Geometry Boost List: [ggl] Difference / xor for multi polygons

Subject: [ggl] Difference / xor for multi polygons
From: Javier Gonzalez (xaviergonz)
Date: 2011-02-27 14:12:06

Also I just noticed doing the inverse difference (poly2 - poly1 instead of
poly1 - poly2) gives another weird result.
It returns two polygons, being the first the poly2 (as expected) and the
second being the inner hole of poly 1, but as an external ring polygon
(unexpected).

On Sun, Feb 27, 2011 at 7:57 PM, Javier Gonzalez <xaviergonz_at_[hidden]>wrote:

> Ok thanks, that worked.
>
> However now I'm facing another problem with the difference combination.
>
> Given (in cartesian coords)
>
> poly1:
> ext ring = -2, 2   2002, 2   2002, -2002   -2, -2002   -2, 2 (clockwise)
> inner ring = 0, 0    0, -2000   2000,-2000   2000, 0   0, 0  (counter
> clockwise)
>
> poly2:
> ext ring = 560, -360   600, -360   600, -400   560, -400   560, -360
> (clockwise)
>
> The difference between them is
> exterior ring = -2, 2   2002, 2   2002, -2002   -2, -2002   -2, 2
> (clockwise)
> with no interior ring
>
> Isn't this obviously wrong?
>
> Actually always I try to make a difference between a holed box and a box
> within it I get the whole external unholed box as result :-S
>
> PS: I'm using the difference_inserter to do the combination, both the new
> version for multi polygons and the old version for polygons give the same
> result.
> Ah, and latest dev version of boost geometry.
>
>
> On Sun, Feb 27, 2011 at 11:48 AM, Barend Gehrels <barend_at_[hidden]> wrote:
>
>>  Hi Javier,
>>
>>
>> On 27-2-2011 1:58, Javier Gonzalez wrote:
>>
>>
>> Unfortunately now upon compilation of the same code (VS2010), even if I
>> only leave the union inserterer, I get this:
>>
>>  1>  BoostGeometry.cpp
>> 1>Programming\C++
>> Libraries\boost_1_46_0\boost/geometry/core/point_type.hpp(42): error C2664:
>> 'boost::mpl::assertion_failed' : cannot convert parameter 1 from
>> 'boost::mpl::failed ************(__cdecl
>> boost::geometry::traits::point_type<Geometry>::NOT_IMPLEMENTED_FOR_THIS_POINT_TYPE::*
>> ***********)(boost::mpl::assert_::types<T1>)' to
>> 'boost::mpl::assert<false>::type'
>>
>>
>>
>> I see. This is not caused by the change of yesterday, but by an earlier
>> (and correct) change.
>>
>> The problematic lines are:
>> (...) typedef vector<polygon_2d> polygon_2d_list;
>> (...) static polygon_2d_list Combine(polygon_2d_list const& subjectPoly,
>> polygon_2d_list const& clipPoly, clip_type clipType)
>> (...)  intersection_inserter<polygon_2d>(subjectPoly, clipPoly,
>> back_inserter(outputPoly));
>>
>> First a polygonlist is declared but it is not a multi-polygon. For output,
>> using it with the back_inserter, this is OK.
>>
>> But for input, it should conform to the multi-polygon concept. A
>> vector<polygon> does conform to most of it, but it still needs the tag to
>> note the library "I'm a multi-polygon".
>>
>> So in the last line it expects a multi-polygon and gets a vector of
>> polygons, and therefore the MPL assertion of point_type goes off.
>>
>> That MPL assertion is relatively new, but I'm surprised that the code
>> earlier compiled because it needs to assure the multi-polygon concept on
>> several places.
>>
>> So three solutions possible:
>> 1) change typedef vector<polygon_2d> to multi_polygon<polygon_2d>
>> 2) change Combine from poylgon_2d_list to polygon_2d (but I don't know if
>> that is what you want)
>> 3) add a traits class to register your polygon_2d_list:
>>
>> namespace boost { namespace geometry { namespace traits
>> {
>>
>> template <> struct tag<polygon_2d_list>
>> {
>>     typedef multi_polygon_tag type;
>>
>> };
>>
>> }}}
>>
>>
>>
>>  I'm using the version from
>>
>> Subversion repository with source code of *Boost Geometry* is hosted in
>> the Boost Trunk available at
>>
>> svn co http://svn.boost.org/svn/boost/trunk/boost/geometry boost-geometry
>>
>>
>>  though I only got the geometry part and used it over the release version
>> of boost 1.46
>>
>>
>> That should be no problem.
>>
>>
>> Regards, Barend
>>
>>
>> --
>> Barend Gehrelshttp://about.me/barendgehrels
>>
>>
>> _______________________________________________
>> ggl mailing list
>> ggl_at_[hidden]
>> http://lists.osgeo.org/mailman/listinfo/ggl
>>
>>
>
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