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I want to detect existence of continuous or closed polygon in binary image. The image may contain polygon of variable number of edges and also there may be no polygon at all.

Here is the positive case:

enter image description here

And,

this one is negative case:

enter image description here

Is there any method in image processing or some algorithm to detect continuous polygon ?

Any kind of help is appreciated.

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  • Does it have to be convex ? Commented Mar 27, 2017 at 13:27
  • both concave or convex Commented Mar 27, 2017 at 13:59

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If you need to detect both concave and convex polygons, then I can suggest you another way to do it, one has already have been told by @mdh

  1. first, you need to find the contour (cv2.findContours)
  2. then compute the area of the contour (cv2.contourArea)
  3. compute the area of the black area in the binary image (compute the number of black pixels in the binary image, openCv has a countNonZero function, that does it for you)
  4. compare the 2 values, if they are close (e.g. their ratio is close to 1, or greater than a value, you should chose it by testing on more images), then the contour you found is closed, so it may be a polygon

Of course there may be some errors in self-intersecting polygons, and/or if there are more than one contour in an image, but this may be fixed by a segmentation picking just the bounding box of the contour and apply the algo on each one.

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I hope you know how to find contours.

Using cv2.isContourConvex(contour) you can identify whether the detected contour is convex or not. By convex, I mean whether the convex is closed or not. You will be able to separate closed and open contours using this function, which return a boolean value True or False.

After separating out the contours you go on to perform further analysis deciding whether the contour has edges or not.

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I don't need concave or convex but, just polygon existence. I know how to find contour there is some other function in opencv to find closed or not. can you explain
You have stated 'there is some other function in opencv to find closed or not'... what is that function? I would like to know.... :D
hey, sorry for that. That was my question actually
I am not getting your point... You have stated you would like to find a closed or open polygon. An open polygon is not a polygon in the first place. So I mentioned a function that checks whether detected contours are closed or not. Now you can perform further analysis whether the detected closed contours are polygons are not. In a nutshell, you have crossed the first hurdle finding closed contours. You just have one more hurdle to find whether they are polygons or not.
Thanks, I understand.
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You may wanna look into shapely.geometry.Polygon, in particular the is_valid method.

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I had a tool to detect region in image and some properties belonging to that region. https://github.com/mribrahim/Blob-Detection

Think that, if it is a polygon , it will be a closed region (positive case).

If it is not closed,it will be a line (negative case).

You can find regions properties with the blob detection tool, I mentioned. And you can use some properties like eccentricity, solidity to distinguish positive and negative cases.

For example; negative case eccentiricity will be closed to 0, because of it is a line

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