On Wed, 9 Nov 2016 03:26:51 +0100 Philippe Verdy <[email protected]> wrote:
> 2016-11-09 0:42 GMT+01:00 Richard Wordingham < > [email protected]>: > > 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 As you pointed out, it most clearly addresses the case of two combining marks with the same canonical combining class, and obviously in such a case the sequence is not 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). I can't guess what you mean: (a) The combining marks in question *follow* the base consonant, but are rendered before it. 'Preposition' is a property of abstract characters, not of codepoints. (b) All characters with an Indic Positional Category of 'left' (or similar) have canonical combining class 0. There is a simple example of the base outwards rule in the Tai Tham script. The only way of encoding Northern Thai /pʰɛː/ 'to chanɡe' with the glyphs of U+1A38 TAI THAM LETTER HIGH PA, U+1A55 TAI THAM CONSONANT SIGN MEDIAL RA and U+1A6F TAI THAM VOWEL SIGN AE acceptable to the Universal Shaping engine is <U+1A38, U+1A55, U+1A6F>, and the visual order is the reverse of the encoding order. Unfortunately, it could be argued that the encoding order is independent of the visual order. Richard.
