message: Clarify flipping of gradient
diff: @@ -89,9 +89,12 @@ static void sampleVolume(
  // compute gradient shading lighting
  if (gsc > 0.0f) {
  - vec3f ns = Volume_getGradient(m->volume, p) * -1.f;
  + vec3f ns = Volume_getGradient(m->volume, p);
  if (dot(ns, ns) > 1e-6f) {
  - ns = normalize(ns);
  + // assume that opacity directly correlates to volume scalar field, i.e.
  + // that "outside" has lower values; because the gradient point towards
  + // increasing values we need to flip it
  + ns = neg(normalize(ns));
  // use scivis shading function
  // Prepare differential geometry structure
  