3D Model Representation

By Shaun Bryant

A 3D model is represented either as a full solid or a shell of a solid. Imagine an old-fashioned wooden toy block as compared to a hollow Lego™ brick. Pretty much all 3D models fall into one of two categories:

  • Solid: These models define the volume of the object or entity they represent (like a cube, for example). Solid models are often used for engineering and medical applications and are usually built with constructive solid geometry. In this book, I show you how Tinkercad utilizes solids to make your life easier as you design.
  • Shell/boundary: These models represent the surface of an object or entity. The boundary of the object is a bit like an eggshell and forms the object’s shell, which is infinitesimally thin. Almost all visual models used in games and film are shell models, with surface properties applied.

Solid and shell modeling can create functionally identical objects, such as the Utah teapot, which is one of the most common models used in 3D graphics education.

Credit: Dhatfield/CC BY-SA 3.0.
A modern rendering of the iconic Utah teapot model developed by Martin Newell (1975).

The differences between solid and shell modeling are the different methods in which they’re created and edited in the various 3D modelers that are used, along with differing conventions of use in various fields.

Another difference is in the types of approximations between the model and reality, such as units of measurement and how the solids, shells, and boundaries are represented.