title: What do the arguments for frustum in OpenGL mean?
body: <p>I've understood that a frustum is a clipping cube inside which the camera can see. I just don't understand the arguments needed to define it. From <a href="http://developer.android.com/reference/android/opengl/Matrix.html" rel="noreferrer">here</a>, it looks like you specify parts of some kind of space. But for example, top and bottom of what? What do ratio and -ratio represent? Here's an example function that I'm talking about:</p>
  
  <pre><code>Matrix.frustumM(matrixHandle, 0, ratio, -ratio, -1, 1, 3, 7);
  </code></pre>