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 16:44:18

Hi again.

On 06/11/2014 11:34 ??, Menelaos Karavelas wrote:
> 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.
>

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.

All the best,

- m.

> - m.
>
>>
>>
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