title: Converting World coordinates to Screen coordinates in Three.js using Projection
body: <p>There are several excellent stack questions (<a href="https://stackoverflow.com/questions/11036106/three-js-projector-and-ray-objects">1</a>, <a href="https://stackoverflow.com/questions/7956442/detect-clicked-object-in-three-js">2</a>) about unprojecting in Three.js, that is how to convert (x,y) mouse coordinates in the browser to the (x,y,z) coordinates in Three.js canvas space.  Mostly they follow this pattern:</p>
  
  <pre><code>    var elem = renderer.domElement, 
          boundingRect = elem.getBoundingClientRect(),
          x = (event.clientX - boundingRect.left) * (elem.width / boundingRect.width),
          y = (event.clientY - boundingRect.top) * (elem.height / boundingRect.height);
  
      var vector = new THREE.Vector3( 
          ( x / WIDTH ) * 2 - 1, 
          - ( y / HEIGHT ) * 2 + 1, 
          0.5 
      );
  
      projector.unprojectVector( vector, camera );
      var ray = new THREE.Ray( camera.position, vector.subSelf( camera.position ).normalize() );
      var intersects = ray.intersectObjects( scene.children );
  </code></pre>
  
  <p>I have been attempting to do the reverse - instead of going from "screen to world" space, to go from "world to screen" space. If I know the position of the object in Three.js, how do I determine its position on the screen?</p>
  
  <p>There does not seem to be any published solution to this problem. <a href="https://stackoverflow.com/questions/11534000/three-js-converting-3d-position-to-2d-screen-position">Another question about this just showed up on Stack</a>, but the author claims to have solved the problem with a function that is not working for me. Their solution does not use a projected Ray, and I am pretty sure that since 2D to 3D uses unprojectVector(), that the 3D to 2D solution will require projectVector().</p>
  
  <p>There is also <a href="https://github.com/mrdoob/three.js/issues/899" rel="noreferrer">this issue</a> opened on Github.</p>
  
  <p>Any help is appreciated.</p>