Photonmapping Bilinear Estimate

Radiance _KD_Tree::EstimateFilterSmartBilinear::filtered_estimate(Ray const& ray_i, Vec3 const& sample_position, HitRecord const& hit_record, _BoundedPriorityQueue const* bpq) const /*override*/ {
    Materials::Material const* material = hit_record.object->material;

    Float n_sc;
    if (_kd_tree->pmap->num_reconstruction_photons_min == _kd_tree->pmap->num_reconstruction_photons_max) {
        n_sc = 0;
    } else {
        n_sc =
            static_cast<Float>(bpq->backing.size() - _kd_tree->pmap->num_reconstruction_photons_min) /
            static_cast<Float>(_kd_tree->pmap->num_reconstruction_photons_max-_kd_tree->pmap->num_reconstruction_photons_min)
        ;
    }
    Float r_sc = Ash::sqrt(bpq->get_max().distance_sq / _kd_tree->pmap->reconstruction_max_radius_sq);
    Float sc = 1 - r_sc + r_sc*n_sc; //bilinear interpolation

    Radiance total_radiance(Radiance::ZEROES);
    for (auto const& dist_rec : bpq->backing) {
        _PhotonComplete const& photon = _kd_tree->_photons_complete[dist_rec.photon_index];

        Materials::Material::BSDF_sample bsdf;
        if (material->evaluate_bsdf(ray_i,hit_record,-photon.direction,&bsdf)) {
            total_radiance += bsdf.value % photon.energy;
        }
    }

    return (sc * static_cast<Float>(1.0/Ash::PI) / bpq->get_max().distance_sq) * total_radiance;
}