title: gluLookAt explanation?
body: <p>Trying to understand gluLookAt, especially the last 3 parameters.  </p>
  
  <p>Can someone please explain ?</p>
  
  <pre><code>gluLookAt(camera[0], camera[1], camera[2], /* look from camera XYZ */ 
            0, 0, 0,  /* look at the origin */ 
            0, 1, 0); /* positive Y up vector */
  </code></pre>
  
  <ol>
  <li>What exactly does it mean by "positive Y up vector" ?</li>
  <li>Is it possible to have the last up-vector 3 parameters as all 1s, e.g. 1, 1, 1 ?  And, if it is possible, what exactly does it mean ?</li>
  <li>Is it possible for the up vector to have value more than 1, e.g. 2, 3, 4 ?</li>
  </ol>
  
  <p>Thanks.</p>