title: In OpenCV, converting 2d image point to 3d world unit vector
body: <p>I have calibrated my camera with OpenCV (findChessboard etc) so I have:
  - Camera Distortion Coefficients &amp; Intrinsics matrix
  - Camera Pose information (Translation &amp; Rotation, computed separatedly via other means) as Euler Angles &amp; a 4x4
  - 2D points within the camera frame</p>
  
  <p>How can I convert these 2D points into 3D unit vectors pointing out into the world? I tried using cv::undistortPoints but that doesn't seem to do it (only returns 2D remapped points), and I'm not exactly sure what method of matrix math to use to model the camera via the Camera intrinsics I have.</p>