I’ve done rigging in blender before, and I’ve used the rigify add-on. Deformation bones are self-explanatory, and Control Bones are easy to understand (Constrain the deformation bones to the Control bones, then make a custom shape.), but Mechanical bones (or Mechanism Bones) is a tricky one for me to make sense of; they don’t deform the movements of a character, but the animator doesn’t have direct control of these bones either. Additionally, I’ve recently learned that custom properties are certain actions to work via MCH Bones. The example of MCH Bones I can understand easily are the switch between IK/FK bones. Otherwise, I have a hard time understanding how MCH Bones work.
On another note, where do the MCH Bones stand on the hierarchy? I’m thinking certain MCH bones should be parented to the Control Bones and some to the Deformation bones. Idk, it’s a tricky 50/50 circumstance that I haven’t fully learned on my own.
In my understanding, “Mechanism Bones” can be conceived as an umbrella term.
Although there exist multiple distinctions of functionalities that MCH Bones can assume in a Rig Setup, they can be generalized because all they are doing is akin to “Helper Bones” concept for the Deform Rig. Except that Mechanism Bones would be “Helper Bones” but for the Control Rig --and how it relates to the Deform Rig.
Ideally, to prevent confusion, I would prefer to have a specialized name for this broad genre of Bones. I just think “Mechanism” is a good candidate for that, but there might exist better conceptual approaches.
A (TGT) “Target” Bone, for instance, is what I personally call a (MAP) ‘Mapper’ Bone. It’s an analogue functionality, although might not be exactly the same. A “Buffer” Bone, is a Bone that will intake and hold Transforms data from certain ‘Source’ Bone(s), and then deliver such data to another/other ‘Destination’ Bone(s). All those are ‘Generic Mechanism Bones’ in the end in this conceptualization of mine. And there are other examples.
The timestamps below are not always accurate unfortunately, but here are some video examples of “Buffer” Bone cases. In my opinion, essentially, “Mechanism” Bones used to convert Transforms data:
In the end, it’s up to the rigger, how to conceive and to establish these named functionalities in Rigging. As long as it is meaningful in some way, and that it works for the project, it should be adequate proposition. Naturally, theoretical investigation can improve conceptualization, and learning or adapting to certain useful conventions can help in this sense.
