pub fn partials(
surface: &Surface,
u: f64,
v: f64,
) -> Result<(DVec3, DVec3), GeomError>Expand description
Analytic first partial derivatives (∂S/∂u, ∂S/∂v) at (u, v).
Rational B-spline derivatives are evaluated in homogeneous space and projected with the quotient rule. This remains stable when valid imported knot domains have large offsets that make finite-difference steps vanish.