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Re: [css3-fonts] font matching of grapheme clusters

From: Eric Muller <emuller@adobe.com>
Date: Tue, 25 Sep 2012 10:29:33 -0700
Message-ID: <5061E9FD.1070100@adobe.com>
To: <www-style@w3.org>

On 5.2, Cluster matching:

In 1.a, the term "completely supported" is a bit ambiguous. One could 
consider that a font without any explicit support for variation 
selectors supports all variation sequences, on the grounds that the 
variation selector is only encouraging a certain display, not mandating 
it. Given 2.a, I suspect that you intend for the font to provide an 
explicit guarantee that the variation sequence is honored. You could add 
"(including variation selectors, if any)" after "completely supported", 
if I understood your intent correctly. I also suspect that you actually 
meant something stronger, i.e. "has entries in the cmap for each of b, 
c1, c2, ..."

Another case to consider is the joiner characters. What does it mean to 
completely support the sequence <f, ZWJ>?

Finally, I am uneasy with the notion that "a font supports or does not 
support x". Text is not presented to fonts but to layout engines. It's 
perfectly possible to have a layout engine that interprets joiner 
characters, and regulates the application of ligating features 
accordingly. In that case, just looking at the font does not tell the 
whole story. Viewed another way, your steps 1.a and 1.b in combination 
are an attempt to describe the layout engine, but I don't think you can 
be exhaustive; if my implementation is more elaborate, e.g. it can deal 
with the sequence <U+00EA ê LATIN SMALL LETTER E WITH CIRCUMFLEX, U+0308 
◌̈ COMBINING DIAERESIS> when the font only maps e and the combining 
marks, do I become non conformant?

Eric.
Received on Tuesday, 25 September 2012 17:30:34 GMT

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