No-Flip Quadruped Foreleg {

Credit to Morgan Loomis for initial leg rigs (Foreleg, Hind leg)

I'd like to share my No-Flip solution to quadruped legs rig. I'll briefly explain the initial leg rigs, but will focus primarily on the No-Flip nature of them. If you want more information on the initial rigs, check out the links above.

As with any IK solver, flipping is a common and troublesome problem. Once the IK handle breaks the IK origin's plane, the knee flips around. This is almost never a desirable effect. So here's my No-Flip solution.

No-Flip Tutorial {

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Start with a joint chain on the XY or ZY plane. This will be the main joint chain.

Duplicate the joint chain, rename to "02"s, and remove "wrist_02_jt". Add an IK RP solver from "shoulder_02_jt" to "foot_02_jt" (kneeIK). Create a locator called "knee_PV" and use it as a poleVectorConstraint with "knee_IK". This will be our driver IK.

Add another RP IK from "shoulder_01_jt" to "wrist_01_jt" and name it "ankle_IK". Group it, place the group as a child of "elbow_02_jt", and snap-drag its pivot (hold d+v) to "foot_01_jt". This allows the wrist to pivot by rotating the "pivotAnkle_grp", while keeping it aligned with the forearm!  This is the main genius behind Morgan's rig.

Now let's include the foot and toe. Create a SC IK from "wrist_01_jt" to "foot_01_jt" and name it "foot_IK". Now create another SC IK from "foot_01_jt" to "toe_01_jt" and name it "toe_IK".

Let's make a control handle. Create a NURBSCircle, name it "foot_ctrl", and snap it (hold v) to "foot_01_jt". Place all the other IKs under it ("knee_IK", "foot_IK", "toe_IK").

We have now completed the Foreleg rig by Morgan! Now let's make it No-Flip!

Create a 2-length joint chain and snap-drag (hold v) it to start at "shoulder_01_jt" and extend to "foot_01_jt". Add a RP IK to it and name it "helper_IK". Now move "helper_02_jt" up until it is half-way up the leg. Since we made the IK first, movement of the joint will be constrained by the IK for us! This joint chain will be used to keep our "knee_PV" in line with the knee at all times.

Create a locator named "helper_PV", snap-drag (hold v) it to "helper_02_jt", and translate it in Z (if your joints are on the XY plane like mine). Now poleVector constrain "helper_IK" with it. This will keep our helper joint chain from flipping, and thus our knee. (To see what I mean, disable this constraint after the last step).

We need the "helper_PV" to stay between the shoulder and foot, and always on the same side of them. So to avoid getting rotations, we'll use a pointConstraint. Group "helper_PV", select "shoulder_01_jt", shift-select "foot_01_jt", shift-select "helper_PV_grp", and Constrain->PointConstraint.

Now that our helper joint chain won't flip, we can use it to drive the knee PV. Group "knee_PV", select "helper_01_jt", shift-select "knee_PV_grp", and Constrain->ParentConstraint. The group allows for animation of "knee_PV" after the constraint.

That's it! Give it a trial test to see exactly what this rig gives you in comparison to a normal IK setup. It should not flip in the XY plane at all! This should give your characters much more freedom of movement forwards and backwards. And it automates the PV's, so no need to animate PV's every step!

I hope this helps with your quadruped rigs! Thanks again to Morgan Loomis for allowing me to use his awesome foreleg rig! I will be posting a No-Flip solution to his hindleg quadruped rig soon!

Feel free to comment if anything is unclear! I'd love to hear some feedback!

-K