code: def hprewitt(image, mask=None):
      '''Find the horizontal edges of an image using the Prewitt transform
  
      image - image to process
      mask  - mask of relevant points
  
      We use the following kernel and return the absolute value of the
      result at each point:
       1   1   1
       0   0   0
      -1  -1  -1
      '''
      if mask is None:
          mask = np.ones(image.shape, bool)
      big_mask = binary_erosion(mask,
                                generate_binary_structure(2,2),
                                border_value = 0)
      result = np.abs(convolve(image, np.array([[ 1, 1, 1],
                                                [ 0, 0, 0],
                                                [-1,-1,-1]]).astype(float)/3.0))
      result[big_mask==False] = 0
      return resulf hprewitt(image, mask=None):
      '''Find the horizontal edges of an image using the Prewitt transform
  
      image - image to process
      mask  - mask of relevant points
  
      We use the following kernel and return the absolute value of the
      result at each point:
       1   1   1
       0   0   0
      -1  -1  -1
      '''
      if mask is None:
          mask = np.ones(image.shape, bool)
      big_mask = binary_erosion(mask,
                                generate_binary_structure(2,2),
                                border_value = 0)
      result = np.abs(convolve(image, np.array([[ 1, 1, 1],
                                                [ 0, 0, 0],
                                                [-1,-1,-1]]).astype(float)/3.0))
      result[big_mask==False] = 0
      return result
docs: Find the horizontal edges of an image using the Prewitt transform
  
      image - image to process
      mask  - mask of relevant points
  
      We use the following kernel and return the absolute value of the
      result at each point:
       1   1   1
       0   0   0
      -1  -1  -1