View Full Version : Broken Chain Kinematics

08 August 2005, 10:22 PM
Can someone explain what this is? How to do it? Why it's important? I've heard about it several times, but can't find any reference for it (other than expensive Alias DVDs I can't afford). Thanks.

08 August 2005, 08:53 AM
lol. Anyone? it's not like a big secret that pro riggers keep, is it?

08 August 2005, 04:40 PM
No, it's not a big secret! The idea with a broken hierarchy rig is to free some of the parts from each other. In a fixed hierarchy rig, touching a joint in the lower spine will cause everything below it hierarchically (shoulders, head, arms, etc.) to move. This is a big pain for animators. If they love the pose they have, but just want to tilt the shoulders a bit, they don't want to be forced to counter-pose everything else back into place.

This type of rig can be hard to animate if you're used to layering your animation, but if your animators work pose to pose and they're good at keeping things on model, they'll love you for it.

There are many ways to build a rig like this, the way we do it at Disney is to essentially isolate the different body parts from each other. If you took the rigs you currently have and just disconnected, say, the hips from the spine, you'd have the basic idea. All of these mini-hierarchies (head, chest, hips, arms, legs) sit under a top node for moving the whole thing at once for positioning, but during posing, if you transform the chest you'll leave everything else behind. This makes the mesh stretch like crazy, needless to say, so a good, even topology on the model is pretty important.

Hope this helps clear it up a bit.


09 September 2005, 07:24 AM
basically what JohnPark said - although i havn't animated broken translation of limbs, only with rigs that have joint groups [limbs] constrained, and the orient constraint can be keyed from a controller. HR body rigging DVD by alias has a good example of this setup.

very useful if you think of something like a head, which tends to move around where a head wants to look and rarely inherits 100% of the rotation from the body. also great with the arms, sometimes regular FK animation is awrkward because you're actually counter-animating against the rotation of the body as the character walks etc. i imagine animating broken traslation would be hell as far as maintaining topology goes... but more useful for cartoony 3d i imagine, not my thing really so can't comment on that :)

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09 September 2005, 07:24 AM
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