I have a scene graph and IK system with hierarchical transformations, but currently it only allows for a segment in the IK system to have a single parent segment. It works fine for something like two arms attached to a torso, as that just has a segment with two children. But for two legs attached to a torso, with the torso being dependent on the angles of the legs, it does not work.

I am wondering if there is a way to calculate the parent transformation of the torso or other similar circumstance when it has multiple parent segments, such that some attachment point that is the average of all parent segments' endpoints is correctly translated and rotated based on the total transformation of the both leg transformations.

An example being, the legs sway to some direction, the torso should reflect the intuitive change of its transformation (I say intuitive because I don't know how to define it in mathematical terms).

My goal is to just have setup in my scene graph and IK solver which can by default automatically handle scene nodes / IK segments with multiple parents.