smooth IK motion (arcs) - how?

Is there a way, at all, to have IK bones move on an arc as FK bones do? I just don’t see it as productive creating bunch of intermediate poses / keys for IK bones. And still, even if I have 6 extra poses, bones interpolate linearly between them. I tried using Fcurves in IPO, but still nothing like FK.

I noticed that 3DS MAX, for example, works with IK a bit better/smoother.

Any ideas? Thanks.

Is there a way, at all, to have IK bones move on an arc as FK bones do?

Just set IK constraint influence to zero. Then you get FK.

If you really want IK target to move in arc, you can probably give it a constraint or parent to move that way. IK target is disconnected object that just float in space you know. Give it a pivot spin from.

Well, if I wanted FK only, I’d not use IK :wink:

But, try it for yourself, cos it doesn’t sound like you understand the issue.

It should take, like, one extra keyframe on the IK handle to turn the movement from a line to an ark. So that between point A and B it goes to C forming an ark. Is that any help?

Here is what I am talking about:

When I key FK, intermediate interpolated keys will have bones following imaginary arc. When I keys IK the same way, intermediate keys will have bones following straight lines between keys.

Inbetween keying is the only way to good arcs with IK. Sometimes requires keys on every frame if doing feature quality animation. This is where IK/FK switching and snapping comes in handy.

Edit: Armature Properties > Ghost and Motion Paths helps when keying arcs.


:frowning: is it just the way IK works or is it only in Blender ?

That’s just the way IK works. If you need arcs, then you need to animate the target in an arc. This is true in any program, not just Blender.

Yeah I first learned about this online from Pixar animators. Computers are stupid.

I edited my last with a tip.


Aye, thanks!

Set the Keys to “auto clamped” to get smoother motion. You can do that by selecting the keys in the dope sheet or curve editor and pressing V then a menu will pop up to select the key type. You can also set the default key type in the preferences.

For all keys for IK bones (or rather the bone that drives IK chain) or for start/end keys?

I would only recommend using IK when the limb is touching another object and putting force on it, say for legs with the ground or hand against the wall, and in all othertimes use FK. otherwise you will always be fighting to get that natural motion. IK is faster to get your initial pose, but from then on you will always be fighting.

IK is faster to get your initial pose, but from then on you will always be fighting.

Ya I see your point. One needs to be able to control with FK and IK for given setuations.

Here is my experiment with IK and FK generated arc movements with three key frames for both. Large gap between IK key framing don’t give nice arc movement. But, it is not straight line movements either. FK arc was created by rotating upper arm bone only, with IK influence set to zero. So IK setup was not removed. You can activate IK any time you want.

Nice R&D!

Well, it’s nowhere near arc shape, thus - straight line :wink:

I have IK/FK switchable arms and legs, it’s just there is no point to switch to FK when I precisely need to animate with IK.

So the only way is to have at least 8 key poses for IK for it to look better.

Popping knees is a subject of another topic :smiley:

What if you make the target of the IK follow an actual curve you create?

How do you propose moving that curve with the character? Also, what does the curve look like for walking/running/jogging foot, forward and backward motion?


It doesn’t need to move with the character (but you can parent it to the character if you want to); you just put the beginning and end of the curve where the keyframes would be (and animate the influence if you got multiple curves to pass it from one curve to the next).

But for figuring out what the curve should be like, i guess you could check some reference footage or try the motions yourself and stuff like that, and of course tweak the curve till it looks right. I don’t think you can really get automatically created natural-looking curves without some more advanced animation system, sorta like that Euphoria thing or that thing they use to figure out how dinosaurs walk based energy usage optimization taking in consideration things like muscle size, tendon attachment positions, bone length etc; or at the very least you need to use an actuated ragdoll instead of just animating without physics.


Ridix was right from the start : you can parent your IK target to a bone whose pivot is anywhere you need the IK to rotate from. If you need several different arcs (I mean with different pivot points), you can always use multiple parents (IK parented to a bone, parented to another bone, et cetera desunt) or animate the parent bone’s location and constraint influence (using visual keying obviously).



PS - Really why don’t you just use an IKFK switch ? I have a good example of this at hand if you like… there’s no pose matching but who needs this anyway !

What you are saying both intrigues and confuses me. Is there a tutorial on this technique somewhere?

Also - is there a way to copy an IK pose to FK?