Setting Up a Foot
The foot always presents problems when creating a manageable skeleton. The big problem is the way the foot rolls along the ground. As you can see in Figure 3, the foot can pivot on any one of three points: the heel, the ball of the foot, and the toes.
Figure 3 The foot can pivot at three points.
Here's one way to use a series of nulls, a grab point and a simple hierarchy to control the foot. It has been tested in 3ds max and Maya.
Start with a simple skeleton. You can see in Figure 4 that the leg is a two-joint chain, while the foot bone and the toe bone are single-joint chains. Make sure that these all exist in the same hierarchy.
Create four nullsone as a grab point and then one each at the heel, the ball of the foot, and the toe. These three nulls should be aligned parallel to the ground plane, as in Figure 6.
Link each IK handle to its respective null, as shown in Figure 7.
Now link the nulls together to form a hierarchy. Make the grab point the parent of the heel (see Figure 8), make the heel, the parent of the toe (see Figure 9), and make the toe the parent of the ball of the foot.
Figure 4 Start with a simple skeleton.
This setup will give you three IK handles: one at the heel, one at the ball of the foot, and one at the toe (see Figure 5).
Figure 5 Create IK handles at the heel, ball of the foot, and the toe.
Figure 6 Create four nulls, as shown.
Figure 7 Link each IK handle to its null.
Figure 8 Make the grab point the parent of the heel.
Figure 9 Make the heel the parent of the toe.
That's it. Lifting and moving the grab point moves the foot, as you can see in Figure 10.
Figure 10 Lifting and moving the grab point moves the foot.
If the ankle goes beyond the IK limit, the foot naturally bends to compensate, as Figure 11 shows.
Figure 11 If the ankle goes beyond the IK limit, the foot naturally bends to compensate.
Rotating each of the other three nulls rotates the foot around each pivot point (see Figure 12).
Figure 12 Rotating the nulls positions the foot.