MHX rig: Shoulders

In this thread Rhynedahll complained about problems with shoulder deformation. That shoulders lead to problems is not surprising, since it is generally considered as the part of the body that is hardest to rig. I was somewhat reluctant to look into the shoulder rig, because it had become a morass which I only partly understood. But eventually I decided to redo the shoulder rigging from scratch.

The purpose with this post is threefold:
1. To document the rig.
2. To describe a shoulder rigging technique which may be of interest to others.
3. To get feedback from more experienced riggers.

The main idea is to use two kinds of deformation bones: the bone bones, which correspond to actual bones in the skeleton, and muscle bones which mimic deformation from muscles moving under the skin. Without muscles deformation tends to be very poor. This idea was suggested to me by someone on this forum, and the idea was used in the previous shoulder rig, too, but the location of the muscles was not anatomically correct and the end result not so good. This time I actually bothered to look up references, and I think the result is quite believable.


The new shoulder rig is controlled by four bones. The main control bones are the upper arm and the clavicle, which is incorrectly called Shoulder. This was a natural name where the bone was displayed as a box at its end. It should change eventually, but since a new name would affect the mocap tool I will postpone it a little. There are also two new bones: the arm location and the deltoid control. More about that later.


The basic deformation is carried out by the bone bones: the spine continuing into the neck, the clavicle, and the upper arm. To facilitate weight painting, there is also a rib bone at the front of the chest. Finally, the scapula gives some extra movement to the back.


At the front of the chest we have the pectoralis muscle. It is represented by two bones to mimic its triangular shape. The bone location is not quite correct and will probably be improved later, but it already gives quite descent deformation. We also see the deltoid muscle.


The back has two bones for the trapezeus and one bone for latissimus dorsi.


The purpose of the deltoid bone is to maintain the volume of the deltoid. This is quite tricky, because the bone must move when the arm bends up, but not when it bends down. Some rigging magic makes this happen, but the movement is not perfect. You can fine-tune the deltoid shape with the C-shaped control bone, although some improvements of the weighting is necessary to make this work well.


Finally, the location where the arm begins can be adjusted with the box-shaped bone. This is of course cheating, but it can really improve a pose.

I believe that the basic idea is fundamentally right. If the bones are placed in the same place as the major muscles, deformation should look good, because the muscle bones obey the same laws.