[Fontconfig-bugs] [Bug 64766] Make fontconfig scanning faster

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=64766

--- Comment #22 from Keith Packard <keithp at keithp.com> ---
I added the outline scanning code because we found many font files that were
generated by subsetting a large font with extensive unicode coverage. The
subsetting process would leave the code point map in place and only elide the
actual glyph data.  So, the only way we could discover the true coverage of the
font was to look at the actual glyphs.

If the failure mode was to simply select the wrong font for some code points,
that might be fine. However, the failure mode is instead to display blank
spaces for the missing glyphs.

These were not hand-hacked fonts, but rather commercial fonts from
semi-reputable font foundaries. The commit when this was added appears to be
lost in time; having occurred in libXft, before fontconfig was split out back
in 2002.

I'd suggest figuring out if FreeType could return this data faster than by
parsing the whole outline; presumably there's a table present which indicates
where the outline would be and how long it is?

As for speeding up re-scanning directories, the obvious solution would be to
re-use the old cached data for fonts which haven't changed and copy that to the
new cache file.

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