[HarfBuzz] Support for Stylistic Sets

[Richard Wordingham]

On Sat, 14 Sep 2019 21:15:04 +0300
Eli Zaretskii <eliz at gnu.org> wrote:

> > Date: Sat, 14 Sep 2019 18:13:25 +0100
> > From: Richard Wordingham <richard.wordingham at ntlworld.com>
> > 
> > I think it's safe to specify the use of unsupported features, in
> > which case this is a luxury feature.  
> 
> you mean, specifying an unsupported feature will not cause hb_shape to
> fail, but instead just use the nominal glyphs?

Essentially yes, i.e. unsupported features will simply be ignored.

> > One complication is that features are provided by a font on a (per
> > script) per language basis.  
> 
> Why is that a complication?  The user who requests the feature should
> do so only for text of a suitable script, no?

It means that support questions need to be framed in terms of both
script and language.  One can't just ask if a font supports feature
'ss05' or if it supports the feature for Cyrillic - the answer will
depend on the script and language.

> Emacs currently leaves it to HarfBuzz to guess the language, so I
> don't think this is an issue.

Does HarfBuzz guess the language?  I would expect it to default to the
'default language', which shall be supported.  As that is all that those
dependent on Microsoft renderers will get, it should be good enough for
many users.

Richard.