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

[bugzilla-daemon at bugs.documentfoundation.org]

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

            Bug ID: 142940
           Summary: Enabling pair kerning breaks rendering of many common
                    fonts
           Product: LibreOffice
           Version: 7.1.4.2 release
          Hardware: All
                OS: Linux (All)
            Status: UNCONFIRMED
          Severity: normal
          Priority: medium
         Component: graphics stack
          Assignee: libreoffice-bugs at lists.freedesktop.org
          Reporter: adam.m.fontenot+docfo at gmail.com

Created attachment 173010
  --> https://bugs.documentfoundation.org/attachment.cgi?id=173010&action=edit
screenshot of problem caused by pair kerning

**Description:**

I was trying to hunt down the source of the (for me) long-time crappy font
rendering in LibreOffice writer. I found
https://bugs.documentfoundation.org/show_bug.cgi?id=103322 and initially
thought that was the issue, but it turns out to be more complicated.

With most common fonts, the rendering gets *significantly* better when I
disable pair kerning. The following discussion references the attached
screenshot.

Both renderings have flaws in character positioning (they're kerned
incorrectly, which doesn't imply that the bug is necessarily in the kerning
implementation). My understanding is that this is probably caused by
https://bugs.documentfoundation.org/show_bug.cgi?id=103322

However, with pair kerning enabled, the kerning immediately becomes
*incredibly* bad, so much so that Writer sucks to write in. I currently write
text in a different editor and copy it over when I need to use LibreOffice.
Some flaws are so obvious they will be immediately apparent even to those who
know nothing about font rendering. See "Earth" and "signifying" in Minion Pro,
"dwelt" in Arial, "met" in Garamond, and many other examples. (A few pairs are
better with pair kerning enabled, which suggests that maybe it's not completely
broken?)

**System Information:**

 * OS: Arch Linux
 * Kernel: 5.10.44-1-lts
 * LibreOffice: 7.1.4-1 (libreoffice-fresh Arch package)
 * Display Server: X11
 * Pango: 1.48.5
 * Desktop Environment: KDE
 * Fontconfig settings: hinting disabled, subpixel anti-aliasing enabled,
dpi=120

**Other:**

This might be the cause of
https://bugs.documentfoundation.org/show_bug.cgi?id=128987 but I can't tell
because I don't use Windows. 

Another possibility is that enabling pairwise kerning greatly exacerbates the
effect of https://bugs.documentfoundation.org/show_bug.cgi?id=103322 at least
on certain systems / monitors (non-default DPI settings?). If that's the case,
it probably needs to be disabled by default until that bug is fixed.

Text in LibreOffice *looks* like it might be hinted to me (it's hard to tell
when the rendering is broken), which would be a problem / bug because I have
hinting disabled in fontconfig. I'm not sure how much of the problem might be
due to poor hinting.

Elsewhere, users have reported bugs that look as if they might be caused by
this issue:
https://askubuntu.com/questions/827033/libre-office-font-spacing-kerning-issue

Incidentally, PDFs exported from LibreOffice look great, the bug doesn't affect
that.

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