View Full Version : FK/IK -rotation madness

03 March 2003, 03:07 PM

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, 08:20 PM
If im hearing you right (mostly from the pic), your saying that the IK's joint rotation is alil off from your FK joint's.

Schleifer has a script thatll orient them. Id send it to ya, but i dont think i can distribute it (it came w/ the rigging withing a pipeline DVD). The basics of it is that it uses Orient constraints, then copies the rotation values in to the "Joint Orient" attirblue and zeros out the normal rotation values.


just duplicate the whole arm and delete the extra joints. Then everything should match.

One more thing.... are you dividing the rotation value of the "ForearmBind" and "WristBind" in relation to the "WristCTRL"?

ForearmBind.rx + WristBind.rx == WristCTRL.rx

03 March 2003, 08:11 PM
Hi Doogie

The problem was that i hadn't drawn the joints in a staight line. That's what was messing the rotation up. Pretty silly mistake.

Also, i'm not dividing the rotations in any way because that also messes up the Contol_wrists rotations. There is probably a solution to this but im not aware of it.

I have Jasons script which i maybe should have used as Y down the joints is useful when attaching and orienting control objects. Just wanted to earn how to do it manually so i fully understood joint orientation and local rotation axis.

One more thing, i posted this message on the maya sub-forum on rigging. Im a Maya user so it makes sense to post there aswell. Problem is you end up getting answers strewn across the two posts. Hmmm...

Thanks for the advice


03 March 2003, 10:37 PM
If you still wanna do that wrist thing, here's a tut that explains it all

He does it w/ the upperarm (humerus), while lots of people just do it with the lower arm.

In the DVD included w/ the script he talks about ungrouping the bone. Next, he aim constrains the bones (using a locator as the "World up object, it's like a pole vector, so that the other 2 axis are pointing uniformly). After that, he deletes the constraint and the locator. Finally he re-parents the bones.

He explains it better on the DVD (w/ graphics!)

The script is much easier to work w/ when doing it more than once.

But now you know what it does (if you got all that).

Oh yeah, sorry if your questions were answered in the other thread... i didnt take the time to find it.

03 March 2003, 07:23 PM
Yeah, i can get the forearm and bicep twist thing to work on its own but i can't get it to work as part of an fk/ik rig. Thats when the rotations mess up.

Which evaluates faster - using the multiply/divide node or using an expression? I've heard it's better to limit the amount of expressions on a rig. Anyway, i'll give it a few more tries at the weekend.

I'm currently backwards engineering Goosh's IKJOE rig. Once i've figured that out i'll probably have a better understanding on how to solve certain problems.


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