On 06/11/2014 11:56 μμ, Will Lucas
wrote:

On Thu, Nov 6, 2014 at 3:47 PM, Menelaos Karavelas <menelaos.karavelas@gmail.com> wrote:

Yet one more comment.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.

On 06/11/2014 11:44 μμ, Menelaos Karavelas wrote:

Hi again.

On 06/11/2014 11:34 μμ, Menelaos Karavelas wrote:

Hi Will.

On 06/11/2014 11:26 μμ, Will Lucas wrote:

On Thu, Nov 6, 2014 at 3:07 PM, Menelaos Karavelas <menelaos.karavelas@gmail.com> 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 μμ, 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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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:

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.

- m.

All the best,

- m.

- m.

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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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