title: Converting image to grayscale
body: <p>I want to convert any image to grayscale, but I don't understand the difference between these implementations.</p>
  
  <pre><code>image = cv2.imread('lenna.jpg')
  gray = cv2.cvtColor(image, cv2.IMREAD_GRAYSCALE)
  
  gray1 = rgb2gray(image)
  
  gray2 = cv2.imread('lenna.jpg', cv2.IMREAD_GRAYSCALE)
  
  image1 = Image.open('lenna.jpg', 'r')
  gray3 = image1.convert('L')
  </code></pre>
  
  <p>When I plot them, I get them in blue scale, green scale, green scale and gray respectively. When I should use each one?</p>