Copy rotation.order when copying Object3D

[luisfonsivevo]

Not sure if there is a technical reason behind why the euler rotation order isn't cloned by Object3D.copy(), but hey now it is.

[trusktr]

There should instead be a line

this.rotation.copy(source.rotation)

that handles it.

[luisfonsivevo]

If we copy the entire euler instead of only copying its order, it won't be necessary to copy the quaternion anymore. Because inside Euler.copy(), the onChangeCallback is called which automatically recalculates the quaternion. So really, it's only necessary to copy either the quaternion or the euler.

This (what I have currently)

this.position.copy( source.position );
this.rotation.order = source.rotation.order;
this.quaternion.copy( source.quaternion );
this.scale.copy( source.scale );

or this (a bit more concise)

this.position.copy( source.position );
this.rotation.copy( source.rotation );
this.scale.copy( source.scale );

will both get the job done.

[WestLangley]

1. three.js is quaternion-based; the Euler angles are provided for user-convenience.

2. Quaternions are not unique -- and neither are Euler angles. Copying object.rotation will result in identical Euler angles, but possibly different quaternions.

That is why the quaternion is copied, and not the rotation.

Failing to copy rotation.order was an oversight.

This PR, as written, is correct.

[luisfonsivevo]

When you say quaternions are not unique, do you mean that two quaternions with different xyzw values can represent the same orientation? I never knew that.

[mrdoob]

Thanks!

[WestLangley]

@luisfonsivevo Unit quaternions q( x, y, z, w ) and q( - x, - y, - z, - w ) represent the same orientations.