title: How to manual convert `BGR` image to `grayscale` (Python OpenCV)?
body: <p>I want to manually convert a RGB image to Grayscale image.
  What I want to know is how to get the Red/Blue/Green values of a RGB pixel ?</p>
  
  <pre><code>img = cv2.imread("images/penguins.jpg",0)
  grey = img
  for i in range(0,grey.shape[0]-1):
      for j in range(0,grey.shape[1]-1):
          img[i,j]=[ ]
  </code></pre>
  
  <p>I dont know what to do next</p>
  
  <p>Thanks,
  Sorry for my bad english</p>