View Full Version : "Breaking" the legs in IK

09 September 2006, 02:56 AM
My animators have recently come upon the idea of intentionally breaking the legs while animating. Problem is I really dont want to set up fk on the legs as that makes my reverse foot setup much more complicated. I was wondering if there is some way I could cause the knee to bend backwards in IK without it fliping the leg. I tried to change the prefered angle and it worked, except the nasty pole vector had to be animated to pass through the knee at the exact moment that the leg flipped orientation... very trickey to pull off. Anyone know of any neat ways of doing this?

09 September 2006, 04:45 AM
Use the traditional FKIK method (3 bones setup). It works with the reverse foot setup.
It's not complicated.

09 September 2006, 03:41 AM
Really? i was using the 3 bone setup but couldnt get the foot to behave as i wanted. I am using a script to switch between the two and didnt want to have to figure out how to make the foot switch. and since reverse foot uses ik to lift the knee theres no way to make it work properly with fk... at least not that I know of. If you do happen to know how to get a reverse foot to work with the fk leg then please let me know..

09 September 2006, 05:23 AM
I'm not sure if I misunderstood it or not.

With the 3 bones setup: If you switch from FK to IK (or vice versa) the position of the joints will return back to where you last keyed it or to its Bind Pose. But the animation blends perfectly.

09 September 2006, 07:09 AM
I think you may have misunderstood a bit, im using the script so the animator can do seamless fkik (switching over 1 frame instead of blending over multiple), anyways.. the real issue is that, maybe it cant be done but I want to know if anyone knows how to get it to bend backwards in ik... I've never seen it done so im pretty sure it cant be done(or at least not easily).. but I just wanted to check

09 September 2006, 01:01 AM
Funny you should mention this. I'm actually working on a plug-in IK solver that does just this (among many other things.) I'm planning on releasing it within the next 24 hours. I'll post back when it's done.

09 September 2006, 01:15 AM
Coolness.. does it need to bhe installed on every computer it's used on or just the computer you create the IK in? beacuse my animators arent so bright... either way total coolness.

09 September 2006, 11:59 AM

I would like to tell one method which I just tried in my machine and i m not sure it suits for production. Why don't we have 4 bone chain for ur requirement. ie. Skin - FK - IK where my knee bends forward - IK where my knee bends backward.
We can create a float attr named ik blend with min 0 and max of 2 on any curve controller and do a sdk for the constrain node weights... ie. 0 is fk, 1 is normal forward knee bending ik and 2 is reverse knee bending ik...

when the animator wants to break the knee but still have ik on the leg - he can switch it back to the reverse knee bending ik and animate...

pls ck this method and give me a feedback...

09 September 2006, 10:54 AM

Could u be a bit more elaborate on ur setup...??U mean to say that u have 4 separate joint chains or one single joint chain with 4 joints...?? I'm not quite sure of what ur tryin to say....

09 September 2006, 05:28 AM
4 sets totally...
1st - main skin set - hip -knee - ankle ( skin set of joints should be perfectly in the straight line, create a 2 cv linear cv curve between hip and ankle and curve snap all three joints to it.)

make 3 dublicates of the whole 1st set
and 1 would be ik where knee bends forward while 2nd would be ik where knee bends backward and the last is regular fk.

for the 1st set before creating the ik, please rotate the knee along the bend axis forward and perform a set preffered angle cmd.. now in channel box reset the rotation, then create the ik

for the 2nd set before creating the ik, please rotate the knee along the bend axis backward and perform a set preffered angle cmd.. now in channel box reset the rotation, then create the ik

orient constrain all the three sets to the respective skin set of joints...

create a float attr "Blender" with min 0 and max 2 and default 1 on any controller...
now as mentioned earlier, do sdk (set driven key) and with src as blender attr and target would be the orient constrains weight.

now animate the blender as required and also use the knee which bends backward for your animation..

hope this helps..

09 September 2006, 05:28 AM
doulbe post - skip this..

09 September 2006, 01:33 PM

can u post that file, we can have a clloooser luk at it ? :)


09 September 2006, 01:46 PM
I feel not skinning to the ik leg and using seperate breakable joints to skin the leg geo on, which can be moved away A.K.A bendy, ribbon,etc (using the free rivet script/hair follicle with a nurbs plane ) ,should be the easiest solution.because breaking joints also involves a slight curvature in most cases, and a bendy setup provides that too.

09 September 2006, 04:30 PM
Hey Sriram....
pretty interestin n innovative method...!

09 September 2006, 12:50 PM
hi, futurecraft !
Thanks :)

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