title: what is the actual difference between gluortho2d and glViewport
body: <p>I made a window sized <code>800x600</code>. I called</p>
  
  <pre><code>gluOrtho2D(-400,400,-300,300);
  glViewport(400,300,400,300);
  </code></pre>
  
  <p>and I drew a line from <code>(-100,-100)</code> to <code>(100,100)</code>. I think I should see a line from <code>(0,0)</code> to <code>(100,100)</code>, but I am getting the whole line. Why is this?</p>