# Geometry :

Subject: Re: [geometry] Ring intersection question
From: Will Lucas (wlucas_at_[hidden])
Date: 2014-11-06 16:56:54

On Thu, Nov 6, 2014 at 3:47 PM, Menelaos Karavelas <
menelaos.karavelas_at_[hidden]> wrote:

> Yet one more comment.
>
>
> On 06/11/2014 11:44 Î¼Î¼, Menelaos Karavelas wrote:
>
> Hi again.
>
> On 06/11/2014 11:34 Î¼Î¼, Menelaos Karavelas wrote:
>
> Hi Will.
>
> On 06/11/2014 11:26 Î¼Î¼, Will Lucas wrote:
>
> On Thu, Nov 6, 2014 at 3:07 PM, Menelaos Karavelas <
> menelaos.karavelas_at_[hidden]> wrote:
>
>> Dear Will,
>>
>> I tried your polygons and the intersection seems to work.
>> Please checkout the attached program, and let us know if you get a
>> similar output. The output that I get is:
>> MULTIPOLYGON(((75 150,250 150,250 75,75 75,75 150)))
>>
>> Best,
>>
>> - m.
>>
>>
>> On 06/11/2014 10:44 Î¼Î¼, Will Lucas wrote:
>>
>> Hi all,
>>
>> I have a question regarding the intersection of ring concepts using
>> Boost.Geometry. I currently have two overlapping rectangles defined by the
>> following WKTs:
>>
>> POLYGON((75 75,75 175,275 175,275 75,75 75))
>> POLYGON((50 50,50 150,250 150,250 50,50 50))
>>
>> When I perform the intersection of these rectangles, I get the
>> intersection points:
>>
>> POLYGON((75 150,250 75))
>>
>> The intersection points do not allow me to compute the area correctly
>> after the intersection. Is there way to get a fully valid ring/polygon out
>> of intersection, so that the area will be equal to the overlapping region?
>>
>> Thanks!
>> Will
>>
>>
>> _______________________________________________
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>>
>>
>>
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>>
>>
> Menelaos,
>
> Thanks for the quick response! I have tested your code, and it correctly
> outputs
>
> MULTIPOLYGON(((75 150,250 150,250 75,75 75,75 150)))
>
> I had to comment out the is_valid calls as I'm running the Ubuntu
> package (boost 1.54.0), which doesn't contain that helper method.
>
>
> Sure, this was just a sanity test.
>
> I'm guess the problem may be my re-mapping of the OpenCV data-types.
> Here is what I currently am working with

>
> Do I need to define a polygon wrapper for the custom Contour type I have?
>
>
> I think that the problem is that your intersection output type should be a
> multipolygon rather than a ring/polygon. Please checkout the relevant doc
> page:
>

>
> Try replacing the intersection output type by a multipolygon, or a
> vector/deque of polygons and see what the output is.
>
>
> Please see the updated attached program. I added one more call to
> bg::intersection specifying a ring as the output (like you do), and I get
> the result you get. I am now convinced that the problem is what I describe
> above.
>
>
> I think what is happening is that the BG code understands your ring as a
> container of points (a multipoint/vector of points/etc), rather than a
> multipolygon, in which case it is setup to output just the intersection
> points.
>
> - m.
>
> All the best,
>
> - m.
>
> - m.
>
>
>
> _______________________________________________
> Geometry mailing listGeometry_at_[hidden]http://lists.boost.org/mailman/listinfo.cgi/geometry
>
>
>
>
>
> _______________________________________________
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> Geometry_at_[hidden]

>
>
So, it sounds as if I should follow along with this custom polygon example?
https://github.com/boostorg/geometry/blob/master/example/c06_custom_polygon_example.cpp

This would hopefully allow me to define a custom multi_polygon. As my
initial attempt to just create an std::vector<dft::Contour> as a
multi_polygon, resulted in a bunch of compiler complaints :D

For my specific problem, I'm not concerned with "holes" that may exist in a
polygon (I actually indicate to OpenCV, to only return the exterior
contours to me). Is there a way to indicate to BG that only exterior
contours are used via the polygon traits?

I suppose worst case I can copy from OpenCV to BG data-structures, but that
doesn't seem as elegant :)

Will

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