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Re: [css-text] Arabic letters connecting between elements with display: inline

From: John Hudson <tiro@tiro.com>
Date: Tue, 27 May 2014 11:28:04 -0700
Message-ID: <5384D934.3020701@tiro.com>
CC: "public-i18n-bidi@w3.org" <public-i18n-bidi@w3.org>, "www-international@w3.org" <www-international@w3.org>, "www-style@w3.org" <www-style@w3.org>
On 24/05/14 6:37 PM, James Clark wrote:

> It's clearly a significant limitation of OpenType at the moment that
> cross-font shaping is not possible.  However, fixing this would be a
> very non-trivial change to OpenType layout.

In the case of basic Arabic joining behaviour, though, cross-font 
shaping isn't actually necessary, because the substitutions involved are 
based on character string analysis and selective application of <init> 
<medi> <fina> and <isol> features. Most of the lookups for these 
features in a typical font will be non-contextual, i.e. the 
substitutions do not rely on glyph-level context statements that would 
break across glyph run boundaries occasioned by changes in font, font 
size, or font style.*

This means that it would be conceivable for layout engines to apply 
these features to obtain basic joining form substitutions based on the 
character string, even if that string were divided into multiple glyph 
runs based on font, size, or style changes.

Of course, such basic shaping might not achieve all of the desirable 
results for a particular typeface design, but for many simple styles of 
Arabic type, it would suffice. More to the point, if the behaviour were 
well-specified, it would be something that font makers could choose to 
target or not, depending how important they or their clients considered 
such layout.

JH


* The exception to this general rule is implementation of the <isol> 
feature for the 'ARA ' language system, which is usually contextual, 
distinguishing between two different isolated forms of the letter heh 
depending on whether the letter occurs in true isolation or following a 
right-joining letter.
Received on Tuesday, 27 May 2014 18:28:39 UTC

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