Physics bound bones: Almost like ragdolls, only not


I’ll be brief today. Does anyone know how to get bones that are bound by physics? It’s kind of like ragdoll physics, only it’s certain parts, like hair. Has anyone done it before? Or is the only option just to animate by hand?

Thanks in advance.

Spline IK?

AFAIK bones themselves don’t respond to physics, but you should be able to use constraints to have them move according to objects that do. It would require setting up a system of physics-controlled objects as well as the armature itself. With constraints, you could enable the physics-control when needed, leaving the rig free for keyframing in other situations.

@ ridix Your idea actually sounds promising, as I’ve never thought of that before! The tip could be just a little delayed than the rest, soit won’t necessarily be a physics model, but it might look convincing enough for a cartoon!

@ chipmasque Sorry! I’m a little stupid, as I never usually go beyond the scope of modelling, I’'m a little uneducated in such an advanced matter. Would you please sort of show me an example?

Thanks people! I’m loving the suggestions.

I’m not at my own computer now but I’ll try to describe a simple case. Say you have a bone the head of which is free to move. Apart from the armature you have an object that responds to physics. On the bone you put a Track To constraint that uses the physics object as its target (what it Tracks To), and the Y axis enabled (this is always the axis along the length of a bone). Now the bone will point at the physics object when the physics object moves. If the physics object is set to respond to a force like gravity or wind, etc., then when it moves, the bone will follow & look also as if it’s being moved by the physics.

Another example would be pretty much the same but using a Copy Location constraint. Then the bone would need to be unconnected, able to move independently of other bones in the armature, which is how bones used as IK Constraint targets are often set up. Say it controls the IK chain for an arm. If the physics object is moved, the arm will respond to that motion, because it tracks the IK Constraint target bone, which in turn tracks the physics object.

These are only the barest sketches of such a setup, so if you haven’t used constraints before, look them and try a few examples to discover how they work. Very powerful animation tools.

It turned out to be a bit more complicated than I thought because I forgot that Object centers don’t transform during physics sims. So I had to add an additional layer of linkage to get the physics objects to actually displace in terms of global coordinates. The attached .blend shows the setups I described.

PhysicsBoundBones.blend (420 KB)

The only diff between what I wrote above is that the physics objects now have two components. The mesh that actually responds to physics is now the parent of a “slave” object via vertex parenting. When the physics object responds to gravity, its object center doesn’t change so it can’t be used as the constraint target for the bone(s). But its slave’s center does change, so it becomes the constraint target, and then it all works as I described above.

Thanks! I’ll have to have a look at that later, when I have time.