Damper/Accordion Rigg - Question

I have to solve a rigg on damper, that looks like an accordion. It has elements our of gum and steel frames. Almost like the music instrument. Now, a deformer won’t work as the steel frames should not deform, they are quit thin, so it would immediately break the mesh.
My approach right now is o use a mix of a spline deformer, where a spline deformers the gum part, and also some simple mesh plains, that I replace with the steel frames in a geometry node.
This works as long as I do not increase the spline length by pushing it. While this works on the frames, it doesn’t on the gum. Because Blenders curve deformer has no option to use the full length of the spline.
So I have to measure the length of the curve in a driver, to scale the gum mesh. However, drivers do not support that, only scripts do. So I’m running into a dead end here.

Has any one a better idea on how to rigg that?
I have tried bendy bones once, but then I have to do that rigg each time I do mesh changes again. And auto rigging didn’t work probably, because you really need to absolute values on the frames, or it get moved wrong. Its a hand job to do right and I want to be free to add more frames later.

I’m not totally sure what you’re after-- some pics or a file would help-- but if I hear “accordion,” I’m thinking series of 2-chain IKs, like this:

The mechanism bone stretches from root to control. IK targets are parented to the mechanism bone.

I wouldn’t use a spline to make this kind of structure. However,

Spline IK can stretch to the full length of the spline. A curve modifier can as well, if you enable “stretch” on the curve in its properties. Might need to also enable “bounds clamp”, I always do that when I’m making a curve for something like that.

So do geometry nodes, which are quite a bit easier. No, you can’t directly control bones with GN, but you can place vertices and then copy position/track by vertex group.

You are right, I can get the curve length in the geo nodes and do the scale transform there.
I’m not using spline IK but the spline deformer. But yes the spline Ik does so. I just would need to define the iterations of the bones before starting rigging, which I do not really want to. Better keep things dynamic.
I was actually hoping geometry nodes get a curve deformer. I found some information about it… have to check.
PS, the animation I want need to bend and stretch, thats why I used a spline.

Ok found that free node group library adding deformers to geo nodes. Totally awesome. I will now do the whole rigging in a single geo node. :slight_smile:

Here is the link:

Not sure what “the spline deformer” is. If you mean a curve modifier, that can stretch, you just need to enable stretch on the targeted curve.

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Ha, thats it!
Stretch and bounds clamp. Always good to learn something new. Many thanks!