title: get rotation from 2d transformation matrix
body: <p>Assuming I have a 2D rotation matrix:</p>
  
  <pre><code>cos(t) -sin(t) 0
  sin(t)  cos(t) 0
    0       0    1
  </code></pre>
  
  <p>How do you retrieve the correct rotation angle?</p>
  
  <p>If you use acos on the first element, you only get values between 0 and pi. The question is what do you do if <code>t</code> is smaller than 0 (of course I do not know t ;) )?</p>