title: Setting the coordinate system for drawing in OpenGL
body: <p>I just started reading initial chapters of Blue book and got to understand that the projection matrix can be used to modify the mapping of our desired coordinate system to real screen coordinates. It can be used to reset the coordinate system and change it from -1 to 1 on left, right, top and bottom by the following (as an example)<br/></p>
  
  <pre><code>glMatrixMode(GL_PROJECTION);
  glLoadIdentity(); //With 1's in the diagonal of the identity matrix, the coordinate system is rest from -1 to 1 (and the drawing should happen then inside those coordinates which be mapped later to the screen)
  </code></pre>
  
  <p>Another example: (Width: 1024, Height: 768, Aspect Ratio: 1.33) and to change the coordinate system, do:</p>
  
  <p><code>glOrtho (-100.0 * aspectRatio, 100.0 * aspectRatio, -100.0, 100.0, 100.0, 1000.0);</code></p>
  
  <p>I expected the coordinate system for OpenGL to change to -133 on left, 133 on right, -100 on bottom and 100 on top. Using these coordinates, I understand that the drawing will be done inside these coordinate and anything outside these coordinates will be clipped.<br/></p>
  
  <pre><code>glMatrixMode(GL_PROJECTION);
  glLoadIdentity();
      glOrtho(-100 * aspectRatio, 100 * aspectRatio, -100, 100, 100, 1000);
  glMatrixMode(GL_MODELVIEW);
      glRectf(-50.0, 50.0, 200, 100);
  </code></pre>
  
  <p>However, the above command doesn't give me any output on the screen. What am I missing here?</p>