title: Blob detection in Python?
body: <p>I'm trying to detect a blob from the following image. I used the skimage and used the 3 different methods explained in the manual, but it's not able to detect the grey blob. Here is the original image:</p>
  
  <p><a href="https://i.stack.imgur.com/doco3.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/doco3.jpg" alt="enter image description here"></a></p>
  
  <p>So I tried the following code:</p>
  
  <pre><code>from math import sqrt
  import cv2
  from skimage.feature import blob_dog, blob_log, blob_doh
  from skimage.color import rgb2gray
  import matplotlib.pyplot as plt
  
  image = cv2.imread("blob800_cropped.png")
  image_gray = rgb2gray(image)
  
  blobs_log = blob_log(image_gray, max_sigma=30, num_sigma=10, threshold=.05)
  
  # Compute radii in the 3rd column.
  blobs_log[:, 2] = blobs_log[:, 2] * sqrt(2)
  
  blobs_dog = blob_dog(image_gray, max_sigma=30, threshold=.05)
  blobs_dog[:, 2] = blobs_dog[:, 2] * sqrt(2)
  
  blobs_doh = blob_doh(image_gray, max_sigma=30, threshold=.01)
  
  blobs_list = [blobs_log, blobs_dog, blobs_doh]
  colors = ['yellow', 'lime', 'red']
  titles = ['Laplacian of Gaussian', 'Difference of Gaussian',
            'Determinant of Hessian']
  sequence = zip(blobs_list, colors, titles)
  
  fig, axes = plt.subplots(1, 3, figsize=(9, 3), sharex=True, sharey=True)
  ax = axes.ravel()
  
  for idx, (blobs, color, title) in enumerate(sequence):
      ax[idx].set_title(title)
      ax[idx].imshow(image)
      for blob in blobs:
          y, x, r = blob
          c = plt.Circle((x, y), r, color=color, linewidth=2, fill=False)
          ax[idx].add_patch(c)
      ax[idx].set_axis_off()
  
  plt.tight_layout()
  plt.show()
  </code></pre>
  
  <p>However, the blob which I'm looking for is not detected:</p>
  
  <p><a href="https://i.stack.imgur.com/hs3bM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/hs3bM.png" alt="enter image description here"></a></p>
  
  <p>This is the output I'm expecting:</p>
  
  <p><a href="https://i.stack.imgur.com/ck2X3.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ck2X3.jpg" alt="enter image description here"></a></p>