converting mesh to brep....painful!
Accepted solution
@Anonymous - this is a topic which comes up over and over again on the Fusion forum. It's worth a bit of discussion about some various types of models in the 3D world. There are really two main types of representation of 3D solids that are popular today (with a few other types that are less common, but exhibit aspects of one of the main types, so we'll stick to those two for now). Those model types are "mesh" and "boundary representation" or "BRep" for short.
Let's look at those two representations:
- A "mesh" is usually understood to be a faceted representation of a solid. The facets usually consist of 3 or 4 sided polygons. Triangle meshes (3 sided) are the most common, because a triangle is always planar, so the math needed to deal with them is simpler. A faceted representation is always an approximation of the actual shape of the solid. The smaller the facets, the more accurate is the representation (but the larger the data size). This type of representation is used as kind of a least-common-denominator representation of a solid. All graphics display systems use a triangle meshes as their representation. 3D printers, also, speak in meshes. In fact, the STL format was invented for the very first 3D printers, called STereo Lithography printers (hence "STL" as the format type). There are 3D design systems which create and edit meshes as their native geometry type. Meshmixer and 3DS Max are two of those, but there are many others. Those types of products are good for certain kinds of designs (characters for video games or movies, highly organic shapes, etc).
- "BRep". BRep uses mathematical representations of the outer surfaces of a body, instead of facets. This allows a much more compact and accurate representation of the solid. For instance, to represent a sphere, all you need to record is the center and radius of the sphere. A plane can be represented with a point and a 3D vector describing the orientation of the plane in space. Of course, it's more complicated than that, because you have to describe how different faces relate to other faces, how the planes or cylinders are trimmed, etc. But, the key here is that this representation can be exact down to the precision of the computer hardware in use. Some operations that BRep is good at include things like creating a fillet along an edge of the solid. In BRep, you can make use of the mathematical properties of the geometry. For instance, you can create a work axis along the axis of a cylinder, because the system understands that cylinder. You cannot do such things in a mesh representation, because that cylinder is just a collection of triangles.
The key here is that each representation is good for some things, not good for others. Generally, BRep is good for representing things for which precision is necessary, like the parts of a machine. I would not want to drive in a car in which the engine had been designed using a mesh modeler 
On the other hand, I would not want to model a monster for a movie using BRep.
Conversions between these two types. This is the crux of your question. Today, there is a 100% reliable conversion from BRep to mesh. If there were not, you would not be able to see a graphical representation of your design on the screen, or 3D print it. However, there is not a good, 100% reliable conversion between mesh and BRep. Think about the problems here: Trying to guess from a set of triangles that this is a cylinder is a very hard problem, mathematically. There is some interesting work going on here: automatic-conversion-3d-models-csg-trees. Someday we may be able to do that, but right now, I am unaware of any system which can do this reliably. Fusion does have the "Mesh to BRep" command, but this is a very dumb conversion. Each triangle is converted into a separate BRep face. This conversion does not produce any of the benefits of a BRep representation. Cylinders are not cylinders afterward. In fact, this conversion produces the worst of both worlds - the representation is inaccurate, and the model is very heavy. I wish we had never introduced this command into Fusion at all, to be honest. It is misleading to customers who are new to CAD, and produces bad results.
Fusion does support editing mesh models, but to a very limited extent. The mesh workspace was created with a few goals in mind: using meshes as visual reference (think about a mesh model of a room, and you want to design a chair for that room), and for reverse engineering a BRep from a mesh. That is, if you have a mesh representation of an object that has some geometric properties (planar and cylindrical faces, etc), using the Mesh Section command and Fit Curves to Mesh Section commands, help the user to construct a precise representation of that object. But, this is far from an automatic conversion. It is highly manual.
Anyway, I hope that this overly complex explanation sheds some light on the nature of meshes and BReps, and helps to set correct expectations about the conversions, and hopes for editing meshes in Fusion. In general, @TheCADWhisperer is right - don't expect to download a design from Thingiverse and expect to make meaningful edits to it in Fusion.
Jeff Strater
Engineering Director
@Anonymous - that is an excellent analogy, thanks for adding it. It does really capture the essence of the difference between these two representations.
Jeff Strater
Engineering Director
@jeff_strater wrote:
Anyway, I hope that this overly complex explanation sheds some light on the nature of meshes and BReps, and helps to set correct expectations about the conversions, and hopes for editing meshes in Fusion.
This isn't at all an overly complex explanation and it is very necessary!
A lot of today's hobbyists, makers, artists and even degreed engineers work with very powerful tools, but In my experience less than 10% really have a good understanding what is going on under the hood of their tools.
If you are again in such a typing mood, you might want to simply consider adding these lines to the Google docs document about meshes I invited you to. Maybe we can motivate each other to make that a more complete document and something that is worth referring new users to 😉
@Anonymous I think the purpose of @jeff_strater's post was to explain why what you are trying to do is not a good idea.
You may proceed at your own risk 😉
