Function evaluate_rational_surface 

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

Evaluate a rational tensor-product surface: homogeneous evaluation, then divide through.

Errors

As evaluate_surface, plus OgeomError::Numeric if the accumulated weight vanishes, which positive input weights make impossible.