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This is a demonstration .pmx skeleton, with a simple little model I made, intended to show how twist can be separated from a bone's rotation so that rotation in different axes can be applied to different bones-- ie, automatic twist even if the original motion didn't use twist bones. As it's intended as an arm, it also shows initialization of a bone to a different local axis (it is horizontal as modelled, in order to use axis limited IK bones, but needs to emulate the typical MMD A-pose in order to respond properly to canned motions.) In order to accomplish this, it tracks the motion of the control bone using two different IK bones, limited in different axes.
I made a proof of concept for this before but somehow the files disappeared or maybe they were never there in the first place, I don't know. Thanks to Littlemisshorror for letting me know. So I made an updated version. If I remember the old one, this should be working better. It's also simpler, reduced to the fewest number of bones necessary for the structure (plus a motherbone parent for the whole structure and a "test" bone that exists only for display purposes, in order to show how the untwisted arm would be pointing.)
Notes: deform tier is usually important when chaining IK bones like this; notice the deform tier increment and pay careful attention to bone order. Non-control bones have been hidden in order to demonstrate what the end-user should see (no interaction with the complicated mechanism) so remember to enable display of hidden bones. There is a singularity when the bone is at certain angles very near the poles (pointed directly to the sky or directly to the ground) that can create some disagreement with a basic model; this has been minimized by increasing the loop count of one of the IK bones to 400.
If you don't understand pmx IK bones yet, you won't understand this either. But I'm always happy to explain whatever I can.
For display purposes, this includes some GPL licensed textures, and so this is released under the GPL 3.0 license, but that shouldn't matter to anyone; it's just intended as educational.
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