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Mastering
3D Studio MAX R3 |
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| NOTE Isoparametric
lines (also known as isoparms or iso lines) are contour lines of a
surface along the U or V axis. These are lines whose parameter is
constant in the NURBS math. They are similar in concept to topographic
contour lines that represent a constant elevation in a landscape.
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When using the NURBS sub-object creation tools, keep in mind that, except
for the two buttons for creating independent NURBS sub-objects, all the
curves and surfaces are dependent on their parent sub-objects until you
make them independent. You can make a curve or surface independent by
clicking the Make Independent button on the Modify tab of any dependent
sub-object.
Converting a Mesh Object to NURBS
Lets try out some of the NURBS tools. First lets see what
we can do with a mesh primitive.
- 1. Reset MAX.
- 2. Create a cylinder in the perspective viewport.
- 3. Click the Edit Stack button and choose NURBS
from the drop-down menu. Open the NURBS Creation Toolbox if it didnt
already open automatically. Our cylinder is now a NURBS object with
NURBS surface sub-objects.
- 4. Click the Sub-Object button and choose Surface
from the drop-down list.
- 5. In the perspective viewport, select the surface
going around the cylinder in wireframe view.
- 6. Rotate the view of the perspective viewport to
see the top of the cylinder and select the top. Then do the same with
the bottom. You can see three separate NURBS surfaces within our one
object.
Using a Surface to Make Curves
Now lets look at those iso lines or iso curves mentioned earlier.
- 1. Click the Create U Iso Curve button and then
click somewhere on the first surface we looked at (going around the
side of the cylinder). A curve is created going up the side of the cylinder.
- 2. Click several places around the cylinder, making
U iso lines up and down the sides.
- 3. Click the Create V Iso Curve button, then click
the side surface of the cylinder. This creates a circle around the cylinder.
Your iso lines should look something like Figure 5.31.
- 4. Now we can use our iso curves for shaping and
editing a different surface, so we no longer need our original cylinder.
While still in the Surface Sub-Object level, marquee-select the whole
object and press the Delete key. The original surfaces are gone, leaving
us with a set of curves.
Shaping Forms with Curves
As with a spline cage in Surface Tools, we can think of a NURBS surface
in terms of the NURBS curves that can be used to define it. Lets
change the shape we will define with our curves by editing the curves.
- 1. Select Curve from the Sub-Object drop-down list,
move the curve at the bottom of the cylinder and the V iso line to the
left, and move the three U iso lines to the right.
FIGURE
5.31 U and V iso lines around a cylinder
- 2. Select Curve CV from the Sub-Object drop-down
list.
- 3. Scroll down to turn on Soft Selection and turn
up the falloff to about 40.
- 4. Move the curve CVs around to warp the curves
into new shapes.
Using Curves to Make Surfaces
Now lets use our new curves to create new surfaces.
- 1. Click the U Loft button and click the three U
iso lines in order. A surface is formed. Right-click to finish.
- 2. Click the U Loft button again, click the two
V iso lines, and then right-click. You can only see the side facing
you.
| 3. Open the Material
Editor, check 2-Sided, and click the Assign Material to Selection
button. Your surfaces should now be shaded on both sides, as shown
in Figure 5.32.
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FIGURE
5.32 U Lofted surfaces
Making a Surface Independent
As mentioned earlier, all surfaces and curves created from other NURBS
sub-objects are dependent until made independent. Lets see what
that means.
- 1. Choose Curve from the Sub-Object drop-down list.
- 2. Select one of the curves and move it. Notice
that, unless youre sitting at a very heavy-duty machine, the screen
doesnt redraw right away. NURBS use a lot of computer resources.
Notice that the surface changed along with its parent curve.
- 3. Choose Surface from the Sub-Object drop-down
list.
- 4. Select the surface that you just changed by moving
a curve, and click the Make Independent button.
- 5. Now choose Curve from the Sub-Object list and
move the same curve that you moved before. This time the surface doesnt
change with it.
- 6. Marquee-select all the curves in the object and
press Delete. Only the independent surface remains.
Understanding the NURBS Method
Obviously, if we really wanted to create the particular surfaces of the
above exercises, we could have just drawn a couple curves and U Lofted
it directly. In this case, we were getting the feel of using meshes to
create surfaces, surfaces to create curves, and curves to create new surfaces.
These are the methods you will use over and over in NURBS.
Most of the work you will do in NURBS will be on sub-objects within a
single NURBS object. In any given sub-object level, you can click the
Plug-in Keyboard Shortcut toggle and use the H key to pull up a Select
Sub-Objects dialog box for the sub-objects of that type in your NURBS
object. Figure 5.33 shows an example of this dialog box, in this case
showing the sub-objects at the surface level. You can also rename these
sub-objects in order to organize your work.
FIGURE
5.33 Select Sub-Objects dialog box
Drawing a Curve on Surface (COS)
Now we want to draw a curve on our surface so we can trim it. Click the
Create CV Curve on Surface button and draw out a closed shape on the outside
surface. If you want sharp points, click three times in the same place.
This creates three coincident CVs, which causes the curve controlled by
those CVs to have a sharper corner than is generally possible with one
CV. An example is shown in Figure 5.34.
Trimming a Surface
You can trim a curve with any curve on a surface, including, for example,
a Normal Projected curve, which projects a curve along a sub-objects
normal onto the surface. In our example, select the curve that you drew
on the surface, scroll down the Modify tab, and check Trim. You are left
with just the surface within the trim. Check Flip Trim, and you have a
hole in the surface outlined by your curve.
FIGURE
5.34 A Curve on Surface (COS)
One thing to keep in mind about trims in NURBS is that they are even
more illusion than the magic of computer graphics in general. The surface
of the hole is still there; its just not displayed.
You can even move the CVs of the surface within the hole.
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| NOTE The principle
of a calculated geometry being selectively displayed and rendered
is also used in the Boolean operations we looked at in Chapter 4.
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Subdivision Surface Modeling
with NURMS
After these journeys into the exotic math of patches and NURBS, we come
back to simple polygons. NURMS is a new feature of MAX R3 that gives some
of the desirable features of NURBS modeling with the low overhead of polygons.
Weve mentioned subdivision surface modeling several times as a
method of organic modeling that outputs polygonal meshes. Subdivision
surface means that the polygons of a model are subdivided and smoothed
in order to make organic curves. In MAX, a surface is subdivided and smoothed
simply by applying the MeshSmooth modifier discussed in Chapter 4. This
modifier subdivides the polygons of low-polygon-count (or low-poly)
models to smooth and round them out into higher-poly versions.
© 2000, Frol (selection,
edition, publication)
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