Function pbrt::core::geometry::vec3_dot_vec3

pub fn vec3_dot_vec3<T>(v1: &Vector3<T>, v2: &Vector3<T>) -> T where
    T: Copy + Add<T, Output = T> + Mul<T, Output = T>, 

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