Re: Display of decomposed characters

[Philipp <p.stephani2@gmail.com>]

> Am 18.03.2021 um 16:01 schrieb Eli Zaretskii <eliz@gnu.org>:
> 
>> From: Philipp <p.stephani2@gmail.com>
>> Date: Thu, 18 Mar 2021 15:16:42 +0100
>> Cc: help-gnu-emacs@gnu.org
>> 
>>> . we check whether the current character should compose with the
>>>   following and/or preceding ones
>> 
>> Is my understanding right that this is the step that comes too late, i.e. after font selection?
> 
> It comes after the font selection, yes.  And it cannot be any other
> way, because the shaping engine must have the font to return any
> meaningful results.  The results of text shaping depend heavily on the
> font and its capabilities and features it supports.

I get that, I'm just saying that in this case it leads to a suboptimal outcome.

> 
>>>> 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.  (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.

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

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).