title: Orbit camera redirection in threejs
body: <p>I have a problem about camera redirection, unsolved for a long time. Briefly, I have a camera whose up vector is always <code>(0,1,0)</code>. I want to move camera's position while keeping a certain 3D point appearing in the same position of the screen.</p>
  
  <p>For example, let <code>P</code> be the camera's position, <code>T</code> be the camera's lookat, and <code>F</code> be the point appearing in <code>(x,y)</code> of the screen. Now <code>P</code> is changed to <code>P'</code> (camera's new position); in order to make <code>F</code> still appear in <code>(x,y)</code> of the screen, <code>T</code> must be changed.</p>
  
  <p>Question is, what is the new value of <code>T</code>?</p>
  
  <p>I believe the problem needs a lot of math. Tell me if you have any idea. Thanks greatly!</p>