Inverted Joint

[themediapuppet]

I'm creating an ik setup for a human leg, where the later skin wil be driven by a muscle dynamics system. Unfortunately, the knee joint's center of rotation is toward the back of the femur. This makes for a slight bend in the wrong direction when I set up Maya's joints. When an ik solver is added to this it obviously want to bend in the wrong direction. How can I force the ik to bend in the propper direction when the initial angle of the joint is opposite the direction it needs to bend?

Thanks.

[failure]

With "rotate plane" solver you can adjust this by editing the pole vector. Most common way is to pole-constrain the ik handle to a locator. That way you can contol rotation plane by moving the locator.

[joojaa]

first you must understand that for a ik solutrion theres a unimmited amount of soulutions. So the solver is costrained to a plane wich limits the solutions to 2. But its still one more than you want.

So what maya does it looks at what way the natural bone is bent and chhooses that one. Normaly its the position at wich you created the bones. But if you did create the bone completely straight youd be in problems since maya still does not have a clue wich of the 2 to choose.

The joint isnt however limitted to the original construction shape tough as theres the set prefeered angle command that can set this. As a added bonus since a shape is stored it can be later asked for and qucikly toggle back to the neutral bind pose. Altough theres nothing that states you must bind in this pose.

Use a pole vector constraint to set and animate the plane. A simple ik solver on the otherhand just uses the bones default orientation for the plane.

[themediapuppet]

Thanks Joojaa. You've definately given me some insight as to what's going on behind the scene's with ik. Which now makes for better wording of my original question. With an ikRP solver, the are two solutions. Maya by default will solve in the direction of the bend in the joint. However, I want the ik to solve in the other direction. How do I tell maya to choose this solution over the defualt?

To better visualize, think of the knee as starting out inverted. I want the ik to bring the knee from being inverted to fully bent like an normal human knee.

[joojaa]

bend the knee thatway and choose animation menuset; Skeleton -> set prefeered angle.

[themediapuppet]

I looked in the help file and tried that after your first post. Once I set the preferred angle, returned the joint to zero orientation, and the applied the ik, Maya flipped the rotation plain so that the solver would be able to get the joint to the preferred angle. This, of course, screwed up the initail placement of the joints.

I'm actually starting to think what I'm trying to achieve is impossible with ik. It doesn't seem like it would be able to solve more a that 180 degrees of rotation on a joint. And what I need is about 190 degrees - start out inverted and going back past being straight to fully bend in the opposite direction.

[joojaa]

Yes youd get a kinematic lock if you did that. use fk. it is however possible to do sswitch wich tells what solution space you want to operate on but might get tedious to use.

[failure]

Did you pole-constrain the ik handle?

[joojaa]

Pole vector cant easily solve this becaus pole vector twist rather than flips.

heres an exmple about a simple bonde controll with flip

[themediapuppet]

Interesting setup. If that is simple, I have a lot to learn, because I never would have though of that. To make sure I understand this right, the enumerators flip and normal drive the condition nodes on the two joints to change their preferred angle?

Thanks for the help!

[failure]

Uh, finally got it. It's not 'flipping' problem at all, so yes, pole vector manipulation is useless.

The thing is; changing prefered angle after creation of ik chain has no effect since ikHandle picks up skeleton data on creation and sticks with it.

I didn't check joojaa's example but I'd do the following: create two exactly same ik chains ( on same bones ) but prior to creation of each one, adjust the different prefered angle. Group the ik handles together for manipulation. Now use ikBlend attribute to switch between them ( when 0 on first handle, 1 on second and vice versa ).

Of course, you should add nice custom attribute to the group node, for switching between two.

[joojaa]

In reply to:
The thing is; changing prefered angle after creation of ik chain has no effect since ikHandle picks up skeleton data on creation and sticks with it.

wrong, it does affect the thing only if its bound to a RP solver the pole vector comes to govern things wich means you cat switch as easily ecept by inverting the pole vector location. (yo can use pole vector to flip but its not as easy.)

And yes that is what i call simple, infact its allmost trivial.

[failure]

Hm, you lost me there.

Here's what I know:

• Changing joint's prefered angle after rp ik chain creation does not affect behavior of the ik chain in any way.

• You can use pole vector to flip the chain. This might look as changing prefered angle but it isn't - local rotation axes will be different. And if there are child joints in the chain after the ik handle ( foot in case of the leg ) they'll be flipped too. And that's not what we want here.

[joojaa]

In reply to:
- Changing joint's prefered angle after rp ik chain creation does not affect behavior of the ik chain in any way.

you know wrong it does if you use a simple chain ik instead of rotate plane one. And even the rotate plane ones orinetation can be changed at later date but its not that easy! Since the rp solver essentialy carries all that info anyway so it realy doenst need the prefeered angle at all in most cases.

See my file.

In reply to:
- You can use pole vector to flip the chain. This might look as changing prefered angle but it isn't - local rotation axes will be different. And if there are child joints in the chain after the ik handle ( foot in case of the leg ) they'll be flipped too. And that's not what we want here.

Yes thats true!

[failure]

I was constantly thinking that we are dealing with rotate plane solver since that was the original question.

Anyway, I tried your example but unfortunately I'm on maya 5. File did load reporting some errors - everything seems ok but it doesen't behave as it should. Both times joint bends to the same side. It must be a version issue.

Nevertheless I took a look at the graph and yes - it's pretty simple setup.

Now here is what I have done with rotate plane solver - switching between two chains thru custom attribute on faked ik handle.

It will break up if you try to change pole vector or other rp related attibutes on either one of the handles. But this can be easily remedied with few extra lines of code.

EDIT:

Forgot to constrain the foot joint. I've fixed it and uploaded new version of the example.

[joojaa]

boilding it manualy anew in maya 5 should work fine!

[themediapuppet]

Thanks for the discussion guys. Not only did I learn, but I'm now inspired to teach myself more about utility nodes and expressions. Especially if these ideas are considered trival.