View Full Version : IK/FK Blend -> weird rotation at 0.5 blend

03 March 2004, 10:25 AM
I have an Arm rig which is animated by both FK and IK. The Elbow has an orientConstraint to a nullgroup, which is parented to a Nurbs circle (the FK control object).

The orientation is exactly the same when the IK/FK blending is at either 0 or 1, but the value in between cause a very strange rotation. When the blend is at 0.5 the joint rotates exactly 90 degrees over the y-axis (the y-axis is pointing to the next joint).

Anybody knows what is causing this weird problem?

I've already tried deleting the IK handle and building the rig again, but no luck with that. The IK handle has a Pole Vector Constaint, and I've used 'set preferred angle' before creating the handle.
Stickyness on or off doesn't help either.
The nullgroup which is in between the FK control object and the lower arm bone is to get rid of the rotation of the FK control object when I apply the orientConstraint.

I did mess around with the local rotation axis of the joint (they need to be set to a certain orientation as an game engine requirement)...could that be the problem?

03 March 2004, 01:07 PM
Are you using Maya 5?
If so, can't you just use the ikBlend Attribute that Maya provides ?

:: Galactor ::

03 March 2004, 01:19 PM
I am using the Maya5 IK/Fk blend attribute...that's what giving me the headaches :)

03 March 2004, 01:45 PM
I've put up a file that shows what's going wrong.

There are two 'arms' in the file. When you scrub in the timeline it blends between IK and FK. The arm that is going all directions when blending is the faulty one. Note that when the blending is at 0 or 1 the orientation is exactly the same....

I started with rigging the two arms the same way, but with the 'bad' arm I rotated the 'local rotation axes' of all the joints before adding the IK and constraints....
My question is: how can I rotate the local rotation axes without this problem happening? :shrug:

03 March 2004, 08:51 AM
I downloaded your file. It looks like you've pinned down the source of the flipping (LRA's) but unfortuately I don't have any answers.

My only advice to scrap the native Maya5 IK/FK blend and go for the old 3 joint chain setup. The new orient contraint interpolation type, 'no flip', will prevent the nasty flipping you are getting, but in order to use it you have to setup the arm the old way. has a great tutorial on this if you don't know already.

As a side note, this isn't the first time I've heard of people having trouble with the IK/FK blend in Maya5. I think I'll stick to the old way until Maya 6 and then, maybe, they will have worked out all the bugs.

03 March 2004, 12:50 PM
The setup with three skeletons sound like a good solution (a bit more work, but worth it when it works). I'll certainly try it out!

Thanks for the help!

04 April 2004, 11:30 PM
I had the same problem :(

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