Re: ligature formation across text chunks

The current css3-text has the following under [1]:

UAs may apply letter-spacing to cursive scripts. In this case, UAs should
> extend the space between disjoint graphemes as specified above and extend
> the visible connection between cursively connected graphemes by the same
> amount (rather than leaving a gap). The UA may use glyph substitution or
> other font capabilities to spread out the letters. If the UA cannot expand a
> cursive script without breaking the cursive connections, it should not apply
> letter-spacing between grapheme clusters of that script at all.

 When the resulting space between two characters is not the same as the
> default space, user agents should not use optional ligatures.


This text provides for the Arabic Script case (a cursive script), by
indicating that the space to be extended is between disjoint graphemes.
Furthermore, this text provides for the case that only optional (but not
mandatory) ligatures be disabled when a letter space would apply between the
characters that contribute to the ligature's component allographs.

I believe this text (the last sentence) may be acceptable in Indic scripts
as well, since it only prevents optional conjunct formation in the case that
letter spacing is non-zero. The only issue then is if an author wanted to
use letter spacing *and* still have (some or all) optional ligatures
(conjuncts) used. The alternative in that case would be to use an authoring
tool that performs its own letter spacing and outputs glyphs at specific
origins.

G.

[1] http://www.w3.org/TR/2011/WD-css3-text-20110412/#letter-spacing

On Thu, May 19, 2011 at 4:19 PM, Cameron McCormack <cam@mcc.id.au> wrote:

> Cameron McCormack:
> > For letter-spacing, css3-text says:
> >
> >   This property specifies the minimum, maximum, and optimal spacing
> >   between grapheme clusters.
> >
> > and does not mention ligatures.  It sounds like for compatibility with
> > CSS text layout we should not disable ligaures with letter-spacing.
> >
> > css3-text has a font-kerning property, which is similar to our kerning
> > property, but which doesn’t take lengths: it only has values auto,
> > normal and none.  Ligature formation is not mentioned in that property’s
> > definition, either.
>
> Which is to say: maybe we should reconsider the rule that ligatures
> can’t be formed when these properties are used.
>
> --
> Cameron McCormack ≝ http://mcc.id.au/
>