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Re: ligature formation across text chunks

From: Glenn Adams <glenn@skynav.com>
Date: Thu, 19 May 2011 22:43:35 -0600
Message-ID: <BANLkTi=8Lc8JUAXD0zmMF3_uXY51PLQPPA@mail.gmail.com>
To: Rik Cabanier <cabanier@adobe.com>
Cc: Cameron McCormack <cam@mcc.id.au>, Tavmjong Bah <tavmjong@free.fr>, Vincent Hardy <vhardy@adobe.com>, "public-svg-wg@w3.org" <public-svg-wg@w3.org>
The cited text indicates that if space between characters is not the default
(i.e., letter spacing has been applied), then the UA should NOT use an
optional ligature. Since 'ff' is an optional ligature, then letter spacing
should block ligature formation in this case.

However, if letter space does not apply, then, on the presumption that the
UA is performing the character to glyph mapping process, it may choose to
use the ligature.

The question here may be whether there should be some mechanism by which the
author can indicate that no character to glyph mapping process be applied,
i.e., other than the nominal CMAP process which is necessary. That is, how
can the author declare that no substitution (including ligatures) should
occur even in the absence of letter spacing.

G.

On Thu, May 19, 2011 at 10:17 PM, Rik Cabanier <cabanier@adobe.com> wrote:

> > > > When the resulting space between two characters is not the same as
> > > > the default space, user agents should not use optional ligatures.
> >
> > I missed that, thanks.
>
> I do not believe that us true. For certain ligatures, the characters will
> be closer together than the individual glyphs.
> See the attached PDF as an example.
> This is also the reason that the auto-generation of ligatures in SVG is not
> a good idea because it will create gaps or overlap with subsequent glyghs.
>
> Rik
>
>
> > -----Original Message-----
> > From: public-svg-wg-request@w3.org [mailto:public-svg-wg-
> > request@w3.org] On Behalf Of Cameron McCormack
> > Sent: Thursday, May 19, 2011 9:03 PM
> > To: Glenn Adams
> > Cc: Tavmjong Bah; Vincent Hardy; public-svg-wg@w3.org
> > Subject: Re: ligature formation across text chunks
> >
> > Glenn Adams:
> > > The current css3-text has the following under [1]:
> > ...
> > > > 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.
> >
> > I missed that, thanks.
> >
> > > 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.
> >
> > That sounds reasonable.
> >
> > > 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.
> >
> > Maybe font-variant can be used to force optional ligatures to be used in
> this
> > case?
> >
> > --
> > Cameron McCormack ≝ http://mcc.id.au/
>
>
Received on Friday, 20 May 2011 04:44:25 GMT

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