Figure 2 shows the arm I want to rig, which includes an accordion-like elbow hinge. In this figure, the armature has been removed and displaced to the left along the green arrows so you can see it better.
Figure 1 shows the [magnified] bones of the same arm. Bone1 and bone2 are the primary bones of the arm, while hollow1 and hollow2 should control the position of the two bands of verts that populate the inside of the hinge. Hollow3 merely helps for parenting and controls no verts.
How do I control the rotation of hollow1 and hollow2, so the two divide the elbow angle into three even angles? I want to have just one bone to rotate and let these two bands of verts go about their business without my help.
I was sure this would come down to constraints, IK tricks, or both but I don’t know how to use constraints. I found them in the UI and researched them but failed to create a rotational constraint between one bone and another from Pose Mode.
Some grueling, painful step-by-step instructions would be appreciated.
In this case could probably just use a Copy Rotation constraint with varying influence (say 0.33 for one secondary bone and 0.66 for the other). This will mean that if they all start out aligned with the main bone, they will automatically rotate at different rates as the main bone is rotated.