Hi Paul,

re:
> I thought, a solid is a solid, isn't it?

Unfortunately no it's not quite so. MoI will report an object as being a Solid if there are no naked edges, meaning all edges are joined between 2 surfaces.

But there are many possible ways to make degenerate or malformed geometry that will report as a solid but cause many problems.

Some of these conditions are having a trimming boundary on a surface that crosses over itself, having surfaces with control points awkwardly pinched together so the surface has a chaotic shape in a small area, having opposite sides of the object push through each other, faces that are degenerate with all their surface area compressed down to a single line or point, and various others.

These various conditions tend to be both difficult and time consuming to try and catch in an automated process.

If MoI tried to screen all objects to have none of these problems before labeling it as a "Solid" you could potentially be forced to wait for several minutes every time you selected an object before it would be able to tell you the object type.

In the future I do definitely want to add different types of analysis and checking tools to help detect these kinds of problems but currently it's more up to the user to avoid making these kinds of messy objects.

So definitely one area that is not good in your case here is like pafurijaz writes above, this area of the green piece is made up of like 170 little slivered flat pieces instead of just one smooth surface:



Then on top of all of that, the offset and shell functions in MoI are not very good when it comes to thickening pieces made up of multiple faces that are all at different angles to each other. The offsets in those cases do not naturally match up with one another and need to be extended and intersected and that does not work well in MoI. So to generate an inner wall for your case it will probably be needed to explicitly model it rather than generate it through offsetting.

- Michael