Only load the the Q.

offline_ray_tracer/include/unstructured_grid_io.hpp

 #ifndef UNSTRUCTURED_GRID_IO_HPP
 #define UNSTRUCTURED_GRID_IO_HPP
 
+#include <optional>
 #include <string>
 
 #include <vtkSmartPointer.h>
@@ -13,11 +14,15 @@ namespace rt
 class unstructured_grid_io
 {
 public:
-  static vtkSmartPointer<vtkUnstructuredGrid> read (const std::string& filepath)
+  static vtkSmartPointer<vtkUnstructuredGrid> read (const std::string& filepath, std::optional<std::string> field_name)
   {
     auto reader = vtkSmartPointer<vtkXMLUnstructuredGridReader>::New();
     auto data   = vtkSmartPointer<vtkUnstructuredGrid>         ::New();
     reader->SetFileName(filepath.c_str());
+
+    if (field_name)
+      reader->SetInputArrayToProcess(0, 0, 0, vtkDataObject::FIELD_ASSOCIATION_CELLS, field_name->c_str());
+
     reader->Update     ();
     data  ->ShallowCopy(reader->GetOutput());
     return data;

offline_ray_tracer/source/main.cpp

@@ -96,7 +96,7 @@ std::int32_t main(std::int32_t argc, char** argv)
       if (!settings.data_filepaths[slice].geometry.empty())
         pd_mapper->SetInputData(rt::poly_data_io        ::read(settings.data_filepaths[slice].geometry));
       if (!settings.data_filepaths[slice].volume  .empty())
-        ug_mapper->SetInputData(rt::unstructured_grid_io::read(settings.data_filepaths[slice].volume  ));
+        ug_mapper->SetInputData(rt::unstructured_grid_io::read(settings.data_filepaths[slice].volume  , "Q"));
     }
 
     std::cout << "Rendering frame " << current_time << ".\n";