Trying to make correctives for cuffs, but I can't figure out how to set up the drivers

[0Poole (Robert Poole)]

I’m making this model where I want her cuffs to be pretty big, and while I thought I could just use corrective shape keys to fix every issue they were having when the hand rotates, I started noticing issues with everything I tried. The base rig is a Rigify rig.

Obviously I just tried making each respective corrective apply when the ORG hand bone rotated in that direction, which looked like it worked, until I realized that twisting the upper IK arm bone (the arrows) changes the rotation angle of the hand, so it was changing which corrective was being applied even when the hand itself wasn’t being rotated.

Then I tried to add a special bone just for editing which cuff corrective is being active, and it does work if you just leave it so you just trigger each one manually, but of course that’s annoying.

But, I noticed I might be able to use the different variable types in the driver for this. If I set it to Rotational Difference where it compares the rotation of the cuff and hand bone, it does trigger the right corrective without being affected by the upper arm bone, but then the issue is that it doesn’t create a negative value when the hand rotates downward, which normally would trigger the Down corrective, so I’m not sure how to specify to make it only activate in that direction without also then triggering the Up corrective. I feel like I might be missing something obvious there, or if one of the other variable types can do something better.

Am I even on the right track with this? I don’t know if there’s some universally agreed upon way of doing this that everyone uses that I just haven’t heard of lol. Any help or ideas would be appreciated. I’m almost wondering if a lattice could make the cuffs deform in the right way.

[thetony20]

I assume you the cuff to stay tightly fitted around the hand/lower arm. If so, my initial thought would be to maybe use a Surface Deform modifier (with or without a vertex group) to keep it all tightly together.

If you do that with a flat mesh, you can then try solidify to add the thickness outward, rather then modelling it all.

[0Poole (Robert Poole)]

No, they follow fine with just data transferring. The issue is making them look good, which is what the correctives are for.

But, I asked someone else and found the solution, so I’ll put it here for others to find:

They added four bones that Stretch To the head of the ORG hand bone, and are parented to the ORG forearm. Then, you set up the drivers so that they use the Y Scale of each respective bone. That way, even if the forearm twists, they don’t change much.

But, you need to set up the F curve of the driver like this since each bone gets pulled along a bit when going in the perpendicular direction, so this will keep it at 0 in those other situations.