title: Reduce opacity of image using Opencv in Python
body: <p>In the program given below I am adding alpha channel to a 3 channel image to control its opacity. But no matter what value of alpha channel I give there is no effect on image! Anyone could explain me why?</p>
  
  <pre><code>import numpy as np
  import cv2
  
  image = cv2.imread('image.jpg')
  print image
  
  b_channel,g_channel,r_channel = cv2.split(image)
  a_channel = np.ones(b_channel.shape, dtype=b_channel.dtype)*10
  image = cv2.merge((b_channel,g_channel,r_channel,a_channel))
  
  print image
  cv2.imshow('img',image)
  cv2.waitKey(0)
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>I can see in the terminal that alpha channel is added and its value changes as I change it in the program, but there is no effect on the opacity of the image itself!</p>
  
  <p>I am new to OpenCV so I might be missing something simple. Thanks for help!</p>