FK/IK arm - rotation problem?!?

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  03 March 2003
FK/IK arm - rotation problem?!?


Just noticed these sub-forums so i'd give my question a shot here.

Im trying to rig an IK/FK arm but can't seem to match the rotations of the control arm to the bind arm. The wristCTRL joint's Y and Z rotations are connected to the wristBind joint's Y and Z rotations. The X rotation of the wristCTRL is connected to the X rotation of the forearmBind to give that radius/ulna effect when skinning.

However, when i rotate the wristCTRL joint it goes out of sync with the wristBind joint. I've tried changing the rotate order but it doesn't seem to make any difference. I gather that the x rotation needs to be the last evaluated?!? Jason Schleifer mentioned this type of thing in a thread a while back. Something about feeding rotations down a chain for smooth deformations.

Anyway, i understand why mine isn't working. I think. Just don't know what the solution is.

Any help is greatly appreciated

  03 March 2003
What is the current rotation order? It should be XZY or XYZ..

jason schleifer
Animator -
Co-Founder -
  03 March 2003
Hi Jason,

I think i've made a schoolboy error with this arm setup. Am i right in saying that the forearm,wrist and palm need to be drawn in an exact straigt line for this setup to work? I redid the setup like this and it now seems to be working fine.

The joints have an XYZ orientation. So, the rotation order for the wrist works with either YZX or ZYX whilst the shoulder needs either an XYZ or XZY.

From what i understand of this the X rotation at the shoulder (twisting the bicep) needs to be the first solved because its rotation is being fed down the chain whilst the X rotation at the wrist(twisting the forearm) needs to be the last solved because its being fed UP the chain. Phew!!! Is this correct?

You mentioned a while back, "If you want to get really fancy, you can make a ton of joints in the middle of the bone and spread them evenly all the way down." Do you still have that example file or could you explain how it works? Mutiply/divide node or expressions?

Just realised yesterday that i missed Animex 2003. :mad: AAARRGGGHHH!!! Could have asked you in person. When i was at Teeside University i used to live just over the road near that derelict building.


  03 March 2003

sorry ya didn't make it to animex.. it was good fun!

yep, you're right.. you do have to keep the joints all in alignment in order for it to work. sounds like you've got it sorted.

In order to get multiple joints to work, simply create the arm with more joints in the forearm, and divide the rotation along them. for example:


then make forearm1 get 25%, forearm2 get 25%, forearm3 get 25% and wrist get 25%

make sense?

jason schleifer
Animator -
Co-Founder -
  03 March 2003
Nice one Jason, I'll give it a shot. This is my first rig BTW so i may be over-complicating things a bit.

My brain really does hurt.

  01 January 2006
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