[Libreoffice-bugs] [Bug 142940] Enabling pair kerning breaks rendering of many common fonts

[bugzilla-daemon at bugs.documentfoundation.org]

https://bugs.documentfoundation.org/show_bug.cgi?id=142940

--- Comment #7 from adam.m.fontenot+docfo at gmail.com ---
(In reply to Ming Hua from comment #3)
> I don't know much about text rendering so all I can add is a screenshot with
> 7.2.0 Beta1 on Windows 10, showing similar differences with or without pair
> kerning.

Thanks for your effort to get this looked at. The issue with using Windows as a
comparison is that by default Windows hints so strongly that much of the
subpixel nuance is lost anyway. I actually find the screenshot you posted very
difficult to read because of the amount of distortion to the glyph shapes.

IIRC, the developers of Microsoft Word agree with me, and they actually wrote
their own text rendering engine for Word so that they don't have to use
ClearType. (It's been a long time since I looked at anything Windows related, I
could be misremembering.)

(In reply to V Stuart Foote from comment #5)
> Cross platform we went to harfbuzz at 5.3.0.3; any improvement to sub-pixel
> rendereing needed to improve font provided hinting for kerning will only
> come with resolution of bug 103322

This was indeed what I was worried about. My thinking, though, is that this bug
shouldn't depend on that one. If LibreOffice doesn't support rendering at
arbitrary subpixel offsets, then pair kerning (which depends heavily on being
able to render at precise offsets) is sort of useless / broken. As far as I can
tell, in LibreOffice's present state, pair kerning is effectively adding or
subtracting a small random value from the desired glyph offset, which then gets
rounded by the renderer into whole pixels. It's really not surprising that the
result is frequently worse than no pair kerning at all.

For that reason, it seems to make sense to consider disabling it until bug
103322 is fixed. Can anyone show that having it enabled confers any definite
advantage to font rendering under Linux at present? My screenshots of
LibreOffice's rendering would seem to suggest that the results of having it
enabled are uniformly worse.

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