From: Peter_Constable@sil.org
Date: Wed Jul 23 2003 - 18:46:04 EDT
Peter Kirk wrote on 07/23/2003 09:24:12 AM:
> From Unicode 4.0 section 3.11,
> http://www.unicode.org/book/preview/ch03.pdf: "The particular numeric
> value of the combining class does not have any special significance; the
> intent of providing the numeric values is /only/ to distinguish the
> combining classes as being different, for use in equivalence
> comparisons. ... The canonical order of character sequences does /not/
> imply any kind of linguistic correctness or linguistic preference for
> ordering of combining marks in sequences." There is therefore no reason
> for combining classes to reflect ordering.
But, the combining classes do have to reflect relative visual positions wrt
the base -- that's what determines which things do or do not interact
typographically. (Things that are above right will interact typographically
with one another because they are competing for the same visual space, but
not with things that are above, above left, below right, etc.) The classes
will define *some* order, and as the classes are visually based, that order
cannot be the logical order for both RTL and LTR text.
> The problem, if there is one,
> is with rendering software which expects to receive an input stream in a
> logical order although Unicode implies that the order is arbitrary,
No, the problem I'm referring to has to do with editing, when certain
protocols recommend (and all but guarantee) that data be stored in a
particular order. While the particular numerical values of combining
classes are of no special significance and were not intended for anything
more than to distinguish between different classes for use in equivalence
comparisons, is just happens that that order will be used for storage order
of data, and that will be the visual LTR order, making it the opposite of
what users will likely think in terms of for RTL text.
So, even if rendering software is performing its own reordering, the issue
I'm trying to point out is that *editing* software may also need to do its
own reordering in order to provide the kind of editing experience that
users expect. Otherwise, users may have a difficult time knowing what order
to type things, and what will get deleted the next time they backspace.
- Peter
---------------------------------------------------------------------------
Peter Constable
Non-Roman Script Initiative, SIL International
7500 W. Camp Wisdom Rd., Dallas, TX 75236, USA
Tel: +1 972 708 7485
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