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'Maya 2008 Character Modeling and Animation': NURBS Curves

In this third of six excerpts from Maya 2008 Character Modeling and Animation, author Tereza Flaxman provides an introduction to the two basic methods for creating and controlling NURBS curves in Maya.

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.

NURBS curves are commonly used in modeling organic shapes. They are very powerful and flexible, but also complex. This excerpt from Chapter 5 describes the types of curve creation tools available in Maya, and explains some of the relatively obscure terminology needed to understand NURBS modeling.

NURBS stands for non-uniform rational B spline. Fortunately, you don't need to understand the name to take advantage of this highly useful modeling method. NURBS modeling allows you to specify smooth curves using a small number of controls. These curves can be used to build up complex organic surfaces. NURBS is commonly used for modeling cars, animals, and human bodies. Because NURBS retains detail when scaled up, it is often used in motion picture production. In this chapter, you will use Maya's NURBS tools to model a fish. You will then learn two new animation techniques to animate the fish: path animation and dynamics. In path animation, you first draw a curve representing a desired movement over time and then attach an object or character to the path. The process works much like direct keyframing, except that all in-between positions are constrained to the path.

Dynamics is Maya's built-in module that simulates real-world physics. Dynamic simulations are very useful for secondary motion (motion that is a consequence of the direct intentional movements of a character). In this case, you will use dynamics to simulate the motion of the fish's fins and tail through the water. A second important use of dynamics is to connect a character to the surrounding environment. Here, you will use dynamics to simulate water movement, including the wake and splashing "caused" by the fish.

NURBS Curves

NURBS curves form the basis upon which NURBS surfaces are built.

NURBS Terminology

NURBS modeling introduces a number of technical terms that may not be familiar to you. Most of these elements have a distinct visual representation in Maya's interface. A single continuous NURBS curve is usually composed of multiple segments, known as spans.

NURBS curves contain two kinds of points: edit points and control vertices.

Edit points (EPs). EPs are markers on the NURBS curve itself that indicate the extent of a span. In Maya, an edit point is represented by an x on the curve.

Control vertices (CVs). CVs are points that control the position of the curve between the EPs. CVs are not necessarily on the curve itself.

[Figure 1] The Preferences window.

NURBS Display Preferences

Depending on your preferences, Maya can automatically display some or all of the components discussed here as you draw your NURBS curves. One additional component that you may find useful is the hull. Hulls are straight lines that connect CV points. They can't be individually manipulated, but they do make it easier to differentiate between CVs and EPs.

To ensure that you have all components displayed, bring up Maya's main preferences window by following these steps.

1. Select Window > Setting/Preferences > Preferences.2. Under the Categories section on the left, select Display > NURBS.3. In the NURBS, Display Settings for New NURBS Objects section, find the New Curves entry.4. Select the checkboxes for Edit Points, Hulls and CVs (see Figure 1).

NURBS Curve Tools

Maya provides two separate tools for creating NURBS curves: the Edit Point (EP) Curve tool and the Control Vertex (CV) Curve tool. Both tools create NURBS curves with equivalent properties containing both EPs and CVs. However, with the EP Curve tool, you place edit points, and Maya automatically creates CVs. With the CV Curve tool, you directly place the CVs, and Maya automatically creates EPs. The difference between the two usually comes down to personal preference. The EP Curve tool is more straightforward to use, but the CV Curve tool gives slightly more interactive control over the curve shape.

[Figure 2] EP Curve tool clicking positions (left). [Figure 3] EP and CV NURBS Curve tools (right). 

Try experimenting with both tools.

  • 1. Make sure Edit Points, Hulls and CVs are checked in the Preferences window.2. Select Create > EP Curve Tool.3. Hold down the X key to snap to your grid. 4. Click alternating grid corners one grid cell apart, as shown in Figure 2. When done, press Enter to complete the curve. The result should be a continuous curve that goes through each location clicked.5. Repeat the same pattern of click locations but use the CV Curve tool (also under the Create menu). In this case, you will also get a continuous curve, but with a distinctly different shape. Note that the overall curve is much flatter, undulating up and down toward the clicked CV locations but never reaching them (see Figure 3).

[Figure 4] Summary of NURBS components.

A Matter of Degree

As you may have noted in the NURBS curve creation tool options, NURBS curves have an important property known as the "degree" of the curve. First-degree curves are linear and thus run flat between EPs. Maya defaults to creating third-degree curves, which is usually what you'll want. You can also create curves of higher degree. This has the advantage of allowing twistier interpolation between EPs but the significant disadvantage that it becomes harder to get local control of the curve.

Summary of NURBS Components

In summary, here are the basic components of NURBS curves.

  • Spans. Segments of a NURBS curve bounded by EPs.

  • Edit points (EPs). Points on a NURBS curve.

  • Control vertices (CVs). Points not on the NURBS curve itself but that control the curvature between edit points.

  • Hulls. Display lines that connect the CVs (see Figure 4).

Understanding curve parameterization is essential for drawing the construction curves needed to model NURBS characters. In the next excerpt you will learn how to edit and work with these curves.

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.