Function pbrt::core::geometry::spherical_direction_vec3

pub fn spherical_direction_vec3(
    sin_theta: Float, 
    cos_theta: Float, 
    phi: Float, 
    x: &Vector3f, 
    y: &Vector3f, 
    z: &Vector3f
) -> Vector3f

Take three basis vectors representing the x, y, and z axes and return the appropriate direction vector with respect to the coordinate frame defined by them.