Multiple Preposed Marks

[Richard Wordingham]

On Wed, 9 Nov 2016 03:26:51 +0100
Philippe Verdy <verdy_p at wanadoo.fr> wrote:

> 2016-11-09 0:42 GMT+01:00 Richard Wordingham <
> richard.wordingham at ntlworld.com>:

> > 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.