SubD Modeling is the computer technique where simple, rectangular shapes are smoothed for display and further editing. The rectangular shapes are subdivided and approximated by smooth surfaces.

SubD Modeling is the next big thing in 3D modeling and works very well for modeling 3D printed objects. Bye bye curves, hello polygons!

Smoothing a curve in 2D

Consider a Polyline versus a Spline in 2D. If you grasp that concept, you can easily see the consequences of smoothing a 3D mesh.

We are all familiair with a typical CAD entity, the Polyline. By clicking a few points on the screen, a set of connected lines is created:	If we click the same points using a Spline tool, the points will not go though these points, but will merely get a sense of direction from the clicked points and will continue smoothly:
A 2D Polyline	A 2D Spline

So how does that work in 3D?

Now consider the same polyline, but extruded into 3D to form a set of rectangular surfaces, a mesh:	Subdivide this mesh and approximate it by surfaces, we get a shape like this:
3D Mesh	SubDivided Surface Model

If a solid shape is smoothed, we get something that could look like this:

3D SubDivided Solid Model

What are the advantages of SubD Modeling

You no longer need to define a shape using curves, surfaces and stitching the surfaces into a solid. You simply work with the rectangular shapes and push, pull, copy these around. The software will automatically smooth the shape for you.

This leads to a freedom of form that is unreachable using traditional CAD modeling techniques. That is why SubD Modeling is great for preparing 3D printed models: a 3D printer prints any shape you want!

3D Mesh Model

3D SubDivided and Smooth Model

What are the disadvantages of SubD Modeling

You tend to have less control over the shape. Where in traditional CAD you could put curves exactly where you want edges to be, using SubD Modeling you need to move polygon edges around and see what the smoothed out shapes look like.

Also, it might be more difficult to get from a smooth surfaced model to a solid shape that can be further edited in traditional CAD. SharkCAD Pro is able to convert a SubD model into a NURB Solid model, Modo offers the Power SubD-NURBS plugin to accomplish this.

Movie overview

This 4 minute movie is well explained and a great way to learn about SubD Modeling.

Thanks to The Guerrilla CG Project