Geometry Boost List: Re: [geometry] Ring intersection question

Subject: Re: [geometry] Ring intersection question
From: Menelaos Karavelas (menelaos.karavelas_at_[hidden])
Date: 2014-11-06 16:34:57

Hi Will.

On 06/11/2014 11:26 ??, Will Lucas wrote:
> On Thu, Nov 6, 2014 at 3:07 PM, Menelaos Karavelas 
> <menelaos.karavelas_at_[hidden] <mailto: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 ??, 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
> https://gist.github.com/wlucas-DFT/0b8e1e07f34ee7e489c6
>
> 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:
http://www.boost.org/doc/libs/1_56_0/libs/geometry/doc/html/geometry/reference/algorithms/intersection.html

Try replacing the intersection output type by a multipolygon, or a 
vector/deque of polygons and see what the output is.

- m.

>
>
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