Hi SurlyBird, you don't have to make things into solids but there are certain tools that are more oriented towards working on solids such as the Booleans.

Booleans are kind of like a batch process of Trim, where it cuts objects up and then decides which pieces to discard and which to keep based on where they are in relation to the volumes.

If you don't have a solid volume, then you'll probably need to use Trim rather than the Booleans, and you'll have to do a few more manual steps like manually picking which pieces of surfaces you want to discard rather than it happening by volume.

Additionally with Solids MoI is able to automatically understand which side of the object should be the "outside" of it, and will orient normals towards that direction automatically. But you may not necessarily care about the orientation of your exports if you are using double-sided materials when rendering.

So working with solids just kind of makes some things happen more automatically and with fewer manual steps, but it doesn't mean you can't get the job done without doing solids. (well, it does also depend on what your final result needs to be, if you want to produce a physical model rather than a rendering then often times a solid is needed)

But you can gain a fair amount of speed by working solids when possible rather than trying to work at more of a surface level all the time.

When you export to a mesh format, you should make sure to join surfaces to one another so they have shared edges though, even if they do not necessarily form a full closed solid, because the mesh generation treats joined edges with special care to ensure that the mesh has a unified vertex structure along those areas.


It may help somewhat if you could post a model file of what you are working on, that would give some more context to your comments and also probably enable giving some more specific advice. It's hard to know whether you would benefit much from working with solids or not without knowing what it is you are trying to do. In some cases you may save a lot of time by focusing on working with solids, and other times you may not.


It can happen pretty frequently that people get overly focused on trying to build an object only one surface patch at a time rather than taking a step back and building a larger solid and then cutting it. That's the kind of thing where working on a solid and then cutting away pieces can speed things up a lot.

Check out this previous thread for a good example of a situation where it was much more difficult to try and fill in a bunch of surface patches rather than building a larger solid and then cutting the solid down:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3105.1

- Michael