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3D viewer seems to remove faces using "back-face culling".
I guess the 3D viewer removes faces because the viewer looks at their back faces. You could either disable back-face culling, or flip the triangles (usually by reverting the order of the vertex indices), see e.g. https://www.asawicki.info/news_1524_how_to_flip_triangles_in_triangle_mesh
Thank you so much for the advice. I didn't know about back-face culling. After some research, I found that most mesh rendering software apply back-face culling, not showing any faces whose normal is not facing the direction of visualization (https://arxiv.org/pdf/2201.06935.pdf). As you said the 3D viewer uses back-face culling and I fixed by reversing the orientation of the normals. I'm glad you wrote this comment because you really helped me and you made me discovering something new.
Wish you all the best!
Mattia
Always happy to help! 🙂
And thanks for making this demo available!
Simon