Hi Mike,

> I pretty much join surfaces to make the whole of the
> combined surface a little easier to select and work with,
> but yes, it makes rendering a little nicer when there are
> no gaps. MoI will "weld" together the edges.

Yup, especially for exporting to a rendering program you want to join together surfaces instead of having them just be totally separate surfaces sitting next to one another. When they are actually joined together then the mesh will be generated with knowledge of that join and the mesher will do extra work to ensure that there is a common shared vertex structure along that edge. If they get meshed totally separately with no knowledge of the connection between them then there can be different vertex structures there and that tends to make for little tiny cracks.

But if I understood correctly, the question was more like "why do you have joined surfaces instead of solids"?

A solid is a bunch of joined surfaces where every single edge is joined to another one, making a completely closed skin that defines a volume.


> Are there any noted advantages on your part when the
> surfaces are left intact?

I think he was thinking of making a solid, not of leaving them as separate individual surfaces?


> You bring up a good point too: Is there a way to control the
> direction of the normals when exporting an .obj to a stand-alone
> renderer like Kerkythea.

That's one of the advantages of making things into a solid instead of leaving any open edges - a solid will automatically have its normals oriented to point towards the outside of the volume. If you have a joined surface that has some unjoined edges within it somewhere (meaning it does not define a volume), then the normals are not oriented automatically in any special way and the orientation will depend on other factors like if you happened to draw some of the original curves in a clockwise or counterclockwise direction.


> Is that what the non-UI command "Flip" does?

Yes, for a non-solid joined surface you can use the Flip command on it to reverse its positive surface normal direction and that will also reverse the direction of the mesh normals that are generated from the surface.


You don't have to make things into solids, but if it's not too much work to do it on any particular model it can be a good idea because then the final mesh that you generate will automatically have outward pointing normals. Also it can be kind of convenient in general to work more with solids since the boolean commands are more focused on working with solids - they decide which pieces to keep and which pieces to discard based on which volume the pieces are located in. So in some cases if you don't have a volume you may need to use Trim to cut it up rather than using booleans. Booleans are kind of like a "high level" version of trim that wraps up doing a Trim + removing particular pieces + joining all as a kind of batch operation so they're kind of more convenient to be able to use when possible instead of doing those steps manually.

But on the other hand it's not necessarily an "error" to have a joined surface instead of a solid either - if it would take too much work to construct the missing pieces to make it a finished up solid then that can outweigh the convenience factor that working with a finished solid would give...

It also depends on what you're doing with your models as well though, because for some kinds of uses like generating 3D printed physical parts, it is mandatory to have a solid, but for just rendering it's not.


re: rendering normals - most of the time with a renderer it is possible to set materials to be "double sided" and once you do that it won't really make any difference which side happens to be the positive surface normal side. So it can be easiest to just set that one property in the render material and then not worry about normals any more after that at all.


- Michael