title: Screen-to-World coordinate conversion in OpenGLES an easy task?
body: <p><strong>The Screen-to-world problem on the iPhon</strong>e<br></p>
  
  <p>I have a 3D model (CUBE) rendered in an EAGLView and I want to be able to detect when I am touching the center of a given face (From any orientation angle) of the cube. Sounds pretty easy but it is not...    </p>
  
  <p><strong>The problem:</strong><br>
   How do I accurately relate screen-coordinates (touch point) to world-coordinates (a location in OpenGL 3D space)? Sure, converting a given point into a 'percentage' of the screen/world-axis might seem the logical fix, but problems would arise when I need to zoom or rotate the 3D space. Note: rotating &amp; zooming in and out of the 3D space will change the relationship of the 2D screen coords with the 3D world coords...Also, you'd have to allow for 'distance' in between the viewpoint and objects in 3D space. At first, this might seem  like an 'easy task', but that changes when you actually examine the requirements. And I've found no examples of people doing this on the iPhone. How is this normally done? </p>
  
  <p><strong>An 'easy' task?:</strong><br>
  Sure, one might undertake the task of writing an API to act as a go-between between screen and world, but the task of creating such a framework would require some serious design and would likely take 'time' to do -- NOT something that can be <em>one-manned</em> in 4 hours...And 4 hours happens to be my deadline. </p>
  
  <p>The question: </p>
  
  <ul>
  <li>What are some of the simplest ways to
  know if I touched specific locations
  in 3D space in the iPhone OpenGL ES
  world?</li>
  </ul>