Re: Composed Sequences (was: Manually parsing char-tables)

[Eli Zaretskii]

> Date: Sat, 26 Feb 2022 00:28:37 +0000
> From: Richard Wordingham <richard.wordingham@ntlworld.com>
> 
> I still haven't found the code where the difference occurs, but I now
> have a better idea of what is going on.  It seems that runs with the
> same value of the composition property ('composed sequences') are
> sequences of clusters for the font that match a regular expression
> given in composition-function-table.

(Please don't use "composition property" in this context, because it's
confusing: the 'composition' text property does exist in Emacs (it's
an old and now deprecated way of composing characters), but it is not
relevant to this discussion, which instead focuses on what is known in
Emacs as "automatic composition".)

> Different renderers give different clusters, and thus, by default,
> different cursor motion!

Not "different renderers", but "different fonts".

And yes, the grapheme clusters produced by the text shaping engine
(HarfBuzz etc.) and displayed by the Emacs display code indeed
crucially depend on the font.

> The reason Arabic seemed different is that when lam+hah appears to
> ligate, what is happening (at least with Amiri) is that substitutions
> are made which give the effect of a ligature, while remaining two
> distinct glyphs.

Yes, I see that as well.  "C-u C-x =" should tell you whether ligation
happened or not.  What you see is normal, I think: Emacs obeys the
decisions of the font designers.