title: iOS Using GLKMathUnproject to find screen point in world space
body: <p>How to use GLKMathUnproject to determine location in world space?
  The user needs to select one of several objects in world space.</p>
  
  <p>In doc it says</p>
  
  <pre><code>GLKVector3 GLKMathUnproject (
     GLKVector3 window,
     GLKMatrix4 model,
     GLKMatrix4 projection,
     int *viewport,
     bool *success
  );
  </code></pre>
  
  <p>but which modelView matrix? I have bunch of models in world. Which model matrix should i use.</p>
  
  <p>My world is 2d, on x,y plane. z is used for camera movement.</p>
  
  <p>If i understood correctly, <code>GLKMathUnproject</code> is used to find a point in model space.
  How do i know which model? Do i need to determine first the model below the fingers or what?</p>