Update position of mesh after rotation of THREE.Group() - three.js

Question

I use a pivot group to rotate my planegeometries instead of each one individually. After rotation of the pivot group-object, I want to find the new positions for each child/planegeometry-mesh to correspond to the positions that is relative to the actual world position.
How do I go about doing this?

Answer 1

The easy way
Like Craig mentioned, getWorldPosition is a function on Object3D (the base class of pretty much everything in the scene), which returns a new Vector3 of the object's world position.
var childPlaneWorldPosition = childPlane.getWorldPosition();
The harder way:
There are two methods for converting between local and world positions: localToWorld and worldToLocal.
These are also functions on Object3D, and take a Vector3. The vector is then (destructively) converted to the desired coordinate system. Just know that it's not smart enough to know if the vector you're giving it is already in the right coordinate system--you'll need to keep track of that.
So, to convert a child plane's position from local to world coordinates, you would do this:
// clone because localToWorld changes the vector passed to it
var childPlanePosition = childPlane.position.clone();
childPlane.parent.localToWorld(childPlanePosition);
Notice that localToWorld is called on childPlane's parent. This is because the childPlane is local to its parent, and therefore its position is local to its parent's coordinate system.
The hard(er to understand) way:
Each childPlane stores not only its local transformation matrix (childPlane.matrix), but also its world transformation matrix (childPlane.matrixWorld). You can, of course, get the world position directly from the matrixWorld property in one step.
var childWorldPosition = new THREE.Vector3(
childPlane.matrixWorld.elements[12],
childPlane.matrixWorld.elements[13],
childPlane.matrixWorld.elements[14]
);
Edit to answer some questions
"If I understand correctly, can I find the "real" position of the meshes in the pivot-group children-array?"
Yes. If you called:
pivotGroup.add(childPlane);
Then that childPlane will be listed in the pivotGroup.children array, which you could use to iterate over all of the childPlane objects
"And clone these to the position object for each meshes?"
If you want the planes to be in world coordinates (in the scene), but you used the above code to add them to the group, then they are no longer direct children of the scene. You would need to re-add them to the scene:
scene.add(childPlane);
And then apply their calculated world positions. That said, why not just leave them in the group?
(You didn't ask this one) "How would you leave the planes as direct children of the scene, but rotate them as a group?"
Well, you wouldn't. But three.js does this group rotation by multiplying matrices to come up with finalized world matrices for each plane. So you could do the same thing manually, by creating a rotation matrix, and applying it to all of your planes.
var rotMat = new THREE.Matrix4().makeRotationMatrix(x, y, z);
for(var i = 0; i < planesArray.length; ++i){ // I guess this would loop over your 3D array
planesArray.applyMatrix(rotMat);
}

Answer 2

Use plane_mesh.getWorldPosition()