No-Flip Quadruped Hind Leg {

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

Link to No-Flip Foreleg post

As before, I'd like to share my No-Flip solution to quadruped legs rig. This time, I'll be doing the hind-leg, as promised. 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.

The Hind-Leg solution utilizes the same techniques as the Foreleg, but the extra ankle joint gives us some more work to do.

Tutorial {

Start with a joint chain on the XY or ZY plane. This will be the main joint chain. For simplicity, I left out the toe joint, but it shouldn't be difficult to add it if you wish.

Duplicate the joint chain, rename to "02"s. Now we're going to use a solver called ikSpringSolver. This will give us the correct bending behavior of a hind-leg. Type "ikSpringSolver;" into the MEL window and press ENTER. This will load ikSpringSolver. Create an ikSpringSolver from "hip_02_jt" to "foot_02_jt" (knee_ik). Create a locator called "knee_pv" and use it as a poleVectorConstraint with "knee_ik". This will be our driver IK.

Now here's the cool part. We're going to create a nested IK under our SpringSolver in order to give us a cool hip-pivot control. Create an IK RP from "knee_01_jt" to "foot_01_jt" and name it "ankle_ik". Create a locator called "ankle_pv", place it behind the knee joint and use it as a poleVectorConstraint on "ankle_ik".

Note: From here on I deviate from Morgan's rig.

Create a null (ctrl-g), snap its position (hold-v) to "hip_01_jt", and Freeze Transforms. Name this group "spring_jt_grp". Parent "hip_02_jt" under this group. Now do the same thing for "hip_01_jt" but name this group "main_jt_grp".

Create a NURBSCircle, name it "Hip_ctrl", and snap it (hold v) to "hip_01_jt". Freeze its transforms. Point constrain the "spring_jt_grp" to the "Hip_ctrl". This avoids rotating the ikSpringSolver, which causes flipping issues later. Parent constrain the "main_jt_grp" to the "hip_02_jt". This gives our main joint the rotation from the spring joint's hip. Now we are able to move the entire hip via the "Hip_ctrl".

Create a new joint chain, snapped from the Hip to the Foot. Name the joints "helper_01_jt" and "helper_02_jt". Add an ikRPSolver named "helper_ik". Now move "helper_02_jt" up along the ik until it is halfway up the leg. Create a locator called "helper_pv", snap it's position to "helper_02_jt", and move it forward along the X axis. PoleVectorConstrain "helper_ik" to it. This will be our no-flipping helper, as in the first rig.

Create a NURBSCircle called "Foot_ctrl". Snap its position to the foot and freeze its transforms. Parent all three iks under it. Now we have a foot we can move around!

Now let's start putting it all together.

Create a null, name it "pv_grp", snap its position to "helper_02_jt", and freeze its transforms. Parent under it "knee_pv" and "ankle_pv". Now parent that entire group under "helper_01_jt".

Try moving the "Foot_ctrl". As you can see, the PVs move with our leg now. But we still have some odd twisting action happening, as seen in the viewport image above. This is because the "helper_pv" is not moving with our leg as it should. It needs to stay evenly between the Hip and the Foot in order to serve its purpose. Let's solve that next.

Create a null, name it "helper_pv_grp", snap its position to "helper_02_jt", and freeze its transforms. Parent under it "helper_pv".

Now in order to keep it between our Hip and Foot, let's simply create a shared pointConstraint. First Create a locator called "Foot_loc", snap it to our Foot, and parent it under our "Foot_ctrl". Do the same with the hip, naming and positioning it accordingly. Then select "Foot_loc", shift select "Hip_loc", and then shift select "helper_pv_grp". Create a pointConstraint. Now our helper_pv will stay where it should.

Try moving our "Foot_ctrl" around to see the improvement. The leg should stay facing the Z axis, with no twisting action at all.

This is the basic finished rig, but as you can see, it's a bit messy and unusable as it is now. Next let's clean it up and add one more nifty control for our animators.

Now all we want to do is connect our hip control's rotation X to our hip joint's Z rotation. We also need to multiply by negative 1 in order to flip it. Simple enough.
Select "Hip_ctrl" and "hip_01_jt". Then enter the Node Graph and press the  button to add them to the view. Press "Tab" and start typing "multiplyDivide" to create a multiplyDivide node. Now connect "Hip_ctrl" Rotate and connect it to the Input of "mulitplyDivide1". It will create a unitConversion node, which you can ignore. Next connect the outputX of multiplyDivide1 and connect it to RotateZ of "hip_01_jt". Now select "multiplyDivide1", and in the ChannelBox, set "Input2x", "Input2y", and "Input2z" to -1. This will invert our value before sending it to our joint.

Now try it out! Rotate "Hip_ctrl" along the axis of the knee. You should see it change the pitch of the leg. This is a great control for our animators to use.

Note: If the nothing happens, try connecting from our hip's rotateZ instead of rotateX. It depends on how your hip control is rotated relative to your joint.

Alright! Now for some simple cleanup.

Create 3 nulls named "rot_grp", "dont_rot_grp", and "HindLeg_rig". Snap all 3 to the hip and freeze their positions. Place everything on the root of the heirarchy EXCEPT "spring_jt_grp" into "rot_grp". Place "spring_jt_grp" into "dont_rot_grp". This is the entire reason we went through all the trouble of separating our spring joint heirarchy. Rotating it causes odd issues.

Now create a giant NURBSCircle named "Main_ctrl". This is effectively your character movement control. Parent constrain "rot_grp" to "Main_ctrl".

Select everything you don't want to see and add it to a layer called "hidden". Set visible to False.

Aaand we're done! 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!

Give yourself a pat on the back for getting through that! This was quite a complex tutorial, so after playing with your new rig a bit, go get some ice cream and enjoy it knowing you've conquered this rig. Well done.

Feel free to comment if anything is unclear! I'm sure I glossed over a few minor details in an effort  to shorten the tutorial. I'd love to hear some feedback!

-K