Last time, we focused on what most would agree to be the single characterizing feature of 3D Studio MAX--the modifier stack. We barely broke the surface of this deep subject, but enough was said to introduce the reader to the underlying concept.

In this lesson, we turn to Lightwave 3D to consider its most distinguishing feature--Metanurbs modeling. Once again, we cannot hope to do more than introduce this profound tool. But, as with the last lesson, this introduction should be enough to grasp the essence of the concept.

Metanurbs modeling in Lightwave is best understood against the background of spline modeling generally. A spline is, for all practical purposes, simply a curve. The word "spline" entered 3D computer graphics from engineering, where it originally referred to a thin strip of metal that could be pinned at points to assume a curved shape. The term subsequently came to mean the types of curves that could be mathematically defined, and therefore could be utilized in computer graphics applications.

Splines follow an important hierarchy. At the bottom is the "natural spline." A natural spline is essentially the abstraction of that physical metal strip we just described. A natural spline is defined by certain control points, and these points are on the curve itself. By moving around these control points, we shape the curve. Control over natural splines is the most limited of all classes of curves, and more subtle curvature can be obtained only by adding more control points to the spline. Lightwave supports only natural splines at present.

Immediately above the natural spline is the familiar Bezier spline. This is the same Bezier curve found in all 2-D drawing programs. The Bezier spline is much more flexible than a natural spline because, in addition to the control points that actually sit on the curve ("interpolate" the curve), there are control points that sit off of the curve that can be moved about to define the curvature. These latter points are often visually connected to the interpolating points by lines to create the common Bezier "handles" used to shape the curve.

The highest class of splines is called B-splines and, following the progression, its control points need not be on the curve at all. As they do not interpolate the curve, they appear to be a rather magical form of control, manipulating the shape of the curve like a puppeteer works a marionette. B-splines are capable of much more subtle curvature than either Bezier or natural splines and subtle curvature is important. The phrase "organic modeling" is much tossed around and much abused. At a minimum, however, it refers to the creation of models having the subtle curvature of living animals and plants. Thus implementation of the higher orders of splines is essential for modeling tools seeking to provide organic modeling.

The acronym NURBS abbreviates "nonuniform rational B-spline." It is far beyond our purposes here to distinguish NURBS from other B-splines, but it is enough to understand that they are the most generally useful form of B-spline. Like other B-splines, a NURBS (yes, it's singular, not plural) is characterized by control points that do not interpolate the curve.

Spline modeling is, not surprisingly, the use of a network of curves to define the shape of the model. To take the simplest example, a spline model of a sphere could be composed of a few intersecting horizontal and vertical circles, forming lines of longitude and latitude. To get to a renderable model, this spline "cage" (as it is called) must be ultimately be converted into a faceted mesh of flat polygons, but so long as it remains composed of splines, the curvature of the model can be fluidly edited by moving the control points.

Enough with the background. What does all this have to do with Lightwave and Metanurbs?