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Subject: Re: [geometry] multiple polygon difference
From: Barend Gehrels (barend_at_[hidden])
Date: 2013-10-30 07:20:27

Hi Stefan,

Welcome to the list.

Stefan Wörthmüller wrote On 30-10-2013 12:09:
> Hello
> i am currently struggling with the process of subtracting small
> polygons from a larger polygon. None of the polygons have holes and
> all small polygones are completely covered by the larger polygone. Of
> course, the result will be a polygon with holes.
> I first tried with boost 1.48, but as this resulted in compilation
> errors i now upgradet to 1.54 as i read in the release notes of 1.54:
> > support for line/polygon intersections and differences.
> My code looks like this:
> typedef boost::geometry::model::d2::point_xy<double> boost_pnt;
> typedef boost::geometry::model::polygon<boost_pnt> boost_poly;
> boost_poly outputPoly = outerPolygon;
> for(...)
> {
> boost_poly innerPoly = ...;
> boost_poly new_output;
> boost::geometry::difference(outputPoly, innerPoly, new_output);
> outputPoly = new_output;
> }
> Compiling thjs with VC2010 results in the line of difference
> >D:\Develop\boost\boost_1_54_0\boost/range/iterator.hpp(63): error
> C2039: 'type' : is not a member of 'boost::mpl::eval_if_c<C,F1,F2>'
> 1> with
> 1> [
> 1> C=false,
> 1>
> F1=boost::range_const_iterator<boost::geometry::model::polygon<boost_pnt>>,
> 1> F2=boost::range_mutable_iterator<boost_poly>
> 1> ]
> etc.
> So my understanding is, that difference() does not work with all
> parameters of type polygon.
> When using std::vector<boost_poly> as last parameter, difference
> compiles and works, but i see no way of converting this back to a
> polygon.
> I can not find a way to to distance in a loop.

Yes, the output should be either a multi-polygon, or a vector of
polygons. The reason for that is that difference can (by theory) create
multiple polygons as output. If you have a rectangle, and subtract a
less wide, but higher rectangle lying in the center of the other one
(like a cross), you get two rectangles, one left and one right, as output.

So the output must be a collection of polygons. If there is only one
output, you have a polygon-result, you can take output.front(). If there
are more, you have to deal with that, it is just the correct result.

Regards, Barend

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