About Spline IK

Spline IK gives you a way of controlling many joints using a curve rather than by rotating them directly. This approach is especially useful when you have many joints in a continuous chain (such as in a spine or a tail), because you can move the skeleton quickly into the shape you desire without rotating each individual joint (Figure 10.35).

Figure 10.35. By creating an IK spline on these joints, you can animate a spine skeleton by moving just one point. Keyframing the rotation of each joint to achieve the same effect would be much more difficult.

To create a Spline IK:

1.	Create a spine skeleton, starting from the bottom and working your way up (Figure 10.36). Figure 10.36. The human spine has a natural curve. When you're creating a spine for a character, it's helpful to match the shape of the character's back.
2.	Choose Skeleton > IK_Spline Handle Tool.
3.	Click the top joint of the skeleton.
4.	Click the second joint from the bottom of the skeleton. Two things are created: the Spline IK handle and a curve. By default, the curve is automatically parented to the joints.
5.	From the Hypergraph or Outliner, select the curve you just created (Figure 10.37). Figure 10.37. It's easier to select the curve in the Hypergraph because the icon can be isolated.
6.	In the Pick mask, select component mode. You can now see the CVs of the curve.
7.	Select and move a CV (Figure 10.38). Figure 10.38. When the CV is moved, the spine bends to form the shape of the control curve. The joints rotate in response and stick to the shape of the control curve.

Tips

• In general, you shouldn't use the root joint of the skeleton as part of your Spline IK chain. If you do, when you move the CVs, the whole skeleton will rotate. To avoid this, choose the joint that's one above the root joint of the skeleton as the base of your Spline IK.

• Because CVs don't have their own node, they don't leave keyframes in the timeline. To see the keys controlling your spline, create a cluster for each CV, and animate the cluster. See Chapter 12 ("Animation") and Chapter 13 ("Deformers") for more information on keyframes and clusters.

Understanding the Hypergraph and Skeletons

When you're creating characters, the Hypergraph plays an important role in hierarchy order and selection. It's the best tool Maya offers to help you understand the way hierarchies are formed as well as how Maya's various elements relate to one another.

The Hypergraph provides two layout options: Freeform and Automatic (the default). If you choose the Automatic option, Maya lays out the Hypergraph for you, with the root node at the top of the hierarchy and each child indented below its parent with a line connecting child and parent. In addition, nodes are locked in place so that the hierarchies remain visually consistent and organized.

In contrast, if you select Freeform from the Options > Layout menu, you can move the nodes around and organize them to your liking. In addition, you can import an image of your character into the background of the Hypergraph so that you can align its joints to their proper positions on top of the image, making it much easier to select individual joints. In a simple hierarchy, this may not be necessary; but when you have hundreds of joints and surfaces, selection can become cumbersome without this aid.

The Hypergraph displays each of your joints with a blue joint icon . When a single joint is selected in the view pane, all of its children are highlighted to indicate their relationship to the selected joint. As children of the joint, they are affected if the selected joint is rotated. Be aware that although the children are highlighted in the view pane, they haven't been individually selected.

Select a joint, and open the Hypergraph. The joint you clicked is selected (yellow), but the joints underneath it (its children) aren't. Because the parent is selected, the children will follow. If, in the Hypergraph, you -select the children and then rotate the joints, each joint will rotate. Although you'll rarely select joints in this fashion, it's important to understand what is and isn't selected in a hierarchy.

You can move joints from one hierarchy to another or disconnect them from a hierarchyall from within the Hypergraph. You use the middle mouse button to move joints in and out of hierarchies. To move a joint (and its children) into another hierarchy, drag the joint (using the middle mouse button) onto the node you want to serve as its parent. Once you do this, the selected node (and its children) is connected to the new hierarchy.

To remove a joint (and its children) from a hierarchy, select its node with the middle mouse button, and drag it away from the hierarchy (releasing the mouse when the node has been moved away from the hierarchy). This technique also works well for removing objects from a group within the Hypergraph.