View Full Version : minor but common joint problem

03-14-2010, 07:39 PM
I have had this problem every time I have gone to animate a mechanical object. That problem being that bones follow the axis orientation of their parent. Unfortunately the next piece in the chain doesn't follow that rotational axis. (see pic 1)

I changed the coordinate system to local and now the axis line up like they are supposed to (pic 2)

I need the joint to only rotate along the pitch axis, but under the new local coordinate system the joint is actually rotating along all three axises. I understand why that happens, but now I can't lock the IK along one axis.

If this were for a still photo I would just rotate the bone around all three axis and not care, but this is going to be animated using Relativity and IK so all the joints have to work appropriately.

Anybody know what to do here? :help:

03-14-2010, 08:05 PM
Use a null as the joint with the bone parented to the null.
Apply IK to the bones as usual use the null to rotate the bone
parented to it.

03-21-2010, 09:14 PM
:) I gave it a shot and it seemed to work, but then my bones screwed up and stopped acting on the weights set them, then the model blew up :D

I will work with it tomorrow and see if I can figure out what I did wrong.

03-22-2010, 12:14 AM
Setup our rig with nulls then constrain your bones to it with "same as item". This will be slightly slower depending on the number of items you need to constrain. But allows for much greater control.

03-22-2010, 05:21 AM
yes, bones parented under nulls in a hierarcy only seem to deform when their actual parent null is operated on... but and movement on parents of that null nix the deformation... seems to be a bug, one that Im preparing a report for as it happens.

Anywhich way... this is easily solved, just orient the bone as it needs to be, and record pivot rotation... your parent space handles will line back up along the bones local axis.


03-22-2010, 06:46 PM
I really wish I knew what I was doing here. I want to be able to do animation and be good at it, but every time I try to do something other than rotate, move, etc, I feel like I am pulling my fingernails off. :/

So I set up the null as a joint like SplineGod said. If I parent the lower bone to the null first and the parent the null to the upper bone, the rotation stays like I want it to. (If I parent it in a different order the model explodes) Unfortunately it only works when the coordinate system is set to local, but when its in local the camera rotates along all three axis whenever I try to position it.

I honestly didn't understand much of what the other two suggested :o

03-22-2010, 07:16 PM
its easy...

first off... forget using local axis mode... it'll only give you gimbal issues, cos no matter which mode you're in LW always records, and proceses motions in parent space... local and world are only really useful for setup taks... no good for animation.

second... forget the whole null parenting thing, its unnecessary, at least in this situation... just select the bone who's parent space handles dont line up with its apparent orientation, and hit the record pivot rotation.

Sometimes rpr can be a bit screwy, especially if used more than once on the same item... if in doubt, use the bone tools rpr instead, its a lot cleaner.

Also, as for the model blowing up... you need to rerest bones after reparenting, or doing an rpr... so select the bone, or all the bones to be doubly sure, and hit "r". Make sure u do this at base pose (frame 0).

03-23-2010, 02:10 PM
:D:boogiedow:dance: Wahoooo that seems to have set me straight, ty :)
I can already see that this model is going to take a lot of pre setup before I can actually animate it, but at least I know how to do it :)