# Geometry :

Subject: Re: [geometry] difference algorithm yields invalid multi-polygon
From: Volker Schöch (vschoech_at_[hidden])
Date: 2014-11-17 11:50:28

Here is another example:
polygonA = "MULTIPOLYGON(((1920 1660,1920 1462,3720 1462,3720 3262,1920 3262,1920 1958,2218 2189,1920 1660),(3718 1561,3360 2233,3718 1957,3718 1561),(2818 2653,2218 2189,2818 3253,3360 2233,2818 2653)))"
polygonB = "MULTIPOLYGON(((1918 2155,1918 1957,2818 2653,3718 1957,3718 2154,2818 3055,1918 2155)))"

difference(polygonA, polygonB, polygonC)
-> polygonC == 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)))

Both, polygonA and polygonB, satisfy is_valid(...), but polygonC doesn't.
Regards
Volker

```--
Volker Schöch | vschoech_at_[hidden]<mailto:vschoech_at_[hidden]>
Senior Software Engineer
We are looking for C++ Developers: http://www.think-cell.com/career
From: Geometry [mailto:geometry-bounces_at_[hidden]] On Behalf Of Volker Schöch
Sent: Montag, 17. November 2014 17:24
To: Boost.Geometry library mailing list (geometry_at_[hidden])
Subject: [geometry] difference algorithm yields invalid multi-polygon
polygonA = MULTIPOLYGON(((1701 985,3501 985,3501 2785,1701 2785,1701 985)))
polygonB = MULTIPOLYGON(((1698 1860,1698 1122,2598 1392,3492 1842,3492 32706,2598 2340,1698 1860)))
difference(polygonA, polygonB, polygonC)
-> polygonC == MULTIPOLYGON(((1701 1122,1701 985,3501 985,3501 2785,3492 1857,3492 1842,2598 1392,1701 1122)),((1701 1861,2598 2340,1669 2785,1701 2785,1701 1861)))
Both, polygonA and polygonB, satisfy is_valid(...), but polygonC doesn't.
Is there anything wrong with the input? Is this a bug?
Regards
Volker
--
Volker Schöch | vschoech_at_[hidden]<mailto:vschoech_at_[hidden]>
Senior Software Engineer
We are looking for C++ Developers: http://www.think-cell.com/career
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```

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