Head-anchored spine IK

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  4 Weeks Ago
Head-anchored spine IK

Something I've been trying to figure out for a while is a way to have spine IK which allows animating the position of the head directly, and just moves the body to follow. That seems useful in general, so you can animate dialogue head movement precisely without having to fight spine IK. A good "stress test" example is sipping from a glass or straw on a table while sitting (everyone's poured a carbonated drink too fast). That's an easy, intuitive movement that anyone can do, bending your back so your head goes exactly where you want it (the glass), while your hip stays planted on the chair.

So far I haven't found a good spline IK setup for this. Typical spine setups are hip-anchored: the hip stays in place, and the head just lands somewhere along an IK curve. I tried reversing the chain: create a joint hierarchy starting at the head and going down to the spine, and setting up spline IK that way. That just inverts the problem: the head goes to the right place, but the hip moves around instead of staying in place.

Of course, a regular single chain IK will reach the start and end points, but we want a spine bend, not an elbow.

(The right approach might actually be anchored at the neck or upper spine rather than the head, since it's a simpler curve and would give better control over head rotation, but that's a detail for later.)

It's really intuitively obvious what should happen, and it feels like this should be an easy variation of a standard IK setup, but for some reason I'm having trouble with it. Anyone have any ideas?
 
  4 Weeks Ago
Hi
Actually with the usual hip upward spiline ik setup, I create another hierarchy from chest to hip (just the control) and constraint to the actual hierarchy and switch on and off the constraints, to get upward or downward hierarchy.
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  4 Weeks Ago
I think that would only give you FK from the top-down. I'm trying to think of a way to get an IK system that'll anchor at the top, but still try to reach the hip position so the hip doesn't shift around when you're just trying to move the upper body...
 
  4 Weeks Ago
Hi
See above I did mention its ikSpline setup of the spine...
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  4 Weeks Ago
You said you create another hierarchy in addition to IK. But Maya spline IK doesn't do this:



On the left, the joint chain and the curve are in bind pose and everything lines up. In the middle, the "head" control is moved, and the top of the joint chain matches (obviously a very exaggerated pose). But, the hip (the bottom joint) has moved far off. I want the hip to stay at the hip control. (It's obvious why that's happening here, spline IK just lines each joint up with the curve.)

On the right (manually positioned) is what I want to happen. The head matches the head control exactly, the hip matches the hip control exactly (if possible), and the bend is distributed along the joints in between.

This is really close to what the IK spring solver does, but it only works on one axis and in one direction, which isn't enough...

A high stiffness nHair can do this, but I don't want to use dynamics for this.
 
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