View Full Version : IK: Unaffected by IK of Ancestors?

05-04-2006, 03:04 PM
I've made a simple human skeleton, with the root at the base of the abdomen. From there I've got three chains, two legs and a spinal cord.

Further up the spinal cord, it branches into a neck and two arms, and I think this is where the problem is. I'm using IK and controlling it all with goal nulls-> two feet nulls, two arm nulls, a body null and a head null.

The problem is that when the Body null is moved, the left arm goes haywire. The right arm simply goes along with the body, and its goal can still be easily moved (I have 'Keep Goal Within Reach' checked). The left null leaps to unexpected positions and is completely undirectable.

I think this must be something to do with LW's IK mathematics getting confused... it seems to be trying to follow through the maths of the spine into the left arm, while ignoring the head and right arm. I want it to follow through into the head, yet there's no way to tell the left and right shoulders not to be affected by the IK of their parent bones.

I can of course simply create new bones from the shoulders to the root, but I can't believe that Lightwave offers no provisions for determining which path an IK chain follows, and I can foresee the problem recurring in future situations where this fix won't be realistic.

Is there anything basic I've missed (am relatively new to LW) and how can I get greater control over the IK, when dealing with several interconnecting chains?

Any help would be greatly appreciated.