Geometry Boost List: [ggl] Multipolygon self intersection with of coincident edges

Subject: [ggl] Multipolygon self intersection with of coincident edges
From: Barend Gehrels (barend)
Date: 2011-10-25 17:11:00

Hi,

Thanks for your clear figures and code sample including WKT, this makes 
researching/responding easy.

On 25-10-2011 6:38, V D wrote:
> Hi,
>
> I'm looking at self-intersection problems when dealing with integer 
> multi-polygons.
> For example, the following 2 polygons as a multipolygon:
>
>
> They have a coincident edge, and therefore doing 
> bg::intersects(this_multipolygon) will return true. Of course, 
> bg::intersects() on each polygon individually is perfectly fine.
>
> Is there any way I could tweak this algorithm so that coincident edges 
> are not considered as self intersection? (can't use algorithms on this 
> kind of multipolygons right now because of that)
> Would this possibly break algorithms ? Should I simply deal with this 
> kind of geometries as a vector of polygons and do algorithms 1 polygon 
> at a time rather than the multipolygon concept ?
>

The bg::intersects returns true which is indeed correct. Though the 
single-argument version is not OGC-compliant, it behaves as though it 
operates on (in this case) two polygons which would return true.

If we would apply this single-argument overloading further you would be 
helped. If there would be a single-argument bg::touches, it would return 
true in your case, and if there would be a single-argument bg::overlaps, 
it would return false because the interiors of the two do not overlap 
each other.

So that would be a nice solution, I think. But it is not yet there (if 
we have overlaps for the two polygons, the single-argument is relatively 
easy).

You could call the single-argument per polygon, for some purposes this 
will be a perfect solution. Then you know that all polygons do not 
self-intersect. But of course they might mutually overlap then...

Regards, Barend

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