title: THREE.js check if object is in frustum
body: <p>i've been tinkering around with three.js and i have a canvas i'd like to use as kind of a GUI. for that i have to check if an object is in the camera frustum.</p>
  <p>my current code:</p>
  <pre><code>camera.updateMatrix(); 
  camera.updateMatrixWorld(); 
          
  var frustum = new THREE.Frustum();
  var projScreenMatrix = new THREE.Matrix4();
  projScreenMatrix.multiplyMatrices( camera.projectionMatrix, camera.matrixWorldInverse );
          
  frustum.setFromProjectionMatrix( camera.projectionMatrix );
          
  if(frustum.containsPoint( mesh.position )){
      //stuff happens...
  };
  </code></pre>
  <p><code>frustum.containsPoint()</code>keeps returning false. what am i doing wrong here?</p>