Thanks for those links. I'll check it out. I have this same post in the Animation and Rigging section. And I came up with a workaround that doesn't require getting into the API. I usually shy away from plugin type solutions just because of the maintenance aspect of plugins.
The following is copied and pasted from my other post:
I still don't know the equation to figure this out mathematically. But I did come up with a work-around. I'm dealing with mocap on a character. Since most character rigs are mirrored with behavior, this automatically sets the joint orient values to make this possible. Mocap rigs do not. Joint orient on mocap joints are all 0 0 0. So when you orient constrain something, usually on the right side, you get crazy values for rotation. And if you want to blend keyframes with mocap, this is a real nightmare.
My workaround is to duplicate the joint in question and zero out the rotation. Then create a rotatePlane ikHandle. I also parent locators to the corresponding mocap joint and to my duplicated joint with a 1 unit offset on 1 of the axis. This is to measure my twist angle. I snap the ikHandle to the mocap joint and measure my twist angle. And that gets me rotation values I can use. Then I delete my duplicated joints and locators. Clunky but once scripted, it just works.
