…
cnts,hie= cv2.findContours(im_binary,
cv2.RETR_EXTERNAL,
cv2.CHAIN_APPROX_NONE)
print(cnts)
…
Shouldn’t a contour have at least three points? 
on what data does this happen? show the data. show where this single-point contour would be in the picture.
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Thank you for your reply! Maybe I have found the answer in the paper “Topological Structural Analysis of Digitized Binary Images by Border Following” 