Customizing IK Using Constraints

CAT's IK is customizable to the extent that you can use all regular 3ds Max constraints, and even write scripts to control a limb. Following are descriptions of two leg setups that you might find useful: Knee LookAt and Orientation Constrained.

LookAt Knee

An effective method when applying IK to legs is to have the knee look at a target. With CAT you can accomplish this by using an extra bone as the target object.

To set up a LookAt Knee:

1. Make sure there are no animation layers in the Layer Manager and return to Setup mode.
2. Select the thigh bone (upper leg).
3. Go to the Modify panel.
4. On the Bone Setup rollout, click Add Bone to create an extra bone.

   The extra bone is created at the end of the bone on which it is created.

   Attention: Create the extra bone on the limb with which it will be associated, because even after an extra bone is relinked to the pelvis, it is still part of the limb group and thus will use the limb weight as its local weight. This is useful when working with feathered weighting.
5. Move the bone out in front of the knee.
   Tip: Select the bone in a wireframe viewport.
6. Link the extra bone as a child of the pelvis.

   The knee LookAt should control the leg; if it remains linked to the leg it will follow the leg, not the other way around.

7. Select the thigh bone again.
8. Go to the Motion panel, add an Absolute layer, and then add a relative layer (Adjustment Local or Adjustment World).
9. Put the rig into Animation mode.
10. Access the Assign Controller rollout and highlight the Rotation track of the current animation layer.
    
11. Click (Assign Controller) and assign a LookAt contraint.
12. Go to the LookAt Constraint rollout.
13. On the LookAt Constraint rollout click Add LookAt Target and choose the extra bone. Right-click to exit.
14. Put the thigh into FK. (Limb Animation rollout IK/FK=1.0)

    This is useful because you can get a much better idea of what is going on.

15. On the LookAt Constraint rollout, turn off Select Upnode World and assign the FootPlatform as the Upnode.
16. Set Upnode Control to LookAt and pick the footplatform as the target.
17. Set the Source Axis to Y and choose Flip.
18. Put the limb back into IK.

Orientation Constrained

When you want a leg to inherit motion from both its parent (the pelvis) and the FootPlatform, use an orientation-constrained setup.

To set up an orientation-constrained leg:

1. Select the upper leg bone.
2. Go to the Motion panel, add an Absolute layer and activate Animation mode.
3. Put the limb into FK.
4. Go to the Assign Controller rollout and highlight the Rotation track in the current animation layer.
5. Click (Assign Controller) and assign an Orientation constraint.
6. On the Orientation Constraint rollout click Add Orientation Constraint and select the FootPlatform.
7. Turn on Keep Initial Offset.
8. The Leg should now be standing on the FootPlatform normally.
9. Blend the leg back into IK.

   Now when you rotate theFootPlatform the whole leg will rotate with it.

To blend the leg rotation between the FootPlatform and the pelvis:

1. Apply an Orientation constraint and assign the FootPlatform as orientation target, as described in the preceding procedure.
2. Add a second Orientation constraint, this time assigning the pelvis as orientation target.
3. Adjust the constraint Weight settings between the two targets as required.

Saving Constraint Settings

To save your IK constraint settings, set up the configuration in a layer and then save the layer as a clip file with no keyframes on it. When you want to add a new Absolute layer with the same configuration simply load in the layer.

Note: Note: Using these constraint systems makes it impossible to blend to FK on the same layer. To blend to FK, blend to another animation layer in which the limb doesn't have this system added.