View Full Version : Backwards IK Problem

07 July 2010, 11:45 AM
I'm having issues rigging a bipedal character, namely with the legs. The problem is with the IK setup. The knee joint is slightly behind the hip and ankle joints. This causes the IK rig to bend the wrong way at the knee. Can anyone think of a way to fix this? I have attached an image that will hopefully explain this better.

Thanks in advance.

07 July 2010, 03:14 PM
Select knee joint and rotate the way you want, then right click and select set preferred angle.

07 July 2010, 04:16 PM
Thanks for the response shinyprem. Unfortunately setting the preferred angle does not help as it just causes the chain to flip in the opposite direction and then it just acts as it would have before. Constraining the angles on the joints to prevent that causes the solver to stop doing anything useful.

I think the solution lies in creating another joint chain that is offset so that it is angled in the conventional way, and then using that to drive the original skeleton. I have not yet figured out a way to keep the endpoints of each skeleton aligned though (as the pivots are in different places, simply using an orient constraint does not work).

07 July 2010, 04:27 PM
You got to delete the IK handle and do the preferred angle settings and then apply the ik

07 July 2010, 04:32 PM
You can also turn off the Ik Chain by setting your IK Blend Attribute to 0 (Zero) then setting the prefered Angle. Then turn the IK Chain back on. This way you will not have to delete the IK chain.

07 July 2010, 04:56 PM
I still can not get it to work. Here is the process as far as I understand it:
1. I create a simple leg skeleton in a new scene (as shown in the image in my first post)
2. Set the preferred angle by rotating the joint so that it is pointing in the other direction (to the opposite side of the hip joint direction).
3. Rotate the knee joint back to its original position.
4. Then when I create the IK handle the skeleton flips 180 on the Y axis. This makes sense to me as Maya is simple orientating the skeleton to maintain the preferred angle.
Am I doing something really wrong?

I'm not sure a simple IK rig will work as I would need the IK handle to cross the line of the hip joint, which I wouldn't think was possible?

07 July 2010, 06:06 PM
there is at least one point you are missing which is the setup for the Rotation Plane Controller or the Pole Vector.

Attached are two types of leg setups. One with the Knee going the traditional forward, and the other with a flipped knee.

I did not have to set the prefered angle with either one of these.

07 July 2010, 09:25 AM
Thank you very much for taking the time to create a test scene for me, Darksuit. Unfortunately this does not resolve the problem I'm having, it's probably my fault for not being clear enough in the first place.

What I'm looking for is something that acts like the forward knee setup, but in the bind position (straight leg) the knee is behind the hip and ankle, a bit like the reverse knee setup but less acute.

So, from the bind position (reverse knee), if I move the foot controller backwards and up, the joint chain should go straight (as it would in the setup you sent). But as it moves further the leg should continue to bend as in the forward knee setup.

I hope this makes sense now. I'm planning on modifying my geometry so that I can just use a simple rig, but if anyone has any ideas, that would be very much appreciated.

07 July 2010, 02:58 PM
first I would ask what is the purpose of the setup, and second can you find any animal or object that acts the way you are describing.

I am attempting to understand the purpose of the exercise. Is this a school project, personal, something you doing for a contract. In any case I would say find an object or animal that moves the way you are describing. The reason is that those structures move a particular way for a reason.

If you can't find an object in the world that moves a particular way, you may not have a clear enough vision or understand the movement correctly.

for instance your description of your leg setup now sounds more like you are looking for something like a mechanical piston setup like that on a train or car engine.

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07 July 2010, 02:58 PM
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