Screw VTK.

vtu_to_vti_converter/source/main.cpp

 #include <cstdint>
+#include <iostream>
 #include <string>
 
 #include <vtkDataObject.h>
+#include <vtkImageData.h>
 #include <vtkResampleToImage.h>
 #include <vtkSmartPointer.h>
 #include <vtkXMLImageDataWriter.h>
 #include <vtkXMLUnstructuredGridReader.h>
+#include <vtkUnstructuredGrid.h>
 
 // Usage: ./vtu_to_vti_converter [VTU_FILEPATH] [X_RESOLUTION] [Y_RESOLUTION] [Z_RESOLUTION].
 std::int32_t main(std::int32_t argc, char** argv)
@@ -16,15 +19,23 @@ std::int32_t main(std::int32_t argc, char** argv)
   auto converter = vtkSmartPointer<vtkResampleToImage>          ::New();
   auto writer    = vtkSmartPointer<vtkXMLImageDataWriter>       ::New();
 
+  std::cout << "Reading unstructured grid.\n";
   reader   ->SetFileName           (filepath.c_str());
   reader   ->SetInputArrayToProcess(0, 0, 0, vtkDataObject::FIELD_ASSOCIATION_CELLS, "Q");
+  reader   ->Update                ();
+  auto input_data  = reader->GetOutput();
 
-  converter->SetInputConnection    (reader->GetOutputPort());
+  std::cout << "Converting unstructured grid to structured grid.\n";
+  converter->SetInputDataObject    (input_data);
   converter->SetUseInputBounds     (true);
   converter->SetSamplingDimensions (std::stoi(argv[2]), std::stoi(argv[3]), std::stoi(argv[4]));
+  converter->Update                ();
+  auto output_data = converter->GetOutput();
 
-  writer   ->SetInputConnection    (converter->GetOutputPort());
+  std::cout << "Writing structured grid.\n";
+  writer   ->SetInputData          (output_data);
   writer   ->SetFileName           ((filepath.substr(0, filepath.size() - 3) + std::string("vti")).c_str());
+  writer   ->SetInputArrayToProcess(0, 0, 0, vtkDataObject::FIELD_ASSOCIATION_POINTS, "Q");
   writer   ->Update                ();
 
   return 0;