Question

I have got a ray and a plane. The ray is intersecting the plane and I would like to know where. How do I get the world-coordinates of this intersetion?

(My particular case I unproject screen-coordinates to the camera and create the ray. The plane is the ground of the scene)

var vector = new THREE.Vector3( mousePosition.x, mousePosition.y, 1 );
projector.unprojectVector( vector, camera );
var ray = new THREE.Ray(
    camera.position,
    vector.subSelf( camera.position ).normalize()
);

Answer 1

This is the usual pattern. You may have to adapt it to fit your particular situation.

var raycaster = new THREE.Raycaster(); // create once and reuse
var mouse = new THREE.Vector2(); // create once and reuse

...

mouse.x = ( event.clientX / renderer.domElement.clientWidth ) * 2 - 1;
mouse.y = - ( event.clientY / renderer.domElement.clientHeight ) * 2 + 1;

raycaster.setFromCamera( mouse, camera );

var intersects = raycaster.intersectObjects( objects, recursiveFlag );

if ( intersects.length > 0 ) {

    console.log( intersects[ 0 ].point; );

}

Here, objects is an array. For example,

var objects = [];

objects.push( plane_mesh );

EDIT: Updated for three.js r.84