title: opencv Laplacian function not work effectively like in document
body: <p>here is my code</p>
  
  <pre><code>img_original=cv2.imread("sudoku-original.jpg",0)
  cv2.imshow("original",img_original)
  laplacian = cv2.Laplacian(img_original,cv2.CV_64F)
  cv2.imshow("laplace",laplacian)
  </code></pre>
  
  <p>I want result like in the document but It don't.</p>
  
  <p>here is the link of document:<a href="https://docs.opencv.org/3.0-beta/doc/py_tutorials/py_imgproc/py_gradients/py_gradients.html#gradients" rel="nofollow noreferrer">https://docs.opencv.org/3.0-beta/doc/py_tutorials/py_imgproc/py_gradients/py_gradients.html#gradients</a></p>
  
  <p><a href="https://i.stack.imgur.com/CkPwU.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CkPwU.png" alt="enter image description here"></a></p>