title: Crop black edges with OpenCV
body: <p>I think it should be a very simple problem, but I cannot find a solution or an effective keyword for search.</p>
  
  <p>I just have this image.</p>
  
  <p><img src="https://i.stack.imgur.com/BSBL9.png" alt="the original image"></p>
  
  <p>The black edges are useless so that I want to cut them, only leaving the Windows icon (and the blue background).</p>
  
  <p>I do not want to calculate the coordinate and the size of the Windows icon.  GIMP and Photoshop have sort of autocrop function. OpenCV does not have one?</p>