How to detect polygon in binary image

Question

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:

And,

this one is negative case:

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

Any kind of help is appreciated.

Answer 1

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.