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Each skin area that surrounds a bone gets encompassed by a capsule-shaped envelope. All vertices within this envelope move along with the bone. When two of these envelopes overlap, their surfaces blend together like skin around a bone joint. Most of the skinning process involves getting the skin vertices into the right envelope. The Skin modifier includes several tools to help make this easier, including the Skin Weight table and a painting weights feature.

Binding to a skeleton

After you have both a skeleton and a skin mesh, you need to bind the skin to the skeleton before you can edit the influence envelopes. The binding process is fairly easy: Simply select the skin mesh, and apply the Skin modifier to it using the Modifiers⇒Animation⇒Skin menu command.

Tip

Before binding a skeleton to a skin mesh, take some time to match the size and dimensions of the bones close to the skin mesh. The skin mesh is bound to the skeleton, and the envelopes are created automatically. If the bones match the skin, then the new envelopes are pretty close to what they need to be. •

In the Parameters rollout, click the Add button above the bones list. This opens a Select Bones dialog box where you can select the bones to use to animate this skin. The selected bones appear in the bones list. The text field directly under the bones list lets you locate specific bones in the list by typing the name. Only one bone at a time may be selected from the list. The Remove button removes the selected bone from the bone list.

Tutorial: Attaching skin to a CAT rig

For human figures, using a biped skeleton saves lots of time. For this example, because he's human in form, you'll bind a biped skeleton to the Future man model.

To bind the skin of a model to a biped skeleton, follow these steps:

1. Open the Marvin Moose CAT rig - final.max file from the Chap 40 directory on the CD.

This file includes the CAT rig built in Chapter 39 for the Marvin Moose character.

2. With the CAT rig aligned to the skin mesh, select Unfreeze All from the right-click quadmenu and disable the See-Through option in the Object Properties dialog box. Select the moose model, and choose Modifiers⇒Animation Modifiers⇒Skin to apply the Skin modifier.

3. In the Parameters rollout, click the Add button. The Select Bones dialog box opens. Click the All button to select all the bones, and click Select. Don't include the Footsteps, Character001, or the Leg Platform objects.

4. In the Parameters rollout, select each of the bones in the list and click the Edit Envelopes button. Zoom in on the highlighted bone (which is the left shin bone), select the cross-section handles for this bone, and set the Radius values to 0.15 near the body and 0.1 for the other end. Figure 40.2 shows the moose skin with all the CAT rig bones added.

FIGURE 40.2

Bone references are added to the skin modifier.

After the Skin modifier is applied and bound to the skin mesh, every bone includes an area of influence called an envelope that defines the skin vertices that it controls. If any of the skin mesh vertices are outside of the bone's envelope or are included in an envelope for the wrong bone, then the vertices are left behind when the bone is moved. This causes an odd stretching of the skin that is easy to identify.

To check the envelopes, select and rotate several of the skeleton's key bones. If any envelope problems exist, they are easy to spot, as shown in Figure 40.3. The incorrect stretching of the vertices for the boot simply means that the envelopes need to be adjusted.

FIGURE 40.3

If the envelopes are off for any of the skin vertices, the skin stretches incorrectly.

Editing envelopes

When the Skin modifier is selected, the Parameters rollout includes an Edit Envelopes button that places you in a special mode that lets you edit the envelope for the selected bone in the bone list. This mode is also enabled by selecting the Envelope subobject under the Skin modifier at the top of the Modifier Stack.

When the Edit Envelopes