Geometry Boost List: Re: [geometry] #8376: difference of non-closed polygons returns closed polygon

Subject: Re: [geometry] #8376: difference of non-closed polygons returns closed polygon
From: Menelaos Karavelas (menelaos.karavelas_at_[hidden])
Date: 2014-10-09 16:57:58

Hi Volker.

On 08/10/2014 08:32 ??, Volker Schöch wrote:
>
> https://svn.boost.org/trac/boost/ticket/8376
>
> I'm confused. This ticket is marked resolved. Yet, in the following 
> instance, difference still returns a closed polygon in 1.56.0:
>
> A:MULTIPOLYGON(((1920 1660,1920 1462,3720 1462,3720 3262,1920 
> 3262,1920 1958,2218 2189),(3718 1561,3360 2233,3718 1957),(2818 
> 2653,2218 2189,2818 3253,3360 2233)))
>
> B: MULTIPOLYGON(((1918 2155,1918 1957,2818 2653,3718 1957,3718 
> 2154,2818 3055)))
>
> difference(a, b) -> closed: MULTIPOLYGON(((*2218 2189*,1920 1660,1920 
> 1462,3720 1462,3720 3262,1920 3262,1920 2157,2562 2799,2818 3253,3043 
> 2829,3718 2154,3718 1957,3718 1561,3360 2233,3718 1957,3360 2234,3360 
> 2233,2818 2653,*2218 2189*)))
>

I tried this example as well (see attached program and output).
What I see is that the output has 18 points (I can see that from both 
the dsv printout of the geometry, as well as from bg::num_points). So 
the resulting multi-polygon is not closed, but when you output it 
through bg::wkt, it is shown as closed.

> What's even worse, the closing point can no longer be removed by 
> applying boost::geometry::correct, boost::geometry::remove_spikes, or 
> boost::geometry::unique. I used to rely on these methods as a 
> work-around to fix the output from difference for boost 1.55.0, but 
> now it seems that
>

I don't get this. There is no point to remove. The resulting 
multipolygon is open, but it seems it is not valid (I tried bg::is_valid 
on it), but have not investigated more. If there is an issue with the 
output, I do not think that this is closedness ot openness.

- m.

> with 1.56.0, not only the original issue isn't fixed but even the 
> work-around isn't available any more.
>
> Am I missing something?
>
> Thanks!
>
>    Volker
>
> --
> Volker Schöch | vschoech_at_[hidden] <mailto:vschoech_at_[hidden]>
> Senior Software Engineer
>
>
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