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Subject: Re: [boost] [Review] We're halfway through the GGL review
From: Fabio Fracassi (f.fracassi_at_[hidden])
Date: 2009-11-16 12:52:49

Barend Gehrels wrote:
> Hi Fabio,
>> Hm, I tried:
>> #include <boost/ggl/ggl.hpp>
>> #include <boost/ggl/geometries/cartesian3d.hpp>
>> #include <boost/ggl/multi/geometries/multi_polygon.hpp>
>> #include <boost/ggl/algorithms/union.hpp>
>> ...
>> typedef multi_polygon<polygon_3d> polygon_set_3d;
>> polygon_set_3d ps;
>> polygon_3d poly1;
>> polygon_3d poly2;
>> union_inserter(poly1, poly2, std::back_inserter(ps));
> There are two things:
> - I forgot one thing, sorry, the inserter-versions always have their
> output type as a required template parameter. That one can, AFAIK, not
> be deduced from the output iterator. And it might be a different type of
> the input polygons. If someone knows if that is possible somehow, I
> would be happy to know. So it should be: union_inserter<polygon_2d>(p1,
> p2, std::back_inserter(ps1));

Ok, one more argument for both an example and a union_ function then ;)

> - You're using the 3d polygons here. The union would make a 2D
> union-operation then of your 3D polygons (so polygons with for all
> vertices a height). It would ignore the third dimension (because it is
> not a polyhedron union). I just tried it, to not give you again wrong
> information. But it is not working yet like this, a.o. because the
> functions convert, area and point-in-polygon which should support 3D
> then. Especially area implementation would probably be different for 3D.
> Though it is used as a helper-function in the union, so its usage could
> be avoided.

I didn't really expect 3d union to work (i.e. create a polyhedron ) I
wanted to find out what would happen. I'm not quite sure what I would
expect (projection on the xy plane like you suggest or another
projection most likely).
But the docu said that 3d was not a focus of GGL yet, so its ok that
there are missing pieces.

I'm content that the design allows for both union_(poly3d_1, poly3d_2,
poly3d_result) and union_(poly3d_1, poly3d_2, polyhedron) to do the
right thing, once the building blocks are in place.

Thanks and Regards


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