Using NURBS to Model

In my animation classes, one of the first exercises we do involves animating flour sacks that jump, walk, and interact with a ball. In past courses, we have used polygon and NURBS types of flour sacks; currently we use NURBs. We use a flour sack because it is simpler to animate than a biped yet more complex than a beach ball. Of course, before modeling, I tried to think about what the flour sack might have to do. I had to model for a variety of motions because the final project for the unit is to come up with a ball/flour sack interaction.

Consequently, my students need wide latitude of motion to the sack. Figure 1.6 shows some of the preparatory drawings I used in planning for these motions.

Figure 1.6: Character sketches of the flour sack

Building a flour sack is a useful first step in building a more complex character, because you can apply many of the modeling techniques to just about any character. In this instance, we stitch "hands" and "feet" onto our flour sack. I use NURBS patches for a number of reasons. First, from this finished model, you can easily tessellate the model into a polygonal model of varying detail. Merging vertices results in a single polygonal mesh. Or, you can also bind the model directly to a skeleton. Modeling with patches gives us instant, controllable smoothing; and surface attachability and detachability. Using the file floursackdismembered . m a on the CD, let's begin.

One problem with patches is aligning the surfaces properly so that they join as smoothly as possible. Stitching edges and global stitching help a lot, but that won't make a perfectly aligned "silk purse" out of these completely incongruous "sows' ears" that we have now. I use the following method:

1. Replace end caps

2. Fillet blend between patch isoparms

3. Delete history on fillets

4. Rebuild fillets geometry

5. Attach fillet geometry to appendage patches

6. Global stitch

In the steps in the following section, I start from the hotbox whenever I choose a menu command. Also, because you'll want to see the wireframe while working, choose Shading Shade Options Wireframe on Shaded if it is not already visible.

Figure 1.7: The Rebuild Surface Options dialog box

The steps in the following sections assume some knowledge of NURBS modeling.

Although we will model the flour sack in this case, these techniques apply to creating any patch model.

I created f l o u r s a c k d i s m e m b e r e d . m a by massaging

NURBS spheres into shape for the body, the "hands," and the "feet." Figure 1.7 shows the settings I use when rebuilding after detaching surfaces. I detach and rebuild immediately, because it is sometimes easy to lose track when working with a patch model. All these surfaces have been rebuilt/reparameterized and are ready for action.