Re: Display of decomposed characters

[Eli Zaretskii <eliz@gnu.org>]

> From: Philipp <p.stephani2@gmail.com>
> Date: Fri, 19 Mar 2021 17:37:31 +0100
> Cc: help-gnu-emacs@gnu.org
> 
> >>>> What if Emacs ignored font lookup for combining characters and always picked the font of the previous base character?
> >>> 
> >>> What would that produce if the font of the previous character didn't
> >>> have a glyph for the accent?  The accent will disappear, or maybe will
> >>> be displayed as "tofu", right?  Does that sound like a good strategy?
> >> 
> >> Can't the shaping engine produce fake compositions in that case?
> > 
> > What do you mean by "fake compositions"? what would they entail, and
> > which glyphs would they use?
> 
> For example, the shaping engine could use U+00A8 (assuming it's available in the font), but place it on top of the base glyph, without horizontal shift.

First, we were talking about the case where U+00A8 is NOT available in
the font.  (If it _is_ available, then this whole discussion is
pointless, because things already work well in that case.)

> (At least that would be a possibility; I don't know whether Harfbuzz actually does that.)
> That would still produce suboptimal results, but probably slightly better ones.

I don't understand what you are describing here.  If the font does
have U+00A8, what you describe already happens.  If the font doesn't
have the glyph, what can the shaper do?

The horizontal shift happens because we use U+00A8 from a different
font.  Placing a glyph from a different font on top of a base glyph is
in general an impossible task, because different fonts have different
ways of describing the points where the accents shall be placed on top
of base characters.

> The optimal approach (for this case) would still be to try out composition before font selection, and use that if it works.

I tried to explain why that's not possible, but I guess I failed
miserably.

> I should note that Emacs is not alone in producing suboptimal results in this case; other GUI programs on that systems appear to either perform the fake composition I mentioned before, or no composition at all (placing the base and combining characters next to each other).

Which should tell us something about the issue and the ways it can and
cannot be solved.