bug#50177: Support U+20DD COMBINING ENCLOSING CIRCLE

[handa]

In article <8335qyx0f3.fsf@gnu.org>, Eli Zaretskii <eliz@gnu.org> writes:
> > I haven't looked at the machinery here at all -- is there a fundamental
> > reason why Emacs can't combine glyphs from different fonts?

> The basic reason is that glyphs from different fonts cannot combine
> well because they were designed to look differently, and so offsets
> don't match.  That is almost certainly the reason when we use our
> fallback composition code in composite.el.  I'm less sure about modern
> shaping engines like HarfBuzz -- we should ask their developers to be
> sure; feel free to open an issue/question on their GitHub.

> CC'ing Handa-san, in the hope that he could explain better why we
> disallow character composition from different fonts.

The main reason is what Eli wrote. An opentype font contains rules to
tell how to compose two glyphs in that font.  But such rules are
specific to that font, and there's no way to combine rules of different
fonts.  So, an opentype rendering engine does not work for different
fonts.

And, when we artificially compose characters from different fonts, there
is a possibility that the resulting image looks like a different
character which I think is worse than not composing.

---
K. Handa
handa@gnu.org