Function pbrt::core::geometry::nrm_dot_nrm

pub fn nrm_dot_nrm<T>(n1: &Normal3<T>, n2: &Normal3<T>) -> T where
    T: Copy + Add<T, Output = T> + Mul<T, Output = T>, 

Product of the Euclidean magnitudes of a normal (and another normal) and the cosine of the angle between them. A return value of zero means both are orthogonal, a value if one means they are codirectional.