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roll bones

hello, so in my character there were bones that just rotated like shoulders, bicep/tricep elbow but not the arm, like vertices around the bone, it was parented to 1 bone only and recently learning motion builder i discovered that those might be roll bones but what they are for?

Simple exercise.

Put you hand out with palm facing down and wave bye bye from the wrist. Now rotate your palm up like someone is going to give you something. See how your hand no longer rotates at the wrist but at the elbow?

In Motion Builder the roll bone is linked to the forearm which calculates the amount of offset rotation when you rotate the hand which you set in the character settings.