Mesh Offset or Alpha Wrapping to inside?

[lslzl3000]

I want to extract a "shell" of a closed mesh, the simple idea is to erode an offset of the mesh/surface, and then do a boolean-operation to combine two parts.

But the alpha_wrap_3 only add offset on outside of a mesh.
Any way to perform a inside only offset to common mesh? or any better way to do a shell extraction?

[MaelRL]

3D alpha wrapping by default wraps from outside. You can also wrap from inside if you set up seeds, see this example.