CGAL users discussion list

[Sebastien Loriot <>]

I guess the outer ccb you mention if of the polygon and not the
arrangement. IIRC, you have to keep only components that have a
winding number of exactly one.

Efi, do you confirm?

Best,

Sebasiten.

On 1/23/21 9:41 PM, Dov Grobgeld ( via 
cgal-discuss Mailing List) wrote:
> Thanks! That helped, and indeed I'm able to use the winding number to 
> get rid of the red loop in my previous post, which gets a negative 
> winding number, and can easily be filtered. But I'm still stumped by 
> another problem of arrangements.
>
> Consider the following closed self intersecting path:
>
> input.png
> If I run the book example chapter6/polygon_repairing.png on the above 
> path I get the following separated paths:
>
> res.png
> The green path is however still self intersecting! Why does the 
> Arrangement return self intersecting paths in the outer_ccb? I guess I 
> would have to access the holes of the unbounded face to get the 
> non-intersecting outer curves. But will that work with the winding 
> number class? I admit that I'm quite confused by this.
>
> Thanks!
>
>
> On Fri, Jan 22, 2021 at 1:59 PM Sebastien Loriot < 
> <>> wrote:
>
>     I think in the arrangement book:
>     http://acg.cs.tau.ac.il/cgal-arrangement-book
>     <http://acg.cs.tau.ac.il/cgal-arrangement-book>
>
>     there is some code to fix self-intersecting polygons using winding
>     numbers.
>
>     The code of the example is also available at the URL above.
>
>     Best,
>
>     Sebastien.
>
>     On 1/21/21 6:10 PM, Dov Grobgeld (
>     <> via
>     cgal-discuss Mailing List) wrote:
>      > Hi all,
>      >
>      > In my efforts to deal with a large variety of fonts in my text to
>     glyph
>      > generator, https://github.com/dov/pomelo
>     <https://github.com/dov/pomelo>
>      > <https://github.com/dov/pomelo <https://github.com/dov/pomelo>>,
>     have I encountered several cases of
>      > "bad" contours that I need to clean. One of the problems can be
>     seen in
>      > the following figure:
>      >
>      > foo.png
>      > The input is the purple polyline on the left. My desired output
>     are the
>      > blue and the green polylines on the right, while dropping the red
>      > polyline. My idea of how to do this was to use arrangements.After
>     doing
>      > an arrangement, so I thought, the generated separated polylines,
>     would
>      > keep the segment orientations of the input. Since I know the desired
>      > direction of the polyline, (I know whether I'm looking for an outer
>      > boundary or on a hole), I can drop polylines whose orientations
>     are not
>      > as expected.
>      >
>      > Unfortunately this doesn't work. I found that boundaries of the
>     faces
>      > that Arrangement_2 generate are all the in the same direction.
>      >
>      > Am I missing something?  How can I prune the red polyline?
>      >
>      > Here are the input coordinates of the input as a svg d path:
>      >
>      > m 457.2054,532.00482 -2.16634,-106.13373 2.48438,-79.73047
>      > 4.94872,-58.86624 15.90675,0.0928 -9.17579,50.94531
>     -7.96093,57.34766
>      > -3.03516,15.03515 -5.07812,10.16406 -7.16537,4.46318
>     -9.10807,1.23214
>      > 8.85783,1.10863 7.45076,4.90699 5.07032,10.11328 3.01562,14.85157
>      > 8.83504,74.12725 z
>      >
>      > Thanks!
>      >
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