title: OpenCV: How to use free scaling parameter (alpha) when dealing with getOptimalNewCameraMatrix and stereoRectify?
body: <p>How to use free scaling parameter (alpha) when dealing with <code>getOptimalNewCameraMatrix</code> and <code>stereoRectify</code> : should one use the same value ?</p>
  <p>As far as I understand it, I guess a few things that led me to this question are worth to be listed:</p>
  <ul>
  <li><p>In <code>getOptimalNewCameraMatrix</code>, OpenCV doc says <code>&quot;alpha   Free scaling parameter between 0 (when all the pixels in the undistorted image are valid) and 1 (when all the source image pixels are retained in the undistorted image)&quot;</code> [sounds to me like 1 = retain source pixels = minimize loss]</p>
  </li>
  <li><p>In <code>stereoRectify</code>, OpenCV doc says <code>&quot;alpha   Free scaling parameter.... alpha=0 means that ... (no black areas after rectification). alpha=1 means that ... (no source image pixels are lost)</code></p>
  </li>
  <li><p>So in the end alpha, seems to be a parameter that may &quot;act&quot; the same way ? (1 = no source pixel lost - sounds like, not sure here)</p>
  </li>
  <li><p>As far as I understand it, after <code>calibrateCamera</code>, one may want to call <code>getOptimalNewCameraMatrix</code> (computing new matrices as outputs) and then <code>stereoRectify</code> (using new computed matrices as inputs) : do one want to use the same alpha?</p>
  </li>
  </ul>
  <p>Are these 2 alphas the same? Or does one want to use 2 different alphas?</p>