Re: Ligature support

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: gregory@heytings.org,  mattiase@acm.org,  emacs-devel@gnu.org,
>   schwab@linux-m68k.org,  stefankangas@gmail.com,  raman@google.com
> Date: Fri, 05 Nov 2021 15:38:12 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > So for having ligatures you need to set up composition-function-table
> > for the characters that could/should ligate.
> 
> Yes.  And to do that, we can parse the tables in the font, apparently.
> It's not obviously clear how, but it looks like harfbuzz has the
> functions needed.

Why do we need to depend on the font?  As I said, if the font doesn't
have a ligature, font-shape-gstring will return nil, and we will then
display the characters as individual glyphs.  Isn't that what is
expected?

> > But please read the TODO item: there are pitfalls here that we need to
> > solve before this could be considered a reasonable user-level
> > feature.  Setting up composition-function-table to produce ligatures
> > for, say, "fi" and "ff" is easy for testing, but if you do that in
> > production, every word with these characters, everywhere on display in
> > Emacs, will display as that ligature, and that is hardly TRT.  We need
> > to make this more fine-tunable.  TODO has the details.
> 
> I guess it needs some more customisation to be a per-font thing?

I'm not sure it's per font.  I think it should be per buffer/mode, and
perhaps also something special for the mode line.

> But first we need to parse the Gsub/Gpos tables so that we can get
> started at all.

I don't think we should or need to do that.  See above.