Buzz’s Blog: On Web 3.0 and the Semantic Web

Oct 31 2010   12:58AM GMT

3D modeling: Straight or curved lines? Surfaces or solids?

Roger King Roger King Profile: Roger King

This blog is dedicated to emerging Web and media technology.  In this posting, we’re going to look at two critical distinctions between two kinds of 3D modeling.

(By the way, I teach 3D animation at the University of Colorado – feel free to download my class videos at:

3D modeling: it’s pervasive.

Powerful 3D modeling tools are becoming ubiquitous.  They are used by artists, product designers, engineers, and scientists.  In fact, a wide and growing spectrum of professions demand some knowledge of 3D modeling.

The first distinction: two kinds of 3D modeling geometry.

3D modeling applications use straight and/or curved line geometry.  In other words, if you look at the outside of a 3D object, like a human or a boat or a caterpillar, it’s made out of things that are not at all 3 dimensional.

There are two basic ways this is done.  The first is with straight line geometry, whereby the surface of an object is made up of flat, 2D polygons.  If you make the polygons small enough, in other words, if you increase the number of polygons that make up the surface of an object, it looks smooth.

The other approach is to use curved line geometry, whereby the surface of an object is defined by taking curved lines and sweeping them through 3-space to make up curved surfaces.  Polygon modeling calls for less computations by the 3D application, but isn’t as smooth.  Surfaces made by curved lines create a more complex computational job for the 3D modeling application, but they are smoother.

Here are examples of both, using Autodesk Maya, an application very popular among artists who create animated movies:

The second important distinction: surface vs. solid modeling.

(Interestingly, all 3D objects, whether they are polygon based or curved line based -or both – have to be displayed, not as lines or polygons or surfaces, but as pixels on a flat 2D screen.  Think about it.  We’ll get back to this in a future blog posting.)

But right now, let’s look at a critical, high level distinction.  With Maya, which is used largely by artists, objects have no “guts”.  They are hollow.  In fact, they aren’t really 3D at all, because their surfaces have no thickness.  They are made up of shells that are infinitely thin. That’s just fine for the artist trying to create an animated character or an entire animated world.

Here are the same two spheres, but with infinitely thin surfaces put on them:

But other 3D users, like engineers, product designers, and scientists, need to model objects with guts.  Their objects have to have insides.  This is so that things like the overall weight of a product being built can be calculated.  It also allows the modeler to estimate the distribution of weight, flexibility, cost of production, strength of materials, and many other factors.  Medical researchers will use solid modelers, as well, to represent tissue and blood flow and the like.

So, if you slice into Manny the Mad Bunny, there’s nothing in there.  But if you slice into a car battery there will be something in there.  

We tend to associate surface modeling with entertainment, and we associate solid modeling with engineering and science.

Popular surface modeling and animation applications are Autodesk Maya, Newtek Lightwave, and the free Blender.  Popular solid modelers are AutoCAD, SolidWorks, and AutoDesSys Bonzai.  By the way, solid modelers tend to also support surfaces modeling.

We’ll look more closely at other issues relating to 3D modeling and animation next…

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  • dwcolli1

    I need to model 3D objects with guts. I need to apply a thickness to my 3D Surface Models made of triangles that form the surface. I create the 3D Geometric Solid Models for 3D Printing. I need to apply a thickness to reduce the amount of Plastic Powder used by the 3D Printer. Can you help me.


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