It is a technical fact that it is not possible to convert from any geometry representation to another geometry representation without loss of data. The concept of “geometric roundtrip” refers to the ability to move between software that support exactly the same geometric representation. Even if one software supports slightly less information, information is lost. This is very hard, as even between the BRep world, different 3D modeling packages can do different tricks with BReps, such as their implementation of a Catmull-clark vs other types of subdivision. A round trip, or DTV, is not to do with converting between representations.
However, and this is the point of the thread: it is a technical opinion, not a fact, that one form of geometric representation is always better than the others. For example, CSG is objectively better for representing the majority of steel members, as inherently the STEP natively represents the manufacturing technique. Equally objectively, if the steel member is a rectangular plate, then a mesh representation does not have any loss of geometric information (there is loss of the semantics of the plate orientation, based on the extrusion direction, though).
On the flip side, meshes, and particularly mesh subdivision surfaces are objectively better at rapid geometry creation during concept phase and design development, sculptural landscape forms, heritage designs, 3D printable designs, scan to BIM, furniture representation. In fact, one may argue that tesselation can be far more advanced than CSG modeling, which is relatively basic parametrically, as tesselations support a far greater number of interesting mesh algorithms (Catmull-clark, un-subdivision, solidification, wave deformations, wireframing, skinning, different triangulation tesselations, …) which artists use to create designs, particles, weights, and vertex information, all of which affect the shape.
The proof of this difference is shown when you see certain fabricators prefer CSG, and certain architects prefer mesh modeling. I have experienced many times when a fabricator needs to remodel a mesh in CSG format because a mesh is not appropriate for their fabrication. Equally, I have experienced many times when a CSG representation is outdated in a BIM model and only used as a placeholder because the real design is done in meshes (furniture is a big example of this).
In short, if you didn’t feel like reading the explanations:
- Design transfer view, or round tripping can only be achieved if each software supports exactly the same representation. It is technically impossible to prevent loss of data if there is a representation conversion
- CSG is better for some things, and meshes are better for other things.
- CSG is not more “advanced” than meshes. Both have a lot of tricks they can do in their own domain. The worldview that meshes are more basic and unworkable is by those inexperienced with the CG industry.
- A DTV MVD that mandates CSG for all things is therefore wrong. Users should be allowed to specify which they expect for which items.
- A practical solution to the round-trip problem is that if a vendor imports a representation they do not support, they should still represent it visually, but if it is not edited, upon export, the original representation must be maintained. Equally, if the user chooses to modify it, the vendor must notify that it is a lossy edit, and provide a function to launch another program that is capable of handling the representation natively.
I have renamed the topic to my original intention for this thread.