RE: [ft-devel] FT_Glyphslot_Embolden is broken at sometimes.

[Turner, David]

Hello,

the problem is that treating such broken glyphs correctly requires
some heavy-lifting algorithms. That's because:

  - any self-intersecting curve cannot be categorized as either
    an "inside" or "outside" curve, since it is both.

  - more generally, any intersections between outlines creates
    odd points that require, for proper emboldening, to basically
    break the curves into non-intersecting ones, which will lead
    you to compute the intersections of liberal bezier arcs, and
    this problem is certainly _not_ easy, is heavy on CPU, and
    prone to significant rounding errors.

  - even if you ignore the problems of intersections, properly
    determining the orientation of non-intersecting outlines is
    equivalent to performing the fill algorithm, i.e. you need
    to compute intersections of bezier arcs with horizontal
    lines, which is also not trivial, and prone to errors.

  - another option is to use heuristics to compute the orientations,
    which is what we currently do, as well as approximation.

It turns out that there is a reason why the Postscript specification
mandates that all outlines are non-intersecting and correctly oriented,
because the PS designed knew their curves pretty well !!

It seems the TrueType designers didn't think they had a problem after
all, since the sub-glyph transform operation allows for outline
reversal :-)

There may be a way to improve the heuristics, but they'll *never*
be 100% accurate. You'll always be capable of finding some badly
designed fonts.However, I've not studied this at the moment.

In theory, we should be able to find quickly wether a given outline
may be problematic or not. It must guarantee 0% false negatives, but
allow for false positives. The "non-problematic" outlines can be treated
with the current algorithm. The "problematic" ones should be passed
into another "reversal" algorithm that tries to compute correct orientations
with a finer heuristic, even if the latter is not 100% foolproof.

However, all of this is rather complex, and I don't have much free
time at the moment. Any one wants to volunteer on this ? You'll need
good knowledge of discrete math and geometry.

Regards,

- David Turner
- The FreeType Project  (www.freetype.org)


> -----Message d'origine-----
> De : address@hidden
> [mailto:address@hidden
> la part de
> sunmoon1997
> Envoyé : jeudi 27 octobre 2005 04:03
> À : Chia-I Wu
> Cc : address@hidden
> Objet : Re: [ft-devel] FT_Glyphslot_Embolden is broken at sometimes.
>
>
> Thanks all  you guys, but it sounds bad if  it's unable to work around
> this in freetype.
>
> Chia-I Wu wrote:
>
> >On Wed, Oct 26, 2005 at 11:00:09PM +0200, Werner LEMBERG wrote:
> >
> >
> >>>    FT_Glyphslot_Embolden works at most of time, but sometimes it
> >>>doesn't(see attachment).  To run the testcase, you need simsun.ttc
> >>>from windows.  Any comments?
> >>>
> >>>
> >>Which FT version?
> >>
> >>Unfortunately, I don't have simsun.ttc so I can't test it.  Chai-I?
> >>
> >>BTW, I don't have cairo either -- can you reproduce the problem with
> >>ftview from the ft2demos package, using the current CVS?
> >>
> >>
> >The character shown in the .png (U+60CA) has a contour of width 0,
> >namely all points have same x value (bug?), in the left top corner.
> >It misleads FT_Outline_Get_Orientation.  Thus instead of emboldened,
> >each stroke got shrinked.
> >
> >Similar issues have already been seen in some free
> traditional Chinese
> >fonts.  They have glyphs consisting of components of different
> >orientations.  I think it's a bug of the font and since
> those fonts are
> >actively maintained, I reported the issue to the font authors, after
> >failed to work around this in freetype.
> >
> >
> >
>
>
> _______________________________________________
> Freetype-devel mailing list
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> http://lists.nongnu.org/mailman/listinfo/freetype-devel
>
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