Generic IK Leg Setup
by Brian Immel
This tutorial covers the three aspects of building a basic IK leg setup through IK joints, control objects, and constraints.
IK Setup
1. Activate the IK Handle Tool. Make sure the IK Handle Tool is set for “Sticky”. If it is not, go to Skeleton > IK Handle Tool > Option Box and make sure the checkbox for Sticky is enabled. If this is not enabled, all the IKs we create will be float around when we animate them instead of staying put like we want them to.
2. Select the Hip joint.
3. Click on the Ankle joint (Figure 1). This creates an Inverse Kinematic chain that controls the rotations and translates of the hip, knee and Ankle joints.
- "Why do we do this?" Two reasons: One- when we move the characters, the legs are earthbound (meaning having to stay in contact with the ground). They must stay put while the rest of the body has the freedom to move about freely (Forward kinematics). Two- its easier to animate one IK handle instead of three FK joints.
4. Name this IK Ankle_IK.
5. Activate the IK Handle Tool again.
6. Select the Toe joint (unless you have a joint inbetween the toe joint and the ankle which you would then select the inbetween joint).
7. Click on the Ankle joint (Figure 2).
- The reason we made a second IK handle is that when you pick up the Ankle_IK, it causes the Ankle joint to rotate which causes the joints below to rotate in opposition. Basically, when the Ankle_IK is lifted up, the foot points down and that is a no-no.
8. Name this IK Toe_IK.
9. Select the Toe_IK.
10. Shift-select the Ankle_IK.
11. Parent the Toe_IK to the Ankle_IK (p key or Edit > Parent).
- We parent the Toe_IK to the Ankle_IK because when we lift up the Ankle_IK, we want the entire foot to stay level. If you wish to make it unlevel, simply move the Toe_IK as you see fit.
Using Pole Vector Constraint to Keep the Knee in Alignment of the Foot
1. Create a locator.
2. Snap/move the locator to the Knee joint (activate the Move Tool and hold down the v key).
3. Pull the locator in front of the Knee joint (2-3 units should work) (Figure 3).
We need the locator to sit in front of the Knee joint otherwise, the joint will act weird when we try to move the foot about.
4. Select the locator, if you do not have it currently selected.
5. Shift-select the Ankle_IK.
6. Go to Constrain > Pole Vector (Figure 4). The Pole Vector helps the IK handle to orient itself to the direction we want it to face. In this case, we want it to face forward (in front of the Knee joint) all the time (Figure 7). Pole Vector Constraints are used to keep IKs in alignment to whatever we assign it to.
7. Rename the locator to Knee_Vector.
Wrapping Up the Foot Control
1. Create a NURBS circle.
2. Snap it to the center of the foot (Figure 5).
3. Shape it the design you like.
4. Rename the NURBS circle to Foot_Control.
5. Select the Ankle_IK.
7. Shift-select the Knee_Vector.
8. Shift-select the Foot_Control.