How does IK/FK switching work and what is its purpose?

Support Basics & Interface
1

Hey all,

I’m putting this in Basics because I see it as more of a conceptual issue, rather than looking for specific help with an animation issue.

Anyway, I recently made a parrot model and rigged it. It has those backward bird legs, and I have noticed this has been giving me trouble.

In any case, the legs are IK only. They have no FK functionality.

I was wondering if anyone could explain to me how IK/FK switching and what the point is? I have noticed that IK makes it very easy to set the general position of a limb, it definitely keeps feet from slipping, at it seems to help the program interpolate between key frames better.

But, I have noticed that I have difficulty getting IK limbs to be exactly how I want them. Every time I get close to the pose I want, something goes weird. The leg spins backward. Or rather than bending more deeply, some other part of the leg moves.

Is IK/FK switching a way to deal with this? Can I make most of the pose with IK, and then use FK to refine the IK pose? Or are they mutually exclusive? Like, does turning off IK return the limbs to their rest position and you have to start all over again with the FK joints?

A brief (or not brief) synopsis on what FK/IK switching, when to use it, what it allows you to do, etc, would be incredibly helpful! Alot of things I have read explain either FK or IK great…but do very little to explain how switching between them works…

2

To answer this properly, I’ll have to cover a bit of ground…

FK (Forward Kinematics) allows you to pose a limb from the “top” down whereas IK (Inverse Kinematics) allows posing from the “bottom” up.

In other words, in FK, you would (for instance) pose the hip, then the knee, then the foot and finish with the toe. This is for animations when the character is NOT walking or running or hopping, whatever. If your character was lying on his back, kicking his legs, that’s when you would use FK.

IK is for those times when the character IS walking, running, hopping or what-have-you. Posing the ankle automatically drags the knee along for the ride.

So, IK/FK switching is pretty much what it sounds like, switching between the IK and FK systems.

Do you have knee pole targets as part of your rig? A knee pole target is an extra bone placed out in front of the knee for most characters. For a bird, because the knees bend in the opposite direction, the knee poles would go out BEHIND the knee. The idea is that wherever you place the pole target, the knee will point at it.

The two systems are, more or less, exclusive. I say “more or less” because some rigs (the Rigify system that comes built into Blender, for instance) have a pair of ‘snap’ buttons, one to snap your FK system to an IK pose, and the other to snap your IK system to an FK pose. These are used for times when (for instance) your character walks up to a wall (IK arms and legs) then leans against it (FK both arms until the hand contacts the wall, then that arm goes to IK so it stays where it is) and stands on one foot (that foot is still IK) and the other foot kicks at a stone (FK).

So, they are mutually exclusive, but you switch back and forth when you want a hand or foot to stick, even for a moment (IK) or move freely (FK).

I hope this helps.

3

Birds actually do have forward facing knees. They are just tucked up into their body a bit. The part that appears to be a backwards knee is actually the ankle.
(edit) I realize this has nothing to do with the actual question…just thought I’d point that out.

4

Yeah, Rontarrant. That does help clarify. And at the risk of making this about my specific problem, I do have knee poles (and more). I am actually thinking about making a thread for that and including the blend and some pics, because it’s not even an extreme pose when the knee goes backwards, its almost like an imaginary “180” line where it hits a divide by zero moment or something, and that line just happens to be in a very unfortunate place. Honestly, it looks a lot like how people describe “Gimbal lock”…but all of my rotations are done in Quaternion mode which is supposed to prevent that. Also, I’m not a scientist so I could be making things up.

Also, Modron, you’re right. I’m falling into the trap of simplicity. It like dog and dinosaur legs…I think the issue is really an extended angle and long toes that people mistake for the entire foot, no?

5

Too true. It’s like the hind legs on a cat or dog, I suppose.

6

What do you have the Pole Angle set to?

Also, double-check all the bone orientations to make sure they’re sane (it’s probably covered in the tutorial you followed). This can also cause the problems you’re having.

We all do. :wink: