Three.js - Rotate around Axis at an **absoulte** angle

Question

I am using this function (from Three JS Pivot point) to rotate an Object around an axis in three.js:

// 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
}

The issue I am having, is that the transformation is persistent, as pointed out in the answer. So when i call the function again the transformation is applied from where it left off. How can I "reset" the transformation so that the provided angle is an absolute value?

The following does not work:

 obj.rotation.y = 0;

Thanks in advance!

Answer 1

The object's transformation is stored under obj.matrix

If you want to reset the object to have the default transformation (i.e. no scaling, rotation or translation), you can do this:

  obj.matrix.identity();
  obj.matrix.decompose(obj.position, obj.quaternion, obj.scale);

or if you just want to reset the rotation to the default, you can do this

  obj.quaternion.identity();

https://threejs.org/docs/index.html?q=matrix4#api/en/math/Matrix4.identity

If you want to return the object to some specific transformation that it had prior to the rotation, you could either.

1. reverse previous the rotation before applying the new one:

  obj.rotateOnAxis(oldAxis, -oldTheta);

or...

2. store off the old transform matrix and restore it before applying the new transformation. Needs to be stored somewhere persistent (so not e.g. a local variable). Various places you could potentially store it, but one that would definitely be available to you would be the object3D User Data.

  obj.userData.oldMatrix = obj.matrix.clone();

and then later re-instate it like this:

  obj.matrix.copy(obj.userData.oldMatrix);
  obj.matrix.decompose(obj.position, obj.quaternion, obj.scale);

https://threejs.org/docs/index.html?q=object3D#api/en/core/Object3D.userData