Model; Closed\Open - What does it mean?

In MED, I create a model, simple cylinders and blocks. I move them close together to make a table with legs. When I use the tools option to test the mesh, I see I have 9 objects, 6 closed, 3 open. Could someone explain to me:

1. what does this mean
2. is it a problem
3. how do I correct or avoid it?

Occasionally I will end up with a model that has all objects open. Using Lithunwrap to weld the model seems to resolve most of those objects, but not all of them. Since I am not sure of what I am doing exactly, it makes me a little uncomfortable. I really am ignorant of the issue here, I know. But I would like to come to an understanding of this. ;-)

Thanks!

A mesh is only closed when every edge is part of 2 triangles.

An open mesh could give problems when you use dynamic shadows.

Perfectly clear. Thanks!

OK, since I have constructed this model out of primitives, and each primitive is perfect in triangular geometry, I can only assume that the openings are being caused by my allowing one primitive to penetrate another? What would be the best manner of using two primitives without opening the geometry? As in butting a round table leg up against a flat table surface? Should I simply make sure they are close but do not penetrate? Is there a better method?

I know this is basic stuff, but I really need to get it clear before I invest a lot of time in constructing meshes.

Again, thanks.

That's strange. Either the description in the manual is wrong and every edge being shared by 2 triangles isn't the only requirement for a mesh to be closed, or your model isn't actually closed.

How have you created your model, have you exported it from an external modeller? If you have, sometimes the vertices are split by the export process. Then there are actually 2 vertices on the same spot, one conected to triangleA and one to triangleB. Try box-selecting a (seemingly single) vertex(vertices) and welding it. If that doesn't help I don't know.

Well, I have spent considerable hours at this now, so I have tried a number of different paths.And yes, I created the model in Milkeshape. It is simple enough I could create it in MED, but I am trying to use this model in other projects as well.

So you are saying that interpenetrating primitives should not affect the mesh closure?

Welding the vertices after importing to MED does seem to reduce the closure count, but I end up with some strange face geometry. Since these models appear to render fine in other rendering environments, I'm guessing it is as you say, the import process into MED.

I have reconfigured the model so each primitive is distinct and separate from the others. I've tried exporting from Milkshape to 3ds, Milkshape Ascii, and I've tried from 3ds to Ultimate Unwrap and into 3dgs MDL. The last seems to work best, I get down to 3 or 4 open faces. The 3DS will work fine for me in Ogre and other renderers, I was just hoping to use 3DGS for prototyping. Importing shouldn't be this difficult. I was thinking it was something I was not understanding about constructing the meshes. But I don't think I want to re-weld 500 models. ;-)That's what the spec calls for.

Given Milkshape, Ultimate Unwrap and MED, can you suggest a possible pipeline to try that routes through those tools ending up with the least damage importing into MED?

Thanks again!

Using open meshes also has the advantage of not having to render unseen polygons which adds to a faster frame rate. Take the case of holding a gun. You can't see the bottom of it, so why bother building faces and using extra vertices in that area when you could avoid them and speed up the rendering (or add extra details to what is visible)? If you can't see the bottom of a table, why bother wasting polygons and vertices to render it?

I agree. But I also see the difficulty of dealing with unruly meshes. No problem really. I am not using dynamic shadows in 3DGS anyway, they seem too fragile. Especially given the import difficulties of models created outside of MED. <g> Works great as a fast prototyper though. The models seem to work fine in other renderers, shadows and all. So no biggy.

Thanks to everyone for clarifying and confirming.