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There are many different types of modeling technologies available to the 3D artist today. There are six common types of model data that can be used to generate a 3D model on a computer, and each of these offer distinct advantages and disadvantages to the 3D artist, regardless of their chosen discipline. This short series of articles will examine each of these types individually. You can find the previous article on Polygons here.

 

NURBs

Image credit: Wikipedia

A NURBs curve in action. Image credit: Wikipedia

NURB, or Non-Uniform Rational B-Splines, are vector based curves (NURBs) that are used to create and define surfaces. Workflows typically involve creating these splines and building surfaces from them using tools such as loft, sweep, and revolve, then joining these surfaces together to create solid bodies.

Sony Cybershot DSC-V1, done in Moment of Inspiration,  by Son Kim

Sony Cybershot DSC-V1, done in Moment of Inspiration, by Son Kim

NURBs, and NURBs derived surfaces have been in use for a long time in the industrial, engineering, and CAD design industries, and prior to the introduction of subdivision surfaces, were also widely used in the VFX animation world. Though they have mostly been phased out, in other studios, Dreamworks does still use NURBs in their pipeline.

A NURBs model of a BMW car. Image credit Benjamin Baron

A NURBs model of a BMW car. Image credit Benjamin Baron

They are extremely useful in the product and car design industries because of the precise control one has over surface continuity; designers are able to maintain control over highlights by specifying surface continuity, or curvature.

There are five types of NURB continuity, from G0 to G4: position, tangential, curvature, curvature and acceleration, and curvature/acceleration in three dimensions. For a more detailed explanation of surface curvatures, you can read here. What this means is that a designer can specify has precise control over how two separate surfaces blend together; from G0, where the fillet is obviously visible as a seperate surface, to G3 and G4, where the three surfaces look like a single unit, with no obvious differentiation.

NURBs allow for arbitrary holes and easy joining of surfaces.

NURBs allow for arbitrary holes and easy joining of surfaces.

For a modeler, NURBs surfaces allow for freedom from polyflows and n-gons, though it still does require some planning to make a proper model. However, unlike poly or subdivision surfaces, a NURB surface has no problem being punched through with an arbitrary hole, or being split by a surface and filleted together. Due to their spline-based nature, they are also excellent for rebuilding models from 2D drawings. With NURBs surfaces you can expect exacting control over surface curvature and part dimensions.

nurblogos-1024x147

 

Popular dedicated NURBs surfacing programs are Rhino and Alias, however programs like SolidWorks and Inventor also include surface editing functionality. Moment of Inspiration (MoI) offers a low-cost NURBs surfacing solution, and Maya/Cinema 4D both offer a relatively limited set of NURBs tools (Cinema 4D is mainly a polygon modeler, similar to Maya).

Below is a sped up video of what it’s like to model with NURBs using Rhino 5.

Pros:

 

  • Precise control over surface continuity and blending
  • No concerns for polyflow
  • Allows for free-form surface creation via manipulation of control points and knots

 

Cons:

 

  • More difficult than Subdivsion surfaces to create and animate organic forms
  • Workflow, while faster than solid modeling, is still slow compared to poly or subdiv modeling
  • Does not require models to be watertight or even physically producible.
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