problem with polyarea method, return NAN

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fahimeh
fahimeh on 13 Oct 2014
Commented: fahimeh on 21 Oct 2014
Hi, I have multiple circles that intersect each other in the static area,I want to find union of them. I use polyarea method to find union,My problem is that this method sometimes return numerical result and sometimes return NAN result.What should I do to not get NAN as a result?
please Help me if you now my answer. Thanks.
  2 Comments
Mohammad Abouali
Mohammad Abouali on 13 Oct 2014
Polyarea is to get the area of a polygon, Why do you use that to get the union area?
fahimeh
fahimeh on 14 Oct 2014
first,I use polybool to get the union,and after that I use polyarea to get numerical result.

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Accepted Answer

Image Analyst
Image Analyst on 13 Oct 2014
You might have to quantize/digitize them into an image, then AND them together. Use poly2mask to turn the vertices into images.
circleMask1 = poly2mask(x1,y1,rows,columns);
circleMask2 = poly2mask(x2,y2,rows,columns);
theUnion = circleMask1 & circleMask2; % This is an image also.
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Image Analyst
Image Analyst on 16 Oct 2014
I have showed you. That's what circleMask1 and circleMask2 are.
fahimeh
fahimeh on 21 Oct 2014
Thank you very much,Your solution solve my problem.

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More Answers (1)

John D'Errico
John D'Errico on 21 Oct 2014
Note that the union of two polygons need not be a simple polygon. In fact, if the polygons are disjoint, then I'll bet that you get a result with a NaN in it. (I don't have the mapping toolbox that contains polybool, so I cannot test that statement.)
One solution is to discretize as Image Analyst has said, however, that will incur some loss due to the digitization into an image. Note that depending on the size of the numbers in your polygon, the digitization process may create a seriously huge image. So be careful!
A second solution is to catch the case when a NaN is returned, and then to perform the area computation as the sum of the two areas.

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