title: OpenNI range of returned coordinates
body: <p>I am using the HandsGenerator class of OpenNI, and I want to use it to track the users' movements.</p>
  
  <p>I've registered my own callback for getting the updated position of the hand, and everything works fine, except I can't find information about the coordinate system etc. of the returned <code>XnPoint3D</code>. Is there a spec somewhere that precisely specifies the X,Y,Z ranges, and perhaps scaling information (so that I would know that say a change of 100 in the <code>XnPoint3D</code>'s <code>X</code> corresponds to a movement of 10 centimeters, or something).</p>