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RE: ISSUE-299: Cluster matching 1b

From: Phillips, Addison <addison@lab126.com>
Date: Fri, 13 Sep 2013 04:25:56 +0000
To: John Daggett <jdaggett@mozilla.com>, Richard Ishida <ishida@w3.org>
CC: W3C Style <www-style@w3.org>, www International <www-international@w3.org>
Message-ID: <7C0AF84C6D560544A17DDDEB68A9DFB5170E33B5@ex10-mbx-36006.ant.amazon.com>
John Daggett wrote:
> 
> > 5.3. Cluster matching
> > http://www.w3.org/TR/2013/WD-css-fonts-3-20130711/#cluster-matching

> >
> > In the numbered list, what should follow if 1b holds true?
> 
> Um, you select that font and great joy ensues? ;)  That's the goal of both
> standard font matching and the font matching for clusters. Steps
> (2) and (3) are prefaced with "if no font was found..." so I don't see any
> ambiguity if that's what you're thinking there's something ambiguous here.

I think we understand what the rule 1b is trying to say. But the text is slightly elliptical. It suggests rather than stating what is plainly intended: if you have a pre-composed or otherwise canonically matching glyph for the grapheme cluster being considered, use it to draw the grapheme cluster. 

> 
> > Or is the meaning that if the font has a glyph for the precomposed
> > character that is canonically equivalent to the sequence of
> > characters, then that glyph should be used (without changing the
> > sequence of characters itself). That would seem to make more sense.
> 
> Yes, that's exactly what section 5.2 is addressing.  For a *sequence* of
> codepoints, a glyph for a canonically equivalent character can be used.  This is,
> in fact, already the norm for most modern layout engines.
> 
> > Otherwise, it's not clear why you would select the font for this sequence.
> 
> ??? Sorry, I don't quite get what you're saying here.

The problem is that rule 1b states:

--
If a sequence of multiple codepoints is canonically equivalent to a single character and the font supports that character, select this font for the sequence.
--

"Select the font" isn't the same as "select the font and use the canonically equivalent character glyph for the sequence". We know (and greatly appreciate!) that most modern layout engines already do this. But if you just select the font and don't render the grapheme sequence with the corresponding glyph you'll get tofu. Perhaps that's overly obvious?

Addison

Received on Friday, 13 September 2013 04:26:35 UTC

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