Solids vs. Surface Modeling: What and why you need to know.

Computer aided design (CAD) isn’t like a car in that you can use it pretty well even if you don’t know how it works. It pays to know what happening ‘under the hood’ when using CAD. It is important to know about surface and solids modeling because it does affect the way you model, and it is important to know if you are switching platforms. It is also very important to know about for rapid prototyping.

Surfaces and solids are the underlying math that defines the geometry of the forms you create. There are three ways to define 3D geometry: solids, surfaces and wireframes. Wireframes don’t play much of a role in CAD, but primarily in digital content creation (DCC) and gaming. The easiest way to understand the difference between surface and solids modeling is to think of a water balloon; the water in the balloon would be solids modeling, while the latex skin would be surface modeling. Need more of an explanation? No problem.

Solids modeling

Solids modeling is defining an object with geometric mass. Solids modeling programs usually create models by creating a base solid and adding or subtracting from it with subsequent features. Features such as extrudes, extrude cuts, revolves, radii, chamfers, etc. Examples of solids modeling programs are Solidworks, CATIA, and ProEngineer. It was originally developed for machine design, and is used heavily for engineering with large part assemblies, digital testing and rapid prototyping.

Surface modeling

Surface modeling is defining an object’s exterior with an infinitesimally thin skin. This skin is created by lofts, sweeps, and NURBS curves - i.e. sculptured surfaces with lots of curvature. The surfaces are either defined by poles or guide curves. A surface is considered a solid only when it is completely enclosed. It is used to make technical surfaces (e.g. air plane wing) or aesthetic surfaces (e.g. car’s hood). It was developed for the aerospace and automotive industries in the late 70s. Rhinoceros 3D and Alias Studio Tools are examples of a surface modeling programs. It is generally considered more difficult than solids modeling, but the models are more robust because the programs aren’t generally feature based. Later changes have to modify the existing geometry as opposed to just editing the original feature, which is more difficult but keeps the model from collapsing when one feature interferes with another.

Hybrid Solids and Surface Modeling

To make thing more complicated, most solids modeling programs incorporate surface modeling. A solid is created by creating a surface, and then filling that surface, much like the previous water balloon example. In true hybrid modelers, the surfaces and solids faces must be able to interact with each other, such as the dimensions of the curves that define a surface referencing the dimensions of solid feature.

My experience is that the surface modeling in solids modeling packages is very temperamental; it creates geometries that are subject to failure and collapse easily. The problem is that changing one dimension ends up changing many dimensions.  For example, modeling the organic legs of an airport seating system I designed in Solidworks was 25% of the time to model the whole thing. I had to approach the surface 5 different ways before finding a way that worked. When I went to add the feet, the surface collapsed.

A superior hybrid modeler would allow you to create free form surfaces as well as sketch driven, parametric surfaces. I don’t know if we are there… yet.

You can also create a complex surface in a surface modeler, and then import it into a solids modeler. This is the workflow in automotive design.

Conclusion

While you don’t necessarily have to understand surfaces vs. solids modeling to create high fidelity renderings, animations or simulations, knowing the limitations and the strengths of both can be very powerful knowledge, and pay big dividends in time and quality.