Arm twisting between keyframes.

[Aminator]

This happens to me alot. I'll start tweaking an animation and move the hand around but when I play it back the arm will unexpectedly twist up.
If I click on the keyframes everything's fine but when I play it she looks like she just got busted trying to steal Steven Seagal's wallet.

I don't have the left arm control added to the scene and I don't know if that's a possible cause. Also there are very few elbow keyframes but I haven't had any success fixing it. I tried changing the curves for that section to smooth local and that didn't help.
Any idea where I'm screwing up?

 

[atani]

If the first and second keyframes of the hands are in very different positions or rotations the joints path taken may lead to that. What I do is add a middle keyframe to ease that transition. The speed of movement also matters.

 

[Aminator]

I tried that before and it just seemed to shift the problem forward or backward. Maybe it's because I need to also keyframes the elbow and shoulder when moving the hand?
What happens if I add the left arm control and select that and then move the hand? Will it overwrite the entire arm and keyframe everything from the shoulder down?

 

[atani]

The more targets you add the more control you have, but then you also need to take charge of those targets, which can be a pain with the extra work. It's a compromise that depends on the animation you plan to make, so it's hard to say what would work well for you.
Poses can also help, but again it all depends on what you want to achieve.

 

[Aminator]

Right now I'm having a problem with the head on an animation I've made with my quest 2. Maybe my head movements were to fast for the capture. The head jerks around and twists and there doesn't seem to be any way to fix it. I reduced keyframes several times and also deleted the neck track but it didn't help.

Right now I'm looking at 3 keyframes spaced about 7 seconds apart. If I play it the head spins after the second keyframe to an unnatural position and back. If I just click on the 3 keyframes one at a time each pose looks fine. If I add a keyframe between the second and third then the spin just happens after that keyframe.

The hands are fine in this one. It's just the head but it seems like the same problem.
Could it have something to do with the glance plugin?

If I record motion capture with a neck and a head track and then reduce keyframes on both tracks will that cause problems? I'm just wondering if the process makes the two tracks less compatible.
I can't remember now if the animation was smooth in the beginning but I doubt it.

 

[atani]

I have no knowledge or experience with motion capture. Perhaps is better to take the issues to Acid's Discord, there you might find someone that can help with this.

 

[Acid Bubbles]

@Aminator I'm aware of a problem called "quaternion continuity alignement", for which I _should_ have a built-in solution. Long story short, any rotation can follow any numbers of paths, and the shortest will be chosen; however that are two mathematical representations of the same rotation, which look the same but are actually on both extremes... resulting in twisting and sudden intense rotation. To work around this problem, there is a toggle (enabled by default) which goes through all keyframes and "inverts" rotations when needed to minimize those cases. While this solves the problem in most cases, I think it's still possible, though I never found a way to reliably reproduce.

If you're able to send privately the scene with the problem (no need for dependencies, just the json) I'll try and take a look.