Re: Overlap removal

On Sun, 29 Jun 2014, Kristof Bastiaensen wrote:
> Since extrema and no-overlap are file format requirements,
> why not make it a default operation before converting to
> a font file?

Because it produces incorrect results that look bad.  :-)  I'm also not
sure overlap removal (as opposed to extrema) really is a file format
requirement; many fonts in the wild have not had it done and they don't
seem to cause problems for people.

Also, these operations require a fair bit of computation (possibly minutes
on a large font), and because they can change the number of spline
segments in a contour, they can screw up external data that may be
depending on the exact arrangement of where all the points are.  If we
load a file, save it, with no editing in between, and the result has a
different number of spline segments from what was in the original, it's
guaranteed we'll get a complaint from someone like Ilyaza about how we
have "corrupted" the font.

There are also some small advantages in a script context to having tight
control over exactly when overlap removal happens.  For instance, in
Tsukurimashou I assemble dozens of small fonts into each single large
font, and I do the overlap removal on the small fonts in a separate script
that only does that, because that way I can do many of them at once on a
parallel machine instead of having to wait to remove overlaps in a
single thread on the large font later; combined with on-demand (and only
on-demand) updates of the small fonts, this makes a noticeable difference
in the latency of my overall development cycle.

Nonetheless, at such time as FontForge's overlap removal and
extrema-adding are trustworthy, I would be in favour of making them happen
during "generate" of file formats that require it, even as a default,
provided it can be turned off by the user.  There is precedent in our
current (optional, but turned on by default) invocation of auto-hinting
during save, which remains a trap for the unwary.

-- 
Matthew Skala
[email protected]                 People before principles.
http://ansuz.sooke.bc.ca/

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