title: From window to opengl coordinate system intuitive explanation
body: <p>I am trying to understand the map from window coordinate axis (origin top-left) to <code>OpenGL</code> coordinate axis (origin left-bottom) when using the mouse function. In relevant book this map is described by the two following lines:</p>
  
  <pre><code>points[count].x = (float) x / (w/2) - 1.0;
  points[count].y = (float) (h-y) / (h/2) - 1.0;
  </code></pre>
  
  <p>I suspect that these two lines depict a scale. Could you please give an intuitive-mathematical explanation of this map?</p>