message: correcting ns direction for gradient shading
diff: @@ -89,7 +89,7 @@ static void sampleVolume(
  // compute gradient shading lighting
  if (gsc > 0.0f) {
  - vec3f ns = Volume_getGradient(m->volume, p);
  + vec3f ns = Volume_getGradient(m->volume, p) * -1.f;
  if (dot(ns, ns) > 1e-6f) {
  ns = normalize(ns);
  