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'Maya 2008 Character Modeling and Animation': NURBS Curve Editing

In this fourth of six excerpts from Maya 2008 Character Modeling and Animation, author Tereza Flaxman explains how to edit and rebuild NURBS curves.

All images from Maya 2008 Character Modeling and Animation: Principles and Practices by Tereza Flaxman.

VFXWorld continues excerpting a new series from the Thomson Course Technology book Maya 2008 Character Modeling and Animation. VFXWorld readers will take on the challenge of animating realistic and compelling characters using the latest version of Maya, the powerful 3D graphics and modeling software. Excerpts will show beginning to intermediate animation students and enthusiasts how to create professional quality characters, explaining the full character animation process from pre-production to final full body and facial animation.

In the prior excerpt, we discussed how to create NURBS curves, which are the basis of NURBS modeling. This excerpt shows how to edit NURBS curves, including attaching, snapping, cutting and rebuilding curve segments.

NURBS curves have a distinct beginning and end. The first CV is represented by a square box, and the second one is represented by the letter U, indicating the curve direction, as shown in Figure 1. Curve directions become very important when using curves to generate surface geometry, as will be described momentarily.

Attaching, Snapping and Detaching Curves

To conveniently create the profile of a complex object, it is sometimes necessary to use multiple NURBS curves. Fortunately, NURBS curves can be attached and detached as necessary.

[Figure 1] Letter U indicating the curve direction.

1. Press Ctrl+N to create a new scene.2. Press F4 to go to the Surfaces menus.3. Select Create > CV Curve Tool.4. In Front view, click eight times anywhere on the screen to create a curve as shown in Figure 2.5. Right-click the curve and select Edit Points.6. Click the third EP to select it, as shown in Figure 3.7. Select Edit Curves > Detach Curves Option Box.8. In the option box, make sure Keep Original is not checked.9. Click the Detach button. Notice that you have two curves now, as shown in Figure 4.

[Figure 2] CV curve created with CV Curve tool (l) and [Figure 3] Curves' third EP. 

Attach Curves

To attach curves, follow these steps.

1. Hold down the Shift key and click both curves to select them.2. Select Edit Curves > Attach Curves Option Box.3. In the Attach Curves Option window make sure Attach Method Blen is checked and Keep Originals is unchecked, as shown in Figure 5.4. Click the Attach button. You should have only one curve now.

Cut Curve

NURBS curves can also be cut at the intersection of two curves.

1. Select File > New Scene.2. Select Create > CV Curve Tool. In Front view, click approximately in the middle of the screen eight times and press Enter to create a horizontal curve.3. Press G to go to the last tool, which in this case is the Create CVCurve tool.4. Click four times from top to bottom and press Enter. You should have curve1 horizontally and curve2 vertically, as shown in Figure 6.5. Press W to change to the Move tool.6. Right-click curve2 and select Control Vertex.7. Click the last CV of curve2.8. Hold down the C key. Notice that the Move tool’s yellow square changes to a circle. That indicates that you are in the Snap on Curve mode.9. Holding down the C key, middle-click-drag approximately in the middle of curve1. The selected CV should snap to curve2, as shown in Figure 7.10. Deselect both curves.11. Click curve1 and Shift-click curve2.12. Select Edit Curves > Cut Curve. Notice that curve1 was cut at the intersection with curve2 (see Figure 8).13. Select and delete the left part of curve1.

[Figure 4] Curve detached (l). [Figure 5] The Attach Curves Options window. 

Rebuild Curves

NURBS curves can also be rebuilt. The Rebuild Curve function is useful to match the topology of another curve, increase or decrease spans or simply to make the spans uniform.

1. Select File > New Scene.2. Select Create > CV Curve Tool.3. In Front view, click eight times approximately in the middle of the screen and press Enter.4. With the curve still selected, right-click the curve and select curve1. The Attribute Editor should open.5. In the Attribute Editor, open the NURBS Curve History tab. Look at the Spans field, and notice that the curve has five spans.6. With the curve still selected, select Edit Curves > Rebuild Curve Option Box.7. In the options window, select Edit > Reset Setting.8. In the Number of Spans field, type 10. This means the curve will be rebuilt with 10 spans, as shown in Figure 9.9. Click the Rebuild button. Look again in the curve Attribute Editor and notice that the spans are set to 10.10. Right-click the curve and select Edit Points. Notice that the spans are uniformly sized.

NURBS curves do not normally render in Maya. Usually, they are used as construction curves in creating renderable NURBS geometry, commonly known as patches.

[Figure 6] Two CV curves (l). [Figure 7] Curve snapped. 


A NURBS patch is a piece of geometry created with two or more curves. A NURBS patch has all of the elements of the component curves used to create it. It also has a new component known as an isoparm. An isoparm is a line connecting two or more editing points. Isoparms allow you to visualize a NURBS surface, even when rendered interactively in a wireframe model. The number of isoparms on a surface can be adjusted to show more or less surface detail.

When you create a NURBS patch, Maya automatically establishes a 2D coordinate system on that surface. The coordinate system has two directions, called U and V. These U and V coordinates are used to texture map surfaces.

Curve editing is an essential technique in creating organic shapes in Maya. Once you know how to cleanly create and connect curves, you can begin to sketch the outlines of complex forms. The next excerpt will teach you two modeling tools, which create smooth surfaces from NURBS curves.

[Figure 8] CV curve cut (l). [Figure 9] The Rebuild Curve Options window. 

Each chapter of Maya 2008 Character Modeling and Animation: Principles and Practices begins by exploring the "why" behind the techniques being presented, followed by step-by-step tutorials to apply your new knowledge. Following a series of hands-on projects, you'll learn how to model, rig and animate, building a comprehensive skill-set as you move progressively through each chapter. Check back to VFXWorld frequently to read new excerpts.

Maya 2008 Character Modeling and Animation: Principles and Practices by Tereza Flaxman. Boston, MA: Thomson Course Technology, 2008. 500 pages with illustrations and CD. ISBN 13: 978-1-58450-556-3; ISBN 10: 1-58450-556-7 ($44.99).

Tereza Flaxman teaches 3D modeling and animation at the Harvard Extension School and Northeastern University. She has been teaching animation for the past seven years at both undergraduate and graduate levels and is a Certified Alias Maya Instructor. Additionally, she has taught at both the School of Film and Animation at the Rochester Institute of Technology (RIT) and at the State University of New York (SUNY). She also works as a freelance animator. Flaxman has more than 15 years of experience with high-end 3D animation software and has used Maya since version 1.0. Her work has been published in several books and magazines and exhibited in shows throughout the U.S. She has an MFA in computer animation from the School of Visual Arts in New York City and a BFA from the University of Oregon.