title: opencv / numpy how to get the sum of each pixel
body: <p>I've got an image 2d array with each pixel containing rgb value (bgr in opencv i guess) and i'm trying to get the a new 2d array which has the sum of each pixel instead.</p>
  <p>e.g.</p>
  <p>start image:</p>
  <p>shape: (1080,1920,3)</p>
  <p>[[[255,255,255], [0,0,0]],
  [[0,120,255], [0,255,0]]]</p>
  <p>result:</p>
  <p>shape: (1080,1920,1)</p>
  <p>[[[765],[0]],
  [[375],[255]]]</p>
  <p>I'm sure there's a simple Numpy solution that I just do not know yet...</p>
  <p>Any help would be greatly appreciated!</p>