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Re: Rendering performace vs. line-spacing

From: Herman, Geza
Subject: Re: Rendering performace vs. line-spacing
Date: Fri, 8 Jan 2021 17:04:44 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.11.0

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.

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