I extracted the boundary of the longer surface and rebuilt the two long bottom edges with 16 points, what rebuild suggested and used sweep2 to make a new tunnel. Looks better. Guess this is not about the surface so I deleted the surface and kept just two curves in the file.
When viewed more from the top, the facetting effect is not visible…
Latest WIP with Direct3D
2025-09-04_Tess_mrtn.3dm (116.3 KB)