Spectrum MirrorBSDF::f(const Vector3D& wo, const Vector3D& wi) {
        return reflectance  * (1.0 / abs_cos_theta(wo));
    }

Spectrum light_L = light->sample_L(hit_p, &dir_to_light, &dist_to_light, &pdf);
            // convert direction into coordinate space of the surface, where
            // the surface normal is [0 0 1]
            Vector3D w_in = w2o * dir_to_light;
            double cos_theta = std::max(0.0, w_in[2]);
            Spectrum f = isect.bsdf->f(w_out, w_in);
            // TODO:
            // Construct a shadow ray and compute whether the intersected surface is
            // in shadow and accumulate reflected radiance
            Ray shadow_ray(hit_p + EPS_D * dir_to_light, dir_to_light, dist_to_light - (EPS_D * dir_to_light).norm(), 0);
            if (!bvh->intersect(shadow_ray))
            {
                L_out += (f * light_L * (cos_theta * scale / pdf));
            }