what's the main difference between B-Rep and Parametric representation

Question

For solid modeling, I heard besides B-Rep, there is parametric representation. Is there any difference between those two?

Answer 1

A BRep is one way (among many) of representing objects in a computer. Other ways of representing objects are voxels and meshes.

The concept of parametric modeling is independent of the representation of object, although it is intimately associated with it. I think the easiest way to visualize the relationship between the two is to think of a parametric modeler as a machine that accepts two inputs:

1. The recipe for a model. An example recipe is: "a block with dimensions length, width and height, with a hole in the center of the top face of radius .25*width.
2. The values for the parameters. For the previous example the values might me length=10 cm, width=5 cm, height=3 cm. radius doesn't need to be specified because it is given by the relationship radius = .25*width.

Given the recipe and set of values for the parameters, the modeler tries to compute a model. If it succeeds, the output is a model that may be represented as a b-rep, but could alternatively be something else such as a voxel or mesh model.

Currently most commercial parametric modelers work with B-reps.

Of course much of the software to implement a parametric modeler is focused on providing tools to specify the recipe, often as a feature list (feature-based modeling), or as relationships (aka "constraints") between objects (variational modeling) or both. These tools are considered part of the parametric modeler, but the heart of the modeler is a compute operation similar to what I described.