message: Builtin PI
  ..and leave a TODO note
diff: @@ -160,7 +160,8 @@ struct kernel
  auto prob = max_element(throughput.samples());
  brdf_pdf *= prob;
  - auto src = surf.shade(view_dir, L, ls.intensity) / S(M_PI) / ldotn;
  + // TODO: inv_pi / dot(n, wi) factor only valid for plastic and matte
  + auto src = surf.shade(view_dir, L, ls.intensity) * constants::inv_pi<S>() / ldotn;
  auto solid_angle = (ldotln * ls.area) / (ld * ld);
  auto light_pdf = S(1.0) / solid_angle;
  