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

[bugzilla-daemon at freedesktop.org]

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

--- Comment #30 from Behdad Esfahbod <freedesktop at behdad.org> ---
(In reply to comment #22)
> 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?

Ok, I did a patch to not check outline for CFF fonts and only check the glyph
offsets in 'loca' table for TrueType fonts.  This has significant speedup
benefit mainly because of I/O.  It ends up reading less than 5 / 10 percent of
the font, so it was a huge boost.

This approach correctly takes care of simple TrueType glyphs with no outline. 
But I found many fonts have composite glyphs that reference a glyph that has no
outlines.  Currently I can't detect that.  Another group was CFF chars mapping
to cid1 which has no outline.  I might be able to special-case that, but am not
very confident about it.

As such, I don't think this approach actually works.  But even with warm caches
it's still a huge speedup.  For some 22k fonts:

BEFORE
real    1m18.376s
user    1m12.497s
sys    0m3.346s
AFTER
real    0m18.118s
user    0m12.997s
sys    0m2.654s

That suggests that there's still possible value in speeding up the check either
withing FT_Load_Glyph, OR, implementing 'glyf' table access in fontconfig,
ignore bitmap fonts, and call into FreeType for CFF fonts.  I'll give that a
try.

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