What subdivision modeling is good for—and what it isn’t.
SubD modeling, or as it is also more properly known, subdivision modeling, is a way to manipulate a simple blob into almost anything, be it naturally occurring, other-worldly, human or alien, natural or man-made. It is used to make organic shapes, models of animals, human faces … objects that would be difficult, if not impossible, to make with a conventional CAD program. And it’s all done by pushing and pulling on points (vertices) on a simple shape.
SubD got its start in character generation for movies, used for making aliens and monsters, shapes for which imagination is rewarded and little accuracy is required. For shapes that require great detail or accuracy, like hair on the head or precisely parallel surfaces of a machined part, SubD would not be the best choice.
Underlying this rather simple concept of tweaking a simple part to make arbitrary shapes are some sophisticated, surface-generating mathematics. Most, if not all, commercially available SubD modelers use the Catmull-Clark algorithm, developed by Edwin Catmull and Jim Clark in 1978. Catmull was the head of Pixar, which used the algorithm to generate the animated characters in its movies. The algorithm works by subdividing (hence the name) a surface into smaller quadrilaterals to approximate the stretching that would occur in an elastic shape that is being pulled or pushed. Pixar generously made the algorithm open source (OpenSubdiv), which led to its widespread use and success in movies and gaming.
With its ability to add vertices and faces and stretch a skin over them, a SubD model appears smooth—but it can also be configured to give creases and edges where needed. Under the smooth skin is a surface mesh or faceted model. SubD has to use considerable computing to produce the type of smooth shapes that solid modelers can create quite efficiently. Solid modelers are based on non-rational uniform Bezier spline (NURBS), which are by their nature, smooth instead of faceted.
But SubD can do what would be impossible with NURBS. Start with a simple sphere and end up with a work of art, as a character artist did by creating a SubD model of Michelangelo’s La Pietà.
SubD modeling underpins creative modeling programs like 3dsMax, Maya, Blender, Cinema4D, ZBrush, Mudbox, Modo and others.
Think of SubD as a clay model with benefits. Clay models can be reduced or added to and then smoothed, something SubD does easily on the fly. Or consider it sculpting with less risk.
No chunks break off to leave jagged edges. You don’t have to start all over with
a new slab of Carrara marble.
SubD has made tremendous progress in special effects, movies and games, their characters and environments. Where organic and irregular shapes are needed, such as nature produces, from leaves on a tree to animals, SubD rules—except where fine detail in quantity is required. The more esoteric the shape, the better a candidate for SubD modeling. It takes considerable artistic skill and patience to tweak SubD programs to produce Earthly things of beauty (think La Pietà, faces and hands), but if you have to make aliens and monsters … for whom exacting aesthetic standards do not exist, SubD is perfect. Warts, other growths and antennae can be pulled out
of heads with ease—and no one can tell you that they’re not quite right.
SubD has made some inroads into mechanical and architectural design. It is available for all major CAD programs, either integrated, as part of the geometry kernel, an as add-on or a parallel modeling program. Dassault Systèmes offers Sculptor, a parallel modeling program to its SOLIDWORKS. Autodesk Fusion 360 uses SubD modeling acquired from T-Splines. NX and Solid Edge have access to SubD modeling as part of Siemens’ Convergent Modeling technology toolbox. Creo users can apply subdivision with the Freestyle command.
SubD can be handy in making curvy shapes (like the Corvette at the
beginning of the article) for which the level of detail is low, such as it would be if the car were simply in the background, or in the garage of a house that was there for flair while the house was
there for design review. SubD can also make shapes that don’t fit the straight and narrow shapes of solid modelers, such as a radome bulge on an aircraft fuselage or cushions on a sofa—shapes that require bending NURBS to your will—the last 10% of detail that
ends up taking 90% of your modeling time with a solid modeler.
Conversely, SubD modeling can be used to make irregular shapes with ease.But it has a difficult time with precise shapes—that which 3D CAD creates with ease.
A simple bracket with holes and exact dimensions would be a piece of cake with a
solid modeler but just creating a precise hole, its circumference held to tight
tolerance, would be a nightmare with SubD modeling.
With the strengths
and weaknesses of SubD and solid modeling, it’s apparent that neither is good
for all sorts of modeling and each good at one. We can only conclude that
both ought to be in your toolbox. A workflow suggests itself. If a
curvaceous shape is needed, start with SubD modeling, then bring the shape into
your solid modeling to scale, trim and drill to exact dimensions. If both tools
are all made by the same vendor, importing shapes from one to the other is all
but assured. And if your shape is created by an independent SubD modeler, you will
be happy to know that SubD to NURBS converters exist.
