setAxisAngleFromQuaternion( q ) {

		// http://www.euclideanspace.com/maths/geometry/rotations/conversions/quaternionToAngle/index.htm

		// q is assumed to be normalized

		this.w = 2 * Math.acos( q.w );

		const s = Math.sqrt( 1 - q.w * q.w );

		if ( s < 0.0001 ) {

			this.x = 1;
			this.y = 0;
			this.z = 0;

		} else {

			this.x = q.x / s;
			this.y = q.y / s;
			this.z = q.z / s;

		}

		return this;

	}
Sets the x, y and z components of this
vector to the quaternion's axis and w to the angle.

@param {Quaternion} q - The Quaternion to set.
@return {Vector4} A reference to this vector.