https://ask.sagemath.org/question/61908/how-to-flip-normals-of-graphics3d-faces/
How to flip normals of Graphics3d faces? - ASKSAGE: Sage Q&A Forum
Is there an easy way to flip the normals for all the faces of a sage.plot.plot3d.base.Graphics3d or sage.plot.plot3d.base.Graphics3dGroup object? I checked the...
how to