Snapping issue in Simple IK rig.

[Eka Juan]

I'm trying to animate with with simple IK rig.

I'm in IK, positioning the arm. And the arm looks twisted like this.
https://prnt.sc/z8_2MGVxyVd3

I snap to FK.
I pose the arm in the correct/natural way.

https://prnt.sc/7Gvctp6T2rAB

But if I'm back to work with IK, it turns back to the wrong pose again.

What to do?

And let’s say, I don’t care about the IK, as long in FK it looks good. When I baked, will it use the keyframe I made in FK, or will it use the IK and overide the FK bones keyframe?

[Alessandro Padovani]

1.7.4.2029

It seems Thomas changed the setup, I don’t know if this is on purpose but this way it doesn’t work. The forearm setup should be as explained in #1708 in point 2 forearm twist, with copy rotation and dumped track on the ik hand, that way the forearm twists automatically when we pose the hand. I do my own rigs and I don’t use the addon rigs myself, though I contributed to them a little, so I don’t check when Thomas changes things.

We have to wait for Thomas to reply when he’s back.

[Thomas Larsson]

For some reason the forearm twist bone copied the rotation of the forearm ik bone rather than the hand ik bone. Fixed now.

[Alessandro Padovani]

Commit 5c4c9d0.

That will not work correctly, because local Y is different for the hand and the forerarm depending on the pose. To align the forearm to the hand we need copy rotation plus dumped track as explained in #1708.

[Thomas Larsson]

No, that doesn’t work, and it was never implemented. The problem is that the forearm moves around strangely for intermediate values of the ik influence. A local copy rotation constraint is not perfect, but it behaves in a much more stable fashion.

The mhx rig avoids this issue by adding an extra bone. The hand is child of hand0, which shares location with hand and is parallel to the forearm.

[Alessandro Padovani]

Usually the ik influence is a toggle, that is, either one or zero, it doesn’t have intermediate values. The keyframes for snapping in ik influence should be set as constant interpolation mode, not linear. Linear doesn’t work for snapping. That said, again I don’t use the addon rigs so if you like that design I’ll close as resolved. Let us know.

[Thomas Larsson]

We use influence rather than the mute property for the ik constraints, to make a smooth transition between fk and ik. Then it is bad if intermediate values are off. Although I mainly do stills myself and interpolation is not a problem in that case.

[Alessandro Padovani]

Mute doesn’t work for animation, influence is right you do have to use it for animation. For snapping you should use constant interpolation because otherwise it doesn’t snap, that is, if there’s intermediate values then it doesn’t snap by definition. I don’t see why “it is bad if intermediate values are off“.

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But again if you like it this way we can close as resolved. Let us know.