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Re: [css-text] Clusters for letter spacing in Thai and other complex scripts

From: Andrew Cunningham <lang.support@gmail.com>
Date: Tue, 24 Sep 2013 23:13:51 -0700
Message-ID: <CAGJ7U-V4_Wa0HA8RxOzA7i50gBRsOac_nKcj0f-q9V15SWjzVA@mail.gmail.com>
To: James Clark <jjc@jclark.com>
Cc: "www-style@w3.org" <www-style@w3.org>
Letter spacing for Myanmar script based on grapheme clusters is ok when
grapheme clusters coincide with syllables, but letter spacing within a
syllable is less than ideal


On 24 September 2013 19:51, James Clark <jjc@jclark.com> wrote:

> I don't think this is the right way to fix the problem.  The UAX29
> definition of extended grapheme cluster works just fine in Thai/Lao
> when what you want is a grapheme cluster.  The issue is that letter
> spacing is not a logical operation on characters; it's a visual
> operation on glyphs. In other words, there are two distinct kinds of
> cluster:
>
> a) there is a _logical_ cluster of _characters_, which is used for
> selection, cursor movement and other editing operations; this is the
> UAX29 extended grapheme clusrer
>
> b) there is also a _visual_ cluster of _glyphs_, which is what you
> need for letter-spacing
>
> In many scripts, these cooincide, but Thai/Lao shows that they don't
> always do so.
>
> So I think the right approach is to fix the definition of
> letter-spacing to say that the units between which you add extra space
> will not always correspond exactly to extended grapheme clusters:
> implementations should do the typographically correct thing for a
> particular script.
>
> James
>
> > On Sep 24, 2013, at 8:07 AM, fantasai <fantasai.lists@inkedblade.net>
> wrote:
> >
> >> On 09/21/2013 10:37 PM, James Clark wrote:
> >> The Editor's Draft for CSS Text says that letter-spacing is applied
> between adjacent "characters", where a "character" is
> >> defined as a UAX29 extended grapheme cluster. This doesn't do the right
> thing in Thai in the case of SARA AM (U+0E33).
> >>
> >> For example, to properly letter-space the word คำ (0E04 + 0E33), [...]
> >
> > The spec does explicitly allow for tailoring; UAX29 is just a
> > baseline requirement. However I'm not sure that the tailoring
> > you're describing is quite in line with the kind that's allowed
> > by UAX29, so I've broadened the wording a bit and added your
> > example here:
> >
> >  http://dev.w3.org/csswg/css-text/#grapheme-cluster
> >
> > If you have a spec that I can point to normatively, I'm happy
> > to do that. :) But otherwise, I think the example will have
> > to suffice.
> >
> > Please let me know
> >  1. If this is satisfactory, or you want something else.
> >  2. If it's ok for us to use your wording verbatim.
> >
> > Thanks!
> >
> > ~fantasai
>
>


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Andrew Cunningham
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(Social and Digital Inclusion)
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Received on Wednesday, 25 September 2013 06:14:20 UTC

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