title: glFrustum, GL_MODELVIEW versus GL_PROJECTION
body: <p>I am trying to learn that but I really don't understand.
  Using:</p>
  
  <pre><code>glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
  glColor3f(1.0, 1.0, 1.0);
  glBegin(GL_QUADS);
  glVertex3f(-0.5, -0.5, 0.5);
  glVertex3f(0.5, -0.5, 0.5);
  glVertex3f(0.5, 0.5, 0.5);
  glVertex3f(-0.5, 0.5, 0.5);
  glEnd();
  </code></pre>
  
  <p>That's the only thing I do in the draw routine. I am drawing a face and it's all OK.</p>
  
  <p>Now I want to see the same face closer and farther from the observer, so I change the Z coordinate. The problem is that the size of the face as it appears on the screen does not change.</p>
  
  <p>I did a try setting a frustum with <code>glFrustum(-15, 15, -15, 15, 0.1, 15)</code>, but then everything disappears (the face isn't in frustum view?).
  All I want is to draw a face, setting a frustum and moving the face along the Z coordinate seeing as the face getting bigger or smaller. I know there are <code>GL_MODELVIEW</code> and <code>GL_PROJECTION</code>, but I don't understand what they are, maybe this is the trouble.</p>