Re: When and how to register various font backends

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: emacs-devel@gnu.org,  YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>
> Date: Fri, 14 Jun 2019 11:14:17 +0200
> 
> Currently under X11 the font backends end up as
> 
> (xft xfthb x)
> 
> Reordering that to put xfthb first is a matter of reordering the
> register_font_driver calls in Fx_create_frame

That's true, but we don't want to have 3 font backends in the list,
because then looking for a font that isn't available on the system
will take much longer (Emacs tries to find the font with each backend
in turn).  We want to have only 2 backends by default.

> Since xfthb is strictly better than xft, as far as I can tell, and we
> want people to actually use it, we should just prefer it for emacs-27,
> and remove the xft and x backends in emacs-28.

I'm not sure removing x (and gdi on Windows) is a good idea, even in
Emacs 28.  I understand (more accurately, was told very recently) that
HarfBuzz was designed to be able to work with any font, not just OTF,
but I'm not sure our integration of HarfBuzz allows that.  We should
actively test that with old fonts, like bitmapped fonts and BDF,
before we make the decision.  For example, I suspect the methods we
currently use for finding fonts suitable for HarfBuzz filter out
non-OTF fonts (at least on Windows, this is definitely so).

> default-frame-alist and similar will still be available for people
> who absolutely want to turn it off

The main issue at hand is how to allow users to specify the
non-preferred backends when they want that.  See Yamamoto-san's
message about that.