combining characters encoded after the base letter and often have non-zero combining classes and are reorderable But evidently this sentence in TUS is not relevant to "prepended" combining marks that are all with combining class 0, here "prepended" meaning: encoded before the base character, but not after it even if they are visually combining before it, as is the case for wellknown Indic vowels that have now non-zero combining classes that allow them to be reordered before other combining marks when normalizing, but still remaining encoded after the base consonnant). What I want to say is that this sentence in TUS is quite ambiguous: it speaks about graphic interaction, but this is not really encoded in text sequences and forgets the the effect of combining classes on combining sequences, which NEVER considers any actual graphic interaction (simply because it is not specified and the actual graphic interactions may depend on font styles (notably in honorific Arabic typography using very complex layouts, but even within the Latin script when using decorated font styles or custom ligatures where complex also interactions occur, including on larger spans than clusters, such as full words).
