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[css3-fonts/text] Ligature and character variant interaction

From: John Hudson <tiro@tiro.com>
Date: Thu, 18 Nov 2010 09:12:29 -0800
Message-ID: <4CE55E7D.1050401@tiro.com>
To: www-style list <www-style@w3.org>
CC: Joel Kalvesmaki <KalvesmakiJ@doaks.org>, Ross Mills <ross@tiro.ca>
Re. http://dev.w3.org/csswg/css3-fonts/


In thinking through implementation of Byzantine sigillographic 
transcription using CSS3 font module properties -- which is turning out 
to be a great test case --, I came up with the following situation, and 
want to check whether this is addressable with the proposed CSS syntax 
for character variants and ligatures.

The situation is that a sequence of characters, e.g. the uppercase Greek 
letters ΚΑΙ (Kappa Alpha Iota), may be represented by a ligature glyph 
/Kappa_Alpha_Iota/, which is implemented in the font using the 
Discretionary Ligatures <dlig> feature and in CSS using the 
font-variant-ligatures property with the additional-ligatures value.

But the ligature representing ΚΑΙ may take two different forms, and the 
font provides a variant glyph /Kappa_Alpha_Iota.2/, which is mapped as 
an enumerated variant in one of the font's Character Variant features, 
say <cv60>, implemented in CSS using the character-variant property. 
Note that this is a variant of the ligature form, not a variant of the 
underlying letters, i.e. the ligature formation precedes the variant 
selection. The variation in ligature form is not dependent on the form 
of the underlying characters, i.e. it is a free variant of the ligature 
representation not influenced by e.g. the particular shape of the Kappa.

Is there the potential for ambiguity in property interaction, i.e. might 
the character-variant property be interpreted as applying to the 
underlying letters rather than to the ligature? Is this something that 
is influenced by the ordering of CSS properties, or is it something that 
will be determined solely by the ordering of the GSUB lookups in the 
font? [In the case of this font, all ligature substitutions will precede 
any character variant substitutions. I can imagine other fonts in which 
the variant form of the underlying letters dictates the ligature form, 
so in that case the character variant lookups would precede the ligature 
lookups. I can also imagine fonts that use a combination of both 
approaches, with staggered lookups.]

JH
Received on Thursday, 18 November 2010 17:13:05 GMT

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