Yet one more comment.

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.


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.



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