Function rational_surface_derivatives 

pub fn rational_surface_derivatives<P: Blend>(
    ku: &KnotVector,
    kv: &KnotVector,
    grid: &ControlGrid<Weighted<P>>,
    u: f64,
    v: f64,
    order: usize,
    tol: Tolerances,
) -> OgeomResult<DerivativeGrid<P>>

Evaluate a rational surface and its partial derivatives up to total order order.

The two-parameter quotient rule. Each mixed partial subtracts the weight’s influence in u, in v, and in both together; dropping the last of those three sums is the classic error, and it only shows up on genuinely rational surfaces with mixed derivatives, which is to say, on exactly the spheres and tori where the answer matters.

Errors

As evaluate_rational_surface.