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.
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| 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.
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| 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.
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