Could someone please explain to me how you would make a mould of a surface of an .STL or OBJ file? For example, if I had a textured bowl and wanted to get an exact copy of the inside of the bowl, but not include the outside of the bowl? I am trying to find a way to make an exact copy of the hip joint surface (acetabulum) without having the rest of the pelvis in my mould. I have tried a couple of methods I have found online but am not having any success. I would sincerely appreciate your help!