title: how to get all undistorted image with opencv
body: <p>I'm unsing <code>cv::undistort</code> but it crops the image. I'd like to have all the undistorted image, so that the undistorted size is bigger then the original one, like this:</p>
  
  <p><img src="https://i.stack.imgur.com/UU6t0.png" alt="enter image description here"></p>
  
  <p>I think I need to use <code>cv::getOptimalNewCameraMatrix</code> but I had no luck with my trials.. some help?</p>