# Geometry :

Subject: Re: [geometry] Ring intersection question
From: Menelaos Karavelas (menelaos.karavelas_at_[hidden])
Date: 2014-11-06 17:10:03

Hi Will.

On 06/11/2014 11:56 ??, Will Lucas wrote:
> On Thu, Nov 6, 2014 at 3:47 PM, Menelaos Karavelas
> <menelaos.karavelas_at_[hidden] <mailto:menelaos.karavelas_at_[hidden]>>
> wrote:
>
> Yet one more comment.
>
>
> On 06/11/2014 11:44 ??, Menelaos Karavelas wrote:
>> 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
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Geometry mailing list
>>>>> Geometry_at_[hidden] <mailto:Geometry_at_[hidden]>

>>>>
>>>>
>>>> _______________________________________________
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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 list
>>>> Geometry_at_[hidden] <mailto:Geometry_at_[hidden]>

>>>
>>
>
>
> _______________________________________________
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> Geometry_at_[hidden] <mailto:Geometry_at_[hidden]>

>
>
> So, it sounds as if I should follow along with this custom polygon
> example?

>

Yes.

> 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
>

I think this is because your vector's value type is a ring rather than a
polygon.

> 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 think that one way is to define your polygon type such that it always
has an empty container for the interior rings (holes). Registering such
a polygon should do what you want.

The other option is to pass to the intersection algorithm only the
exterior ring of your polygons. Please take a look at the documentation
for bg::exterior_ring (there is a const and a non-const version):
http://www.boost.org/doc/libs/1_57_0/libs/geometry/doc/html/geometry/reference/access/exterior_ring/exterior_ring_1.html
http://www.boost.org/doc/libs/1_57_0/libs/geometry/doc/html/geometry/reference/access/exterior_ring/exterior_ring_1_const_version.html

Best,

- m.

>
> 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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