I just ran an OffsetSrf on a sphere, and I presumed the output would be two separate surfaces since Rhino doesn’t call such non-manifold objects single solids. But it did…which I guess is alright as a way of answering the frequent questions about such offsets, but it also didn’t didn’t recognize it as “non-manifold,” which seems like a bug. SR12.

I’m interested in the definitions of manifold etc. I’ve got by using rhino for several years without really understanding some of the fundamental terminology. Perhaps it exists somewhere already?

Normally in Brep (Boundary Representation) geometry, every edge is either part of only one surface (open or “naked” edge) or is shared by exactly two surfaces (“joined” edge). In this way shells are created that if closed can create a single unambiguous volume with a defined interior and exterior. The “shell” of surfaces is the boundary between the interior and exterior. This construction is considered “Manifold”.

Non-manifold objects occur when more than two surfaces share a common edge, as in a “T” structure. The definition of what is interior and what is exterior then becomes ambiguous, and although they are considered “legal” structures, most 3D programs have difficulty with them.