title: Blob Detection in openCV works well, but for some reason it fails for a specific image
body: <p>I have managed to find circles quite easily thank to the built-in functionality of SimpleBlobDetector_create in openCV (4.2.0.34), I made sure the background would be white for easy recognition. 
  In the following image, 3 circles were found as I would expect:
  <a href="https://i.stack.imgur.com/0ViGf.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0ViGf.png" alt="enter image description here"></a>
  <strong>But for some reason strangely when I apply the same code on the image below,</strong> </p>
  
  <p><a href="https://i.stack.imgur.com/HvLxQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HvLxQ.png" alt="enter image description here"></a></p>
  
  <p><strong>This perfect circle doesn't get recognized. how come??</strong> </p>
  
  <p>here below is my short code.</p>
  
  <pre><code>img = cv2.imread(filename='img1.png')
  
  cv2.imshow(winname="Original", mat=img)
  cv2.waitKey(0)
  
  params = cv2.SimpleBlobDetector_Params()
  
  # set Circularity filtering parameters:
  params.filterByCircularity = True
  # 1 being perfect circle, 0 the opposite
  params.minCircularity = 0.8
  
  # create a detector with parameters
  detector = cv2.SimpleBlobDetector_create(parameters=params)
  
  keypoints = detector.detect(img)
  
  print("Number of circular Blobs: " + str(len(keypoints)))
  </code></pre>
  
  <p>Thank you all for any help!</p>