title: passive motion detect
body: <p>How can I detect mouse passive motion with opengl ? In other words, how can I understand whether it is moving forward, backward, left, right ?</p>
  
  <p>I have done </p>
  
  <pre><code>glutPassiveMotionFunc ( func ) 
  
  void func ( int x, int y ) {
  
     // x and y always positive, I wait it should be negative if it goes left 
     //   acc. to  my coordinate system determined in glLookAt.
  }
  </code></pre>