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

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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