How to find the rotation of an object relative to the camera

Question

I'm having trouble figuring out the angle of an object relative to the camera, I'm trying to code a spaceship with a camera following it. I have the camera following the ship but the rotation of the camera sometimes is a little off, here's my camera code:

var focalpoint = new THREE.Vector3(
    actor.position.x,
    actor.position.y, 
    actor.position.z + 14
);

//move camera closer to the object if it gets too far away
var calculatedDistance = distance(camera.position, actor.position);
var cameraTolerance = calculatedDistance - this.cameradistance.min;
var closingvelocity = cameraTolerance * 0.02;

if(calculatedDistance > this.cameradistance.max)cameravelocity.z = -closingvelocity;

if(calculatedDistance < this.cameradistance.min)cameravelocity.z = closingvelocity;

//slow down the camera
if(calculatedDistance < this.cameradistance.max && calculatedDistance > this.cameradistance.min){
    cameravelocity.z = 0;
}

camera.translateX( cameravelocity.x );
camera.translateY( cameravelocity.y );
camera.translateZ( cameravelocity.z );
camera.lookAt(focalpoint);
camera.rotation.z = 0;

Now I need to limit the rotation of the spaceship (actor) so it doesn't start flying towards the camera, and to fix the camera flipping over problem. So I need to figure out how to find the rotation of the actor relative to the camera, I have absolutely no idea where to start calculating or even how.

Answer 1

found the answer, by inversing the target's rotation, then multiplying like so: var rotationOffset = actor.quaternion.clone().inverse(); var rotation = camera.quaternion.clone().multiply( rotationOffset );