To keep this both as short and as informative as I can:
The character I'm animating has bones in his arms set up like this: The lower arm has a regular IK constraint with a hand controller as a target (chain length 2), and the upper arm has a regular IK with an elbow controller as a target. Both targets are custom shaped helper bones.
(I didn't use the "pole target" function because it introduces undesired twisting, and doesn't fix this problem anyway)
- The problem I have is that the elbow updates in a weird way during animation, as if the IK calculations were struggling to keep up. When you go to any frame, the elbow first appears in a wrong position. And only after about a third of a second it snaps into correct place by itself. Sometimes the difference is minimal, other times the elbow faces the other way completely. Because it takes that short while for the bone to correct itself, mostly the wrong positions show instead of the good ones when the animation is played normally in viewport.
I've made 3 animated actions in total with this same armature, and 2 of them work perfectly fine. Only this one has this problem, which is why I have no idea what the cause could be. So far I've tried to fix this by restricting bone rotations, re-rigging the arms to use the pole target function, changing position of the elbow targets, disabling animation of the elbow targets, and also baking all of the animation. None of that has however solved the issue.
If anyone will be able to make any sense of this and tell me what's wrong, I will greatly appreciate it. The blend file can be downloaded from here: Blend file on Google Drive