title: Opencv - Grayscale mode Vs gray color conversion
body: <p>I am working in opencv(2.4.11) python(2.7) and was playing around with gray images. I found an unusual behavior when loading image in gray scale mode and converting image from BGR to GRAY. Following is my experimental code:</p>
  
  <pre><code>import cv2
  
  path = 'some/path/to/color/image.jpg'
  
  # Load color image (BGR) and convert to gray
  img = cv2.imread(path)
  img_gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
  
  # Load in grayscale mode
  img_gray_mode = cv2.imread(path, 0)
  
  # diff = img_gray_mode - img_gray
  diff = cv2.bitwise_xor(img_gray,img_gray_mode)
  
  cv2.imshow('diff', diff)
  cv2.waitKey()
  </code></pre>
  
  <p>When I viewed the difference image, I can see the left out pixels instead of jet black image. Can you suggest any reason? What is the correct way of working with gray images.<br/><br/>
  <em>P.S. When I use both the images in SIFT, keypoints are different which may lead to different outcome specially when working with bad quality images.</em></p>