Comparing CSG and B-rep Methods

To determine if the B-rep model of the handle was actually smaller than models created using the CSG method, a sample handle was created using both of these methods. The CSG method produced a model that used 50% more disk space than the B-rep method handle. It was also more difficult to construct, and required more constraint equations and variables. Clearly, the B-rep method was indeed the best choice for this model.

Next, a similar study was conducted for the connector model. Again, the same model was created using CSG and B-rep methods. The CSG method did have to use a profile and circular sweep to construct the countersink on the bottom of the connector. For this set of models, the CSG method had a 10% smaller file size, and was easier to construct. This is most likely due to the complex profile that had to be created for the B-rep method. By constructing this part as a series of co-axial cylinders that all start at the same point, but have different heights and diameters, the process was significantly simpler than the original design. The original design for the CSG model used cylinders stacked on top of each other, and the inside part of the connector was not hollow. The decision to create a hollow center in the connector complicated the B-rep method significantly, but simplified the CSG method. Due to this change, the connector model would probably have been better created using the CSG method from this test.

The third family was simply a profile, so those parts could not be made with CSG methods. For the fourth family, the main body of the part was again constructed with B-rep and a combination of B-rep and CSG methods. Again, the method that used CSG proved to be a better choice. A CSG box primitive of the proper size was inserted. Then, a B-rep rotate face operation was performed on two of the faces. The pure B-rep method required that the cplane first be changed from the default top cplane to the right cplane. The profile had to be inserted in two parts and then unioned. The profile was extruded, and the cplane changed back to the top cplane. This required six commands, and multiple attempts before the profile could be constructed correctly. The B-rep method also resulted in a 10% larger disk file.

Last Modified: Wed Aug 28 14:41:29 EDT 1996