Function fit_points_2d_at 

pub fn fit_points_2d_at(
    parameters: &[f64],
    points: &[Point2],
    degree: usize,
    tolerance: f64,
    tol: Tolerances,
) -> OgeomResult<Fitted<BSpline2d>>

Fit a pcurve at fixed parameters: the source curve’s own.

The fixed-parameter twin of fit_points_2d, and the difference is the contract: parameter correction is what makes a free fit’s residual honest, and it is exactly what a same-parameter fit must never do, because the parameters are not a guess to be improved; they are the 3D curve’s own, and drifting them is how a pcurve ends up evaluating away from the curve it annotates. Here the parameters stay put, refinement adds knots where the error says, and the reported error is the true same-parameter deviation in the chart.

Errors

As fit_points_2d, and the parameters must be strictly increasing and as many as the points.