title: How to compute opposite view from a quaternion rotation?
body: <p>I have a quaternion rotation, as usually described by 4 values: <code>a b c d</code>.
  Lets say it transforms the x axis so that i look at some object from the front. Now i want to change this rotation so i look at the object from the back. 
  So basicly i want to change the viewpoint from front to back, but do that using this rotation. </p>
  
  <p>How can the opposite rotation be computed? </p>