title: Don't understand glutPostRedisplay and glIdleFunc
body: <p>I don't quite understand what the <code>glutPostRedisplay()</code> is used. It seems like when I call that in my function, at next frame <code>display()</code> function will be called. However I don't know how does that function relate to animation. We often register a function for physical computation as <code>Idle</code> function and do some computation there. However, if we change the parameters, like rotation, position of the objects that have animation, even if we don't call that <code>glutPostRedisplay()</code>, the <code>display()</code> function itself will redraw the scene. Why do we need to call <code>glutPostRedisplay()</code>?</p>
  
  <p>The second question is about <code>glutIdleFunc()</code>. My question is: is that possible the CPU is so busy that we don't have idle time at each frame? </p>