javascript - Three JS Pivot point

Question

What I'm trying to achieve is a rotation of the geometry around pivot point and make that the new definition of the geometry. I do not want te keep editing the rotationZ but I want to have the current rotationZ to be the new rotationZ 0.

This way when I create a new rotation task, it will start from the new given pivot point and the newly given rad.

What I've tried, but then the rotation point moves:

// Add cube to do calculations
var box = new THREE.Box3().setFromObject( o );
var size = box.getSize();
var offsetZ = size.z / 2;

o.geometry.translate(0, -offsetZ, 0)

// Do ratation
o.rotateZ(CalcUtils.degreeToRad(degree));

o.geometry.translate(0, offsetZ, 0)

I also tried to add a Group and rotate that group and then remove the group. But I need to keep the rotation without all the extra objects. The code I created

var box = new THREE.Box3().setFromObject( o );
    var size = box.size();

    var geometry = new THREE.BoxGeometry( 20, 20, 20 );
    var material = new THREE.MeshBasicMaterial( { color: 0xcc0000 } );

    var cube = new THREE.Mesh( geometry, material );

    cube.position.x = o.position.x;
    cube.position.y = 0; // Height / 2
    cube.position.z = -size.z / 2;

    o.position.x = 0;
    o.position.y = 0;
    o.position.z = size.z / 2;

    cube.add(o);

    scene.add(cube);

    // Do ratation
    cube.rotateY(CalcUtils.degreeToRad(degree));

    // Remove cube, and go back to single object
    var position = o.getWorldPosition();

    scene.add(o)
    scene.remove(cube);
    console.log(o);

    o.position.x = position.x;
    o.position.y = position.y;
    o.position.z = position.z;

So my question, how do I save the current rotation as the new 0 rotation point. Make the rotation final

EDIT I added an image of what I want to do. The object is green. I have a 0 point of the world (black). I have a 0 point of the object (red). And I have rotation point (blue).

How can I rotate the object around the blue point?

Answer 1

I wouldn't recommend updating the vertices, because you'll run into trouble with the normals (unless you keep them up-to-date, too). Basically, it's a lot of hassle to perform an action for which the transformation matrices were intended.

You came pretty close by translating, rotating, and un-translating, so you were on the right track. There are some built-in methods which can help make this super easy.

// obj - your object (THREE.Object3D or derived)
// point - the point of rotation (THREE.Vector3)
// axis - the axis of rotation (normalized THREE.Vector3)
// theta - radian value of rotation
// pointIsWorld - boolean indicating the point is in world coordinates (default = false)
function rotateAboutPoint(obj, point, axis, theta, pointIsWorld){
    pointIsWorld = (pointIsWorld === undefined)? false : pointIsWorld;

    if(pointIsWorld){
        obj.parent.localToWorld(obj.position); // compensate for world coordinate
    }

    obj.position.sub(point); // remove the offset
    obj.position.applyAxisAngle(axis, theta); // rotate the POSITION
    obj.position.add(point); // re-add the offset

    if(pointIsWorld){
        obj.parent.worldToLocal(obj.position); // undo world coordinates compensation
    }

    obj.rotateOnAxis(axis, theta); // rotate the OBJECT
}

After this method completes, the rotation/position IS persisted. The next time you call the method, it will transform the object from its current state to wherever your inputs define next.

Also note the compensation for using world coordinates. This allows you to use a point in either world coordinates or local space by converting the object's position vector into the correct coordinate system. It's probably best to use it this way any time your point and object are in different coordinate systems, though your observations may differ.