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

Thank you for the great feedback, James and Andrew.

I agree that the letter-spacing property in CSS Text Level 3 still has a lot of work until it becomes really a great tool for all over the world. East Asian scripts are in the similar situation too; Japanese for instance has complex rules to apply different spacing by adjacent code points. We once tried to define it in Level 3, but that didn't fit well and we decided to defer it to future levels.

I also tried an idea to define expansion opportunities for justification either in W3C or in Unicode, which may or may not be applicable to letter-spacing too I don't know, but I couldn't find good direction there either. Maybe it's too complex to define even at Unicode level given James's explanation.

So, the best we could do at this point is, as fantasai suggested, show one baseline, but allow any tailoring, and put links to as many references to script-specific rules as possible to help implementers to find them if they want to.

CSS Text Level 3 has a lot of improvements over CSS 2.1 in several areas. I would like to put this spec into a stable state so that people can enjoy it, and seek for better justifications and spacing in future levels.

Does this sound reasonable?

/koji


On Sep 25, 2013, at 3:08 PM, Andrew Cunningham <lang.support@gmail.com<mailto:lang.support@gmail.com>> wrote:

In Cham, Leke, Myanmar and other scripts, possible break points in not at owrd boundaries would occur at syllable boundaries, and considering the occurrence of possible existence of final consonants for some languages in those scripts, just operating on the basis of a grapheme cluster can be problematic, since a syllbale may not be limited to a single grapheme cluster.
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On 24 September 2013 19:51, James Clark <jjc@jclark.com<mailto: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<mailto: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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