Each of the faces in this set shares at least one edge with another. There are two types of Shell objects: open and closed. Open - An edge on at least one face does not have a partnering face. Closed - The edges and faces are topologically closed and enclose an unambiguous volume. This type of Shell can further be classified as enclosing either material or a void. When a Shell is classified as a void, it must have another outer Shell that completely contains it. This outer Shell must necessarily enclose material. With both open and closed Shells, all the faces have their normals consistently oriented. This means that along any shared edge, the normals of the adjacent faces point toward the same side. A void Shell has all its face normals pointing toward its enclosing volume (away from the material side).
