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Tutorial: Animating a spider's leg with the IK Limb solver

As an example of the IK Limb solver, you should probably animate a limb, so I created a simple spider skeleton with not two limbs, but eight. I created this skeleton fairly quickly using four bones for the abdomen; then I created one limb and cloned it three times. Then I used the Bone Tools to connect the leg bones to the abdomen bones, and finally I selected and mirrored the bones on all four legs to get the opposite legs. The hardest part was naming all the bones.

To animate a spider skeleton's leg using the IK Limb solver, follow these steps:

1. Open the Spider skeleton.max file from the Chap 38 directory on the CD.

2. Click the Select by Name button on the main toolbar (or press the H key) to open the Select Objects dialog box. Double-click the RUpperlegBone0l object to select the upper leg bone object.

3. With the upper leg bone selected, choose Animation⇒IK Solvers⇒IK Limb Solver. A dotted line appears in the viewport. Press the H key again to open the Pick Object dialog box, and double-click the RFootBone0l object to select it.

This bone corresponds to the foot bone, which is the end of the limb hierarchy.

4. With the IK Chain01 object selected, click the Auto Key button (or press the N key) and drag the Time Slider to frame 100 (or press End). With the Select and Move button (or by pressing the W key), move the IK chain in the viewport.

The leg chain bends as you move the end effector.

Figure 38.16 shows the spider's leg being moved via the IK Limb solver. The IK Limb solver provides a simple and quick way to add an effector to the end of a limb, giving you good control for animating the spider's walk cycles.

FIGURE 38.16

You can use the IK Limb solver to control limbs such as legs and arms.

Spline IK solver

The IK Limb solver works well for arms and legs that have a joint in the middle, but it doesn't work well for tails. Tails are unique because they require multiple bones to deform correctly. The Spline IK solver works well for tails, but it also works well for rigging tentacles, chains, and rope.

To use the Spline IK solver, you need to create a chain of bones and a spline path. By selecting the first and last bone and then selecting the spline, the bone chain moves to the spline. Each control point on the spline has a dummy object associated with it. By moving these dummy objects, you can control the position of the bones. At either end of the spline are manipulators that you can use to twist and rotate the bones.

The easiest way to use this IK solver is to select SplineIKSolver from the drop-down list in the IK Chain Assignment rollout while you're creating the bone structure. After the bone structure is complete, the Spline IK Solver dialog box appears. With this dialog box, you can select a name for the IK chain, specify the curve type, and set the number of spline knots. The curve type options include Bézier, NURBS Point, and NURBS CV. You also can select to Create Helpers and to display several options.

Another way to use this IK solver is with an existing bone structure. To do this, you need a spline curve in the scene that matches how you want the bone chain to look. Select the first bone where you want the solver to be applied, and choose Animation⇒IK Solvers⇒Spline IK. In the viewports, a dragging line appears; move the line to the last bone that you want to include, and then drag a second time to the spline that you want to use.

The bone structure then assumes the shape of the spline curve. A helper object is positioned at the location of each curve vertex. These helper objects let you refine the shape of the curve.

Tutorial: Building an IK Spline alligator

The IK Spline solver is perfect for creating long, winding objects like snakes or an alligator's tail. For this example, you take an existing bone structure and, using the Spline IK solver, make it match a spline.

To create a bone structure for