Re: How to use "smart kerning" font features in Emacs using composition-function-table

[Steve Molitor]

> Do you really need the space character there? What happens if you omit
> it?

If I type:

  x mm

Then the second "m" is supposed to get shifted away from the first m. If
I take away the space character that stops working. Commit Mono looks at
3 character sequences to do the kerning.

> And what about '?' and '!'?

Commit Mono doesn't seem to take them into account when kerning. Other
fonts like Github's Monaspace might, however.

For ligature support, I do need something like this for Commit Mono:

    (set-char-table-range
     composition-function-table
     t
     `([" [,-.:;A-Z_a-z><=!&|+-?/\\]+" 0 font-shape-gstring]))

(I'm probably missing some characters but common ligatures and the
kerning work. I could finesse it to support mode-specific ligatures.)

> Character composition in Emacs was never meant to be used for ASCII
> characters, nor, more generally, for a significant fraction of buffer
> text, except in several scripts (such as Arabic) where all the text
> must be shaped to be legible.

Does performance suffer a lot in those languages, like Arabic?

Thanks again for the responses, I appreciate it. I have more understanding
of how character composition works in Emacs now.

On Sun, Nov 26, 2023 at 2:16 PM Eli Zaretskii <eliz@gnu.org> wrote:
>
> > From: Steve Molitor <stevemolitor@gmail.com>
> > Date: Sun, 26 Nov 2023 13:49:13 -0600
> > Cc: emacs-devel@gnu.org
> >
> > Thanks. This indeed does work (matching against the characters Commit
> > Mono applies its kerning effect to):
> >
> > (set-char-table-range
> >  composition-function-table
> >  t
> >  `(["[ ,-.:;A-Z_a-z]+" 0 font-shape-gstring]))
>
> Do you really need the space character there?  What happens if you
> omit it?
>
> And what about '?' and '!'?
>
> > > But -- and it's a very significant "but" -- displaying text via the
> > > shaping engine is expensive -- it involves calls from the display
> > > engine to Lisp, which then calls back into C. So rules that match any
> > > sequence of ASCII letters will slow down redisplay of almost any text
> > > in Emacs. This is why Emacs by default doesn't use the text-shaping
> > > engine except for scripts that cannot be displayed without non-trivial
> > > character compositions.
> >
> > Which scripts use the text-shaping engine?
>
> Arabic, Korean (Hangul), various Indian scripts (Devanagar, Bengali,
> Gurmukhi), and some others.  Grep for composition-function-table in
> lisp/language/*.el, and you will see them all.
>
> Typically, only specific sequences of codepoints are composed, with
> rare exceptions, even if a script needs compositions.  This is again
> for performance reasons.
>
> > Is it possible to optionally use the text-shaping engine and bypass
> > Lisp in say the "English" script?
>
> No, not the way character composition was designed and implemented in
> Emacs.  Character composition in Emacs was never meant to be used for
> ASCII characters, nor, more generally, for a significant fraction of
> buffer text, except in several scripts (such as Arabic) where all the
> text must be shaped to be legible.  The calls to Lisp are so that
> complex logic of composition and shaping required by some scripts
> could be easily implemented and maintained.