message: throw an error if a volume wasn't put on an isosurface geometry
diff: @@ -44,6 +44,11 @@ namespace ospray {
  model = (VolumetricModel *)getParamObject("volume");
  + if (!model) {
  + throw std::runtime_error(
  + "the 'volume' parameter must be set for isosurfaces");
  + }
  +
  if (!isovaluesData->compact()) {
  // get rid of stride
  auto data = new Data(OSP_FLOAT, vec3ui(isovaluesData->size(), 1, 1));
  