Hi! Thought to create some objects in openSCAD, to print later.I do not have my own 3D printer (yet). One I talked to, wanted to get the drawings in .STL or .3MF. (I don't know the difference). If I had my own printer, have I been able to print directly from openSCAD? (Without some export?) In the openSCAD, no errors or problems occur.butThe drawings I tested to export from openSCAD have given one or another error message.One message .:WARNING: Object may not be a valid 2-manifold and may need repair!  Another message .:WARNING: No top level geometry to render  What to do?