[cairo] Path fill bug
cworth at east.isi.edu
Thu Aug 12 16:40:01 PDT 2004
On Fri, 06 Aug 2004 15:29:56 -0600, Charles Tuckey wrote:
> We have discovered a bug in the way cairo (or its backends) fill a
> particular concave polygon. Using glitz this polygon has some extra fill
> on its left side; using xlib or pixman the polygon lower interior is not
> filled. Stroking the polygon works fine.
> I have attached three cairo snippets that illustrate the problem:
Thank you for the excellent bug report along with precise snippets for
The problem you've seen is a known bug in the current CVS version of
the code. It's been there much longer than I would like while we are in
the midst of re-writing the tessellation routine within cairo.
To work around your problem, I suggest you back off to the most recent
cairo snapshot which, as I just checked, does not exhibit this bug with
your test cases:
I don't know if you're using glitz or not, but if so then you'll need to
back up a few snapshots before you find one that is compatible with
The current skew between glitz and cairo is precisely because I do not
plan to make a new snapshot until the new tessellator is finished and
this bug is licked.
 As seen in cairo/BUGS:
The polygon tessellation routine has problems. It appears that the
following paper has the right answers:
[Hobby93c] John D. Hobby, Practical Segment Intersection with
Finite Precision Output, Computation Geometry Theory and
Applications, 13(4), 1999.
Recent improvements to make the intersection code more robust (using
128-bit arithmetic where needed), have exposed some of the weakness in
the current tessellation implementation. So, for now, filling some
polygons will cause "leaking" until we implement Hobby's algorithm.
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