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Re: [css-text] I18N-ISSUE-333: 'letter-spacing' and Arabic

From: John Hudson <tiro@tiro.com>
Date: Fri, 30 May 2014 11:08:10 -0700
Message-ID: <5388C90A.5020401@tiro.com>
To: Richard Ishida <ishida@w3.org>, Koji Ishii <kojiishi@gluesoft.co.jp>, Jonathan Kew <jfkthame@googlemail.com>
CC: Najib Tounsi <ntounsi@emi.ac.ma>, "Phillips, Addison" <addison@lab126.com>, "CSS WWW Style (www-style@w3.org)" <www-style@w3.org>, www International <www-international@w3.org>
On 30/05/14 5:08 AM, Richard Ishida wrote:

> Is it really expected that implementations decompose optional ligatures
> when 'stretching' Arabic text? Are we just making assumptions here, or
> is this based on some typographic tradition?

The whole notion of 'required' and 'optional' ligatures in Arabic is 
extraneous to the script tradition. Indeed, a ligature is a particular 
(obsolete) technological mechanism (a glyph/sort representing more than 
one character), and a function to disable 'optional ligatures' presumes 
that digital fonts provide ligatures that could be disabled. An 
increasing number of digital fonts do not (and not only for Arabic) and 
instead use contextual variants to shape text, even to shape 'required 
ligatures'. Obviously, you can't 'turn off ligatures' if a font contains 
nothing that indicates a particular typeform is a 'ligature'.

Given that what is referred to as 'ligation' in Perso-Arabic scripts is 
actually just the usual contextual behaviour of specific sequences of 
letters according to the conventions of particular styles of text, 
trying to disable them at a mechanical level, regardless of the style of 
text or how it has been implemented in a given font.

> (I know that high end justification systems may create ligatures as part
> of the justification process, but) from what I've seen it doesn't
> necessarily follow to me that there is always a logical first step to
> the stretching of arabic text that dismantles optional ligatures.

Right. If it is a viable option, it would be so in the context of a 
particular style and a particular font implementation, not something to 
be done at a higher level independent of that context. It's the sort of 
thing that the OpenType JSTF table could conceivably assist, but work 
needs to be done to define both font and software expectations in this 
regard.

JH
Received on Friday, 30 May 2014 18:08:43 UTC

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