bug#21028: Slow font rendering in emacs

[Eli Zaretskii <eliz@gnu.org>]

> Cc: 21028@debbugs.gnu.org
> From: Ralf Jung <post@ralfj.de>
> Date: Sat, 22 Apr 2017 10:54:56 +0200
> 
> >> Of course this is just an excerpt of the characters we use.  From all I
> >> can tell, the general rule is "if the character is supported by Fira,
> >> use that font; else of it is supported by DejaVu, use that font; else do
> >> <no idea what it does>"
> > 
> > As you have seen, this does work as you want, but it's slow.  We are
> > talking about getting you the same functionality, but faster.  That
> > comes for a price of more accurate fontset setup.
> 
> Well, we also have a patch fixing that slowness, so it doesn't seem to
> be an inherent problem, just some implementation artifact.

That patch has been applied, in case you weren't tracking this bug.

> >> I don't think the font is to blame here.  After all, other applications
> >> manage to deal with exactly the same fonts just fine.
> >> Unfortunately, I don't know *how* everybody else is selecting fonts, I
> >> only know they do a better job at it than emacs, and they have no
> >> problem dealing with fonts that only partially support some blocks.
> >> Probably fontconfig is doing most of the work here, but I am really just
> >> guessing.
> > 
> > Most other applications don't deal with multi-lingual text, so their
> > job is easier.  Emacs attempts to solve a harder problem here.
> 
> What exactly does this mean?  I sure would expect all these editors to
> correctly display text that mixes Latin, Greek, Cyrillic and Japanese
> characters.

Most editors assume each file will only ever include text in a single
language.  Emacs explicitly tries to do better when several languages
and scripts are mixed in the same file/buffer.

Emacs also has some rules for selecting fonts based on cultural
preferences, so it could use different fonts for the same Unicode
codepoints in different locales.

> I believe they can handle this, but have to admit I did not try that
> (mostly for lack of a personal use-case).  Is there an example that
> can be used to test this?

I'd begin with the HELLO file.