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There are many different types of modeling technologies available to the 3D artist today. There are six common types of model data that can be used to generate a 3D model on a computer, and each of these offer distinct advantages and disadvantages to the 3D artist, regardless of their chosen discipline. This short series of articles will examine each of these types individually. You can find the previous article on NURBs here, Polygons here, and Subdivision surfaces here.

B-Rep (Boundary Representation) Solids

Image credit: Andrew L DuBuc

Ion Paintball gun made in Inventor using Solids. Image credit: Andrew L DuBuc

The last of the commonly used model types, B-Rep solids are versatile, powerful, and widely used in the CAD and design markets. If you want to make something that you’re sure can be manufactured in the real world, this is the best option for you.

In this rendered image, the cube on the left demonstrates how in a B-Rep solid, model boundaries are computed via the intersection of surfaces.

In this rendered image, the cube on the left demonstrates how in a B-Rep solid, model boundaries are computed via the intersection of surfaces.

Where NURB surfaces are calculated from curves defining the edges of the surface, B-Rep models models are calculated from the intersection of infinite surfaces with each other. Models are created by determining the volume, and then creating surfaces from that volume. A solid model is thus represented by a series of connected surface elements, and additional changes are made by subtracting or adding to that solid, thus changing it’s profile, limits, or boundaries.

This might be a bit confusing, so here’s an example; if I wanted to create a coffee cup lid in a B-Rep solid, I would start by creating a solid cylinder, and cutting away at it with other boolean operations. Then I would shell the solid to give it a thickness. This ensures that the model will be watertight and physically manufacturable. The B-Rep model is determined first by creating a volume (and using the intersection of faces to define that volume); then it is modified by cutting away at it with other solids to change it’s boundaries.

Meanwhile, if I was using NURBs, I would create the cup by using different surfaces to trim each other. At the end, I would offset the outer surfaces inwards and add the lip manually. Stitching them all together would create a solid that could then be manufactured. There is no guarantee, however, that the resulting part is manufacturable; one side could poke through the other, for instance; these sorts of errors in a large part could take quite some time to fix and diagnose. B-Rep solids make sure this and other errors don’t occur.

solidlogos

B-Reps are used in many CAD programs, including SolidWorks, Inventor, and Pro-Engineer. They are especially popular as the basis for parametric modeling, where a model’s history is tracked over time and can be changed at any point, updating the rest of the model down stream.

 

Pros:

  • If you want to make sure it can be made, make it in B-Reps. They don’t let you get away with much.
  • History-based workflows are extremely powerful for later adjustments.
  • Simple boolean operations, extrusions, etc. are very easy to do
  • Very compatible with NURB surfaces

 

Cons:

  • Models can require a lot of forethought and planning to make sure the history tree works
  • Depending on the software, you don’t often get the same level of surface control that you do in NURBs
  • Can be difficult to learn how to do anything more than simple extrusions and boxy shapes
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