Function fit_surface_grid_closed_v 

pub fn fit_surface_grid_closed_v(
    rows: &[Vec<Point>],
    degree: usize,
    tolerance: f64,
    tol: Tolerances,
) -> OgeomResult<Fitted<BSplineSurface>>

As fit_surface_grid, with the v direction closed into a smooth loop.

The rows must form a loop (the first row repeated at the end), and the join across it is made C1 the way fit_points_closed makes a curve’s: the constraint is eliminated inside the shared solve, so the two border control rows are equal and the loop crosses its own seam smoothly.

Errors

As fit_surface_grid, and additionally if the rows are not a loop.