What are the advantages of multi-skeleton rigs?

[1000101-]

I hear alot of people talking about needing seperate skeletons for IK and FK; and typically 1 to bind to; and I've sometimes heard about 1 more to control it all.
The advantages of binding to a seperate skeleton seem kinda obvious to me; but what about the other purposes.
Whats the advantage behind having an IK skeleton, an FK skeleton, and a control skeleton? and how would one go about setting such things up?

It seems to me like it'd be creating more work having to deal with all of the extra stuff. Any thoughts?

[wcy93]

its a flexibility thing really... basically, for characters that get touched by multiple people, multi-skeleton rigs are pretty useful. different people like to use different mathods for animating. some prefer FK arms, some IK. sometimes, you really need an FK approach for a particular animation, while other times you need an IK solution. for example, animating someone bowling. FK would be the easiest way to animate a bowler. While someone doing handstand lends itself to IK arms.

so basically, a flexible rig has FK/IK switching. While maya has built in FK/IK switching, its not blendable. multi-skeleton rigs enable you to blend between full FK and full IK.

having said all that, it is a lot more work to set up, and some people just don't need it. So they're my main reasons for using multi-skeleton rigs.

[1000101-]

So how would one go about setting all this up? Anyone out there documented a procedure?

[wcy93]

its pretty straight foward really... just setup your skeleton as per usual, and skin your geometry to it.

setup your IK on that skeleton, and create a controller object for the IK handle.

now duplicate the parts of the skeleton that you want to have FK/IK on. ie duplicate the arm heirarchy. then on the duplicate heirarchy, create 2 locators in the exact same position as your IK controller object, parent one of them to the original heirarchy, and the other to the duplicate heirarchy. then constrain the IK controller to both of the locators. now when you change the weighting of the constraints, the IK controller gets pulled between the two locators. the thing that makes it cool is that when its attached to your duplicate heirarchy, the IK handle is being controlled by an FK skeleton. so although the arm is still being controlled by an IK chain, its moving as if it were FK.

make any sense? oh yeah, the other thing you have to do is make a similar setup for your elbow look-at. so two locators, one parented to your duplicate heirarchy, and one either parented to your original skeleton, or to the world (depending on how you like to control your elbow) and then create the constraint on the elbow controller to flick between the two locators.

now you can just create an attribute on your arm controller that switches between FK and IK.

anyway, thats a pretty rushed overview, but it should give you a pretty good idea of where to start.

there is also another method which is more of a true FK/IK switchable skeleton, which I believe is attributed to chris landreth of bingo fame. I'm sure that method is documented somewhere on the web. do a search. his method requires 3 skeletons though, not 2, like the method above.

best of luck!
-zz

[1000101-]

Excellent! thank you for taking the time to explain it to me. I'll look for info on his skeleton as well. I think this evening when I have some spare time I'll build an arm rig as you've described to see how it works out for me.

[webmonkey]

Hey there,

Thought you might want to check out this link about fk/ik switching. Certainly helped me out!

All the best,

WebMonkey

[1000101-]

hrmm should the FK skeleton follow the IK skeleton when moving via IK?
if not then I don't see where the advantage is since it seems I'd have to repose the guy anyway.
or maybe that is the advantage...

or maybe I'm doing something horribly wrong

[wcy93]

no, that would kinda defeat the purpose of the IK/FK blending.

i don't think i understand your question...

or are you talking about when you switch from IK to FK? in that case, yes, there is a little bit of pain indeed. But that can be lessened to some extent by writing a simple little script that snaps the FK bones to the IK bones when you're switching to FK...

at the end of the day, its not usually a problem, as most of the time, an entire shot will use either IK or FK, and not the combination. its pretty rare for you to need to switch within a single shot which indeed begs the quesion why bother with blending. it is useful in games however. or for short films where one has multiple shots within a single scene.

[1000101-]

Force of habit.
used to the max/mirai way of posing and keying. i.e. you can move an arm via IK; set a key; then go back and tweak the elbow in case the IK didn't solve exactly right without having to go through the fuss of turning off IK or setting an IK/FK key.

Though after I checked out that URL and played with that setup; it seems to make a lil more sense to me now. just something I need more experience with.

[wcy93]

yeah, i see what you mean. I was actually trying to write a script that would help emulate that way of animating. being able to pose FK skeletons via IK.

I gave up after getting frustrated with a few mel limitations, but I might look at it again. It was in 4.0 that i was writing it, so I don't know if there is anything new in 4.5 that may help...

good luck.