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Tutorial: Attaching eyes to a melting snowman

When part of a model is deformed, such as applying the Melt modifier to a snowman's body, smaller parts like the eyes either get left behind or get the full weight of the modifier applied to them. If the Melt modifier weren't applied to these items, they would stay floating in the air while the rest of the snowman melted about them. This problem can be fixed with the Attachment constraint, which causes the eyes to remain attached to the snowball as it melts.

Cross-Reference

The tutorial where the Melt modifier is applied to the snowman is included in Chapter 35, “Using Animation Layers, Modifiers, and Complex Controllers.” •

To constrain the solid objects to a melting snowman, follow these steps:

1. Open the Melting snowman.max file from the Chap 22 directory on the CD.

This file includes the melting snowman file from the previous chapter with the Melt modifier applied to all objects.

2. Select the left eye object in the scene. In the Modifier Stack, select the Melt modifier and click the Remove Modifier button to throw that modifier away.

3. With the left eye still selected, select Animation⇒Constraints⇒Attachment Constraint. A connecting line appears in the active viewport. Click the top snowball to select it as the attachment object. This moves the eye object to the top of the snowball where the snowball's first face is located.

4. In the Attachment Parameters rollout, change the Face value until the eye is positioned where it should be. This should be around face 315. Then change the A and B values (or drag in the Position graph) to position the eye where it looks good.

5. Repeat Step 5 for the right eye and for any other objects in the scene that you want to attach.

6. Click the Play button (/) and notice that the snow melts, but the eye objects stay the same size.

Figure 22.1 shows the resulting melted snowman.

Surface constraint

The Surface constraint moves an object so that it is on the surface of another object. The object with Surface constraint applied to it is positioned so that its pivot point is on the surface of the target object. You can use this constraint only on certain objects, including Spheres, Cones, Cylinders, Toruses, Quad Patches, Loft objects, and NURBS objects.

FIGURE 22.1

The Attachment constraint sticks one object to the surface of another.

Caution

Because the Surface Constraint works using the parametric nature of the attached surface, only certain object types can be used with the surface constraint, including primitive objects like a sphere, cone, cylinder, patch, and NURBS objects. •

In the Surface Controller Parameters rollout is the name of the target object that was selected after the menu command. The Pick Surface button enables you to select a different surface to attach to. You also can select specific U and V Position values. Alignment options include No Alignment, Align to U, Align to V, and a Flip toggle.

Note

Don't be confused because the rollout is named Surface Controller Parameters instead of Surface Constraint Parameters. The developers at Autodesk must have missed this one. •

Tutorial: Rolling a tire down a hill with the Surface constraint

Moving a vehicle across a landscape can be a difficult procedure if you need to place every rotation and position key, but with the Surface constraint, it becomes easy. In this tutorial, you use the Surface constraint to roll a tire down a hill.

To roll a tire down a hill with the Surface constraint, follow these steps:

1. Open the Tire rolling on a hill.max file from the Chap 22 directory on the CD.

This file includes a patch grid hill and a wheel object made from primitives.

2. Create a dummy object from the Helpers category, and link the tire object to it as a child. This causes the tire to move along with the dummy object. Position the dummy object's pivot point at the bottom of the tire and the top of the hill. The pivot point