I was thinking about 3D again. I have subdivided quads into triangle pairs and I want to do the same with a parallelepiped https://en.wikipedia.org/wiki/Parallelepiped into six tetrahedrons.

I could further subdivide the tetrahedra into 24 daughters and group them by the original vertices to get volumes around those vertices, if they were stars and interstellar states with territories for a poor man's 3D voronoi diagram/political map.

But I want this to get around marching cubes. I can find contour lines or surfaces in a volume of a triangle fairly easily, I think. Need to try it.

With a triangle, if any of the vertices are different in height, such as land and sea, the shore is an edge at 0 altitude and we can find that make a triangle and quad we can subd into two more triangles, easy peasey. The same would happen in three with a surface inside the tetrahedron.