View Full Version : ik chain jitter

03 March 2003, 07:18 PM
just curious... what causes IK chain jitter? is it incorrect info being passed to the engine?

also, why is it a problem especially in human sized proportions?

i am experiencing it a little in subtle arm movements... normally i would use fk on the arm, but i am playing around with the acs biped with full ik right now.


Mangled Poly
03 March 2003, 07:21 PM
Ik always seems to jitter thats just the fact of it there are some ways to make it jitter less... such as a really good coorectly prebent model, and not using the joint limits.... As far as that IK tends to jitter and thats why people say it has some set backs :)

03 March 2003, 07:24 PM
I've experienced IK jitter when parenting the goal object to the IK chain itself. If the goal object is parented to a second null, and the second null is parented to the mesh or bone system, then I won't have the jitters. Ultimately, trying to keep the IK chain as short as possible.

I hope this can help you.

03 March 2003, 07:51 PM
yes, thanks for the input. At least i know im not necessarily at fault for this one (i know, i know... a carpenter doesnt blame their tools, blah). :)


03 March 2003, 12:10 AM
I have found that jitter somtimes occurs when:
1. Mulitple goals are used in the same chain or there is not a well defined beginning and end to the IK chain.
2. The object containing the bones is very large of very small
3. The object containing the bones is far away from the origin
4. Keep goal within reach is activated
5. The IK goals are moving in straight lines rather then arcs. This can stress the IK solver when the goal forces the bones into very tight poses.
6. Rotation constraints are used but not properly "tuned"
7. Goals are parented to bones.
8. Expressions or motion modifers are mixed with IK
9. Other weird things are setup :)

03 March 2003, 12:15 AM
I had some jittering with my last rig, I tweaked the constraints and it went away.

03 March 2003, 12:20 AM
If I see jittering or popping I pose that limb with the goal to the point right where it starts to do its weirdness then I go to the contstraints and start tuning the value til it stops.
Ive also found for me at least that I get better results if I dont use 0 in the constraints. For example if the elbow is set with a rotational constraint of 180 and 0 as a max and min Ill really set it to 180 and 3. Its a lot like prepbending bones in that it seems to make things more stable.

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