View Full Version : bicep twist problem

06 June 2008, 02:28 AM

I'm having a problem with an arm setup. I've implemented forearm twist without a problem, but the bicep twist is an issue when I rotate the FK shoulder control in all three axes. As soon as I'm rotating in three axes, the controller and the shoulder start rotating in different directions.

This setup uses three skeletons... an IK arm, an FK arm and a bind arm (the bind arm has the twist joints). The IK and FK arms are exact duplicates of the bind arm, minus the twist joints.

The arm has two bicep twist joints between the shoulder and elbow joint. I'm using multiply/divide nodes to divide the rotation of the FK control shoulder and spread them over the bind shoulder and twist joints.

I'm guessing this problem is because the rotation axes are drifting apart because the bind shoulder isn't rotating as far in X as the control shoulder, but I don't know what to do about it. The rotate order of each joint and their corresponding controller object is the same. Has anyone run into this problem before? How do you drive bicep twist joints this way without the rotation axes getting messed up?

06 June 2008, 09:45 PM
okay nevermind, I think I fixed my own problem but I'll write it here just in case anyone else has run into this.

The reason the control shoulder and bind shoulder were rotating in different directions is because X wasn't first in the rotate order. If the rotate order of all of the shoulder-related joints (IK, FK, bind, and the controller) is XYZ or XZY, everything seems to work fine.

06 June 2008, 05:55 AM
Rotation orders are often the culprit with issues on the forearm/bicep. Thanks for posting your solution :)


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06 June 2008, 05:55 AM
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