Re: Rendering performace vs. line-spacing

["Herman, Geza" <geza.herman@gmail.com>]

On 2021-01-08 16:46, Eli Zaretskii wrote:
>> Cc: emacs-devel@gnu.org
>> From: "Herman, Geza" <geza.herman@gmail.com>
>> Date: Fri, 8 Jan 2021 16:38:10 +0100
>>
>> I disabled redrawing caused by overlapping by commenting out the line
>> which sets the "row->overlapping_p" flag (xdisp.c:21752), and the
>> problem is gone away. But my fonts are clipped (not a surprise). But, it
>> fixed the consolas case as well, and I don't see any clipping with this
>> font. Maybe overlapping detection is too strict (I mean, emacs considers
>> cases overlapping which are close to overlapping, but they aren't)? Or
>> maybe I just don't notice the clipping.
> It could be that in the case of Consolas the overlapping doesn't
> actually happen, but some metrics we get from the font tell us they
> are.  Or maybe they do happen, but only when very few characters from
> the font are used.  Or maybe this also depends on the font backend,
> like whether you build with or without Cairo.
>
> This calls for some font expert(s) to study the Consolas case and see
> why the problem happens there.  Perhaps we could speed up at least
> that case, I don't know.
Okay, thanks for the help! I adjusted my font's ascent/descent, and now 
I don't see any clipping with this font either (for the record, I'm 
using Cairo).

If I have the time, I may have a look at this part of emacs's code and 
try to understand what's going on exactly.