/**
* Make a relative 3D vector of a limb out of a skeleton and the two joints
* in the limb.
*
* @param skel The skeleton to acquire the relative vector from.
* @param joint_combo The joint combo to make the relative vector from,
* where the first joint is the origin and the second
* is where the relative vector points to.
* @see moveRelative()
* @return the 3D relative vector between the base joint and the
* extended joint.
*/
private double[] moveJointCombo(Types.Skeleton skel,
Tuple<string, string> joint_combo)
{
double[] a = skel.getJointPositionByName(joint_combo.Item1);
double[] b = skel.getJointPositionByName(joint_combo.Item2);
return moveRelative(a, b);
}