On Sat, Sep 24, 2022 at 9:44 PM Eli Zaretskii <eliz@gnu.org> wrote:

> > From: समीर सिंह Sameer Singh <lumarzeli30@gmail.com>
> > Date: Sat, 24 Sep 2022 21:20:03 +0530
> > Cc: emacs-devel@gnu.org
> >
> > On Sat, Sep 24, 2022 at 8:52 PM Eli Zaretskii <eliz@gnu.org> wrote:
> >
> >  > From: समीर सिंह Sameer Singh <lumarzeli30@gmail.com>
> >  > Date: Sat, 24 Sep 2022 20:32:26 +0530
> >  > Cc: emacs-devel@gnu.org
> >  >
> >  > no, the same would work
> >
> >  Then I don't understand how will HarfBuzz know to apply the first tag
> >  to 'i', but the second one to 'l'.
> >
> > The font file has a GSUB table for this purpose
>
> I'm sorry, I still don't understand.  (I'm not an expert on fonts, so
> I'd appreciate a slightly more detailed explanations.)  Are you saying
> that each tag can be used by a single character in a font?  IOW, no
> feature tag can ever be shared by two or more characters in a font?
>

IIUC, then yes.


> The HarfBuzz docs says:
>
>   The hb_feature_t is the structure that holds information about
>   requested feature application. The feature will be applied with the
>   given value to all glyphs which are in clusters between start
>   (inclusive) and end (exclusive).
>
> This seems to say that we need to set START and END of each feature to
> buffer positions of the characters to which we want the feature to be
> applied.  That is why I said that START and END should be set
> separately for each feature, according to the positions of the
> corresponding characters in the buffer submitted to hb_shape_full.
>

I don't think that is the case, I have attached a slightly modified
harfbuzz-tutorial[1],
where I have defined four features for a,g,i and l respectively with the
same start and end variables.
It works as expected.

You can provide it with the path to a firacode font file and the text which
it has to display and it will produce a png.

[1] https://github.com/harfbuzz/harfbuzz-tutorial