IK Rig - Limb IK Solver - Knee Bending in Wrong Direction

Development Character & Animation
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1

Hi All,

I am following the instructions for the Leg IK Setup in the docs

However, my knee bends backwards instead of forwards

General Setup of the Skeleton:

  • Made in Blender
  • Ensured that each bone’s X axis is forward looking (Y-Axis was default and didn’t work neither)
  • Leg has 4 bones: Upper Twist, Upper, Lower, Foot (Arm has 5 bones and works as desired)
  • No IK Constraints in Blender and no IK bones on the skeleton

Insights from IK Rig in UE5:

  • Lower Leg Bone seems to be stiff (When I include the entire leg bone chain, the upper leg bone (“middle of the upper leg”) bends like a knee would, but when I exclude the top bone, the knees bend backwards…)

From other forum posts:

  • I have read about leaving this as is and then, modify the retargeted animations in the AnimBP with the Two Bone IK node, but in UE5, the effector target is a pin and no object in the viewport anymore - any thoughts on this approach?

Any help is highly appreciated.

2

The issue seems to be the even number of bones (as suggested in OP). Reparenting the Lower Leg bone to the Upper Leg one (and not the Upper Leg Twist bone) fixed the issue.
Now the bone chain has only 3 members and the engine knows how to bend the knee properly (even with wrong bone orientation)

Nonetheless, this solution doesn’t seem very feasible for more complex skeletons… Any other ideas on how to fix the issue?