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Questions about bones in the TSM2 rig


Roger

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  • *A:M User*

Why exactly are there two upper arm bones and two lower arm bones in the TSM2 rig? I'm not really sure why there would be a need for two. Wouldn't it make more sense to have one of each, or is there something I'm missing here?

Also, the leg is the same way.

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I've often wondered this. Could be something to do with the FK/IK . TSM2 installs SO MANY bones, it is quite a complicated rig that is easy to use, add to- and install... so I have always had a 'pay no attention to the man behind the curtain' approach to it.

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Why exactly are there two upper arm bones and two lower arm bones in the TSM2 rig?

 

try this experiment...

 

Hold your arm in front of you and rotate your hand to face your palm up and down. You can imagine the motion as either your hand turning on the end of your forearm bone or as your forearm bone turning and your hand going along for the ride.

 

The second is closer to the truth, your hand moves because it is at the end of your twisting forearm.

 

Now roll up your sleeve and watch the skin on your forearm as you turn the hand back and forth some more. The skin at your wrist turns almost as much as your hand does but the skin at your elbow hardly turns at all. And the skin in the middle of your forearm turns about half as much as the skin at the wrist does.

 

In the TSM2 scheme (after rigger has been run and added all it's constraints) the first bone of the forearm turns not at all if you rotate the hand but the second bone follows it completely. This makes it easy to simulate that progressively twisting skin action by CP weighting or fan-boning between those two basic bones.

 

For example (warning: this is an extremely simplified example) if your model's forearm had 5 spline rings...

 

the one nearest the elbow would be weighted completely to the first of the two bones,

the second spline ring would be 75% to the first bone and 25% to the second bone

the middle ring would be weighted 50-50

the fourth ring woudl be weighted 25% to the first bone and 75% to the second bone

the last ring (at the wrist) would be weighted 100% to the second bone

 

By spreading the turn along the entire forearm it avoids an obviously fake looking twist concentrated at either the wrist or the elbow.

 

The upper arm has a similar twist spread out over its length as do both the upper and lower legs although not as obviously as the arm action.

 

There must never be a bend between the first and second bones of a limb segment, they're purpose is to be exactly in line with each other. Scale, move and turn them as a unit by using the standard manipulators on the first bone only and never adjusting the second bone separately.

 

Personally i find this weighting or fanbone adding to be easiest BEFORE I have run Rigger, when the basic geometry bones are still visible and no non-geometry bones have been added yet.

 

I open up the model in an Action and use the rotate manipulator to turn bones one at at time as needed to see how my weighting is doing. You can call up the CP weight editor while you are in an action and changes you make will be instituted in the model.

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