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I'm a long-time reader and asking for the first time. Fairly new to blender with no prior schooling in 3D, I would like to rig an armature to a human skeleton. However, my knee joint (consisting of 3 bones, femur, knee and tibia) makes the tibia clip into the femur, when the leg is folded (as in: When the heel touches the butt).

Here is a snapshot of the clipping knee...

ShinClippingIntoFemur

I have tried everything and cannot figure out, how to solve this puzzle. But: If there would be a way of making the knee bone rotate 1.5x the amount of the tibia, the problem would be solved.

Here's an illustration of what I mean by rotating the Knee bone more than by the IK: ShinNotClippingWhithMoreRotationOnKneeBoneOnX-Axis

As far as I know, that is not possible in an IK with parented and (even worse) connected bones, right? So what's the solution? The human leg "travels around an arc", rather than rotate around a pivot-point. How can I incorporate that "travel" in an IK-rig.

Thanks a lot in advance for all your help. Also thank you all for this wonderful ressource of information in the past.

If ever needed, I'll try and provide a slimmed-down .blend file (new to this).

EDIT: Added the .blend file here:

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  • $\begingroup$ have you tried a Copy Rotation constraint with a low influence? Also maybe look into drivers and shape keys for the mesh (but it's a different approach)? $\endgroup$
    – moonboots
    Commented Aug 3, 2021 at 19:21
  • $\begingroup$ Thank you for your quick reply! Yes, I tried copy rotation and transforms, yet the priority is the IK. The shapekeys would modify the shape (or relative position) of the object, as far as I understand. Wouldn't it also influence "attached" objects (such as ligaments, muscles, etc.)? $\endgroup$
    – Roro
    Commented Aug 3, 2021 at 19:25
  • $\begingroup$ oh yes you're right you can't give constraint to the bones in the IK chain, so what about deforming the mesh with shape keys instead of rotating a bone? I know it's a bit cheating but... $\endgroup$
    – moonboots
    Commented Aug 3, 2021 at 19:29
  • $\begingroup$ I'll give it a try and report back. In the meantime, can you confirm or dismiss the idea of making a bone in an IK chain rotate 150% instead of the "100%" it would caused by the IK chain? - Again: Thanks for your quick replies. I really appreciate them. $\endgroup$
    – Roro
    Commented Aug 3, 2021 at 19:43
  • $\begingroup$ I've never heard about that but I have a limited knowledge, maybe you could organize your armature differently, for example make an IK chain of 2 only and give another IK for your thigh, etc, but I guess what you want is something rather automatic $\endgroup$
    – moonboots
    Commented Aug 3, 2021 at 20:03

1 Answer 1

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Thank you both for your information.

The answer was a bit more than just modifying one aspect.

What I did, in order to solve the issue was: -Unparent the mesh from the armature -Unparent the shin-bone from the knee-bone -Unparent the knee-bone from the femur-bone

In edit mode I seperated the knee-bone from the armature.

Since the knee-bone had constraints and mesh connected to it, I parented two empties to the tail and head of the femur and shin respectively. Then I added a copy-location transform to the knee and had it follow the empties.

In Edit-mode I set a new position for the femur with the tail exactly on the rotational axis of a humanoid femur. I adjusted the shin-bone's head to the exact location (select tail of the femur, SHIFT-S "Origin to selected", then selected the shin-bone's head, SHIFT-S "Selection to cursor") and parented the shin-bone to the femur (connected).

But still I had the problem of parts of the shin-mesh clipping into the femur.

I solved the issue by the bone roll of the femur. Once the bone twist and the twis-relation between femur and shin was resolved, I was able to bend the knee without clipping.

Thanks for both your contributions!

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