multiplyQuaternions method
Implementation
Quaternion multiplyQuaternions(Quaternion a, Quaternion b) {
// from http://www.euclideanspace.com/maths/algebra/realNormedAlgebra/quaternions/code/index.htm
var qax = a._x, qay = a._y, qaz = a._z, qaw = a._w;
var qbx = b._x, qby = b._y, qbz = b._z, qbw = b._w;
_x = qax * qbw + qaw * qbx + qay * qbz - qaz * qby;
_y = qay * qbw + qaw * qby + qaz * qbx - qax * qbz;
_z = qaz * qbw + qaw * qbz + qax * qby - qay * qbx;
_w = qaw * qbw - qax * qbx - qay * qby - qaz * qbz;
onChangeCallback();
return this;
}