title: Why cant I do blob detection on this binary image
body: <p>First, now i am doing the blob detection by Python2.7 with Opencv. What i want to do is to finish the blob detection after the color detection. i want to detect the red circles(marks), and to avoid other blob interference, i want to do color detection first, and then do the blob detection.</p>
  
  <p>and the image after color detection is <a href="http://i.stack.imgur.com/eRCe1.png" rel="nofollow">binary mask</a></p>
  
  <p>now i want to do blob detection on this image, but it doesn't work.
  This is my code.</p>
  
  <pre><code>import cv2
  import numpy as np;
  
  # Read image
  im = cv2.imread("myblob.jpg", cv2.IMREAD_GRAYSCALE)
  
  # Set up the detector with default parameters.
  
  params = cv2.SimpleBlobDetector_Params()
  # Change thresholds
  params.minThreshold = 10;    # the graylevel of images
  params.maxThreshold = 200;
  
  params.filterByColor = True
  params.blobColor = 255
  
  # Filter by Area
  params.filterByArea = False
  params.minArea = 10000
  
  detector = cv2.SimpleBlobDetector(params)
  
  
  # Detect blobs.
  keypoints = detector.detect(im)
  
  # Draw detected blobs as red circles.
  # cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS ensures the size of the circle corresponds to the size of blob
  im_with_keypoints = cv2.drawKeypoints(im, keypoints, np.array([]), (0,0,255), cv2.DRAW_MATCHES_FLAGS_DRAW_RICH_KEYPOINTS)
  
  # Show keypoints
  cv2.imshow("Keypoints", im_with_keypoints)
  cv2.waitKey(0)`
  </code></pre>
  
  <p>I am really confused by this code, because it work on this image <a href="http://i.stack.imgur.com/c0AXO.jpg" rel="nofollow">white dots</a>
  I think the white dots image is quiet similar with the binary mask,but why cant i do blob detection on the binary image?could anyone tell me the difference or the right code?</p>
  
  <p>Thanks!!</p>
  
  <p>Regards,
  Nan</p>