Three.js Transform 3D coordinates to screen coordinates and back in perspective projection - http://catchvar.com/threejs-game-transforming-isometric-screen-co

perspectiveProject.js

var
    projector = new THREE.Projector(),
    p3D = new THREE.Vector3(25, 15, 9),
    p2D;

p2D = projector.projectVector(p3D, camera);
p3D = projector.unprojectVector(p2D, camera);

//need extra steps to convert p2D to window's coordinates
p2D.x = (p2D.x + 1)/2 * window.innerWidth;
p2D.y = - (p2D.y - 1)/2 * window.innerHeight;

var popout = document.getElementById('popout');
popout.style.left = p2D.x + 'px';
popout.style.top = p2D.y + 'px';