code: def circular_mask(size):
      
      r = size/2
      c = (r,r)
      y,x = np.ogrid[-c[0]:size-c[0], -c[1]:size-c[1]]
      mask = ~(x*x + y*y <= r*r)
      return mask
docs: Create a circular mask for an array
      
      Useful when sampling rasters for a laser shot