title: Calculate a Homography with only Translation, Rotation and Scale in Opencv
body: <p>I do have two sets of points and I want to find the best transformation between them. 
  In OpenCV, you have the following function:</p>
  
  <pre><code>Mat H = Calib3d.findHomography(src_points, dest_points);
  </code></pre>
  
  <p>that returns you a 3x3 Homography matrix, using RANSAC. My problem is now, that I only need translation and rotation (&amp; maybe scale), I don't need affine and perspective. </p>
  
  <p>The thing is, my points are only in 2D.</p>
  
  <p>(1) Is there a function to compute something like a homography but with less degrees of freedom?</p>
  
  <p>(2) If there is none, is it possible to extract a 3x3 matrix that does only translation and rotation from the 3x3 homography matrix?</p>
  
  <p>Thanks in advance for any help!</p>
  
  <p>Isa</p>