title: How to use GLKMathUnproject
body: <p>I've created a simple 3D world using GLKit. I'm trying to find a specific point in my 3D world based on a screen tap. It seems that I would want to use GLKMathUnproject. The signature of the method is:</p>
  
  <pre><code>GLKVector3 GLKMathUnproject (
     GLKVector3 window,
     GLKMatrix4 model,
     GLKMatrix4 projection,
     int *viewport,
     bool *success
  );
  </code></pre>
  
  <p>A few things confuse me about the params it requires. First shouldn't the window coordinates be GLKVector2? And most importantly wouldn't projecting a point on the screen into 3D space create a ray instead of a single point. 
  When you project points in 3D space onto a 2D plain (aka the screen) you loose one dimension of information about that point. So how would it be possible to transform a 2D screen point into a 3D world point without specifying at what depth you'd like to 'unproject' your point to? There is absolutely no comment about this method anywhere on the internet (besides apple docs).</p>