VTK is very unintuitive for real.

vtu_to_vti_converter/source/main.cpp

@@ -2,6 +2,7 @@
 #include <iostream>
 #include <string>
 
+#include <vtkDataArraySelection.h>
 #include <vtkDataObject.h>
 #include <vtkImageData.h>
 #include <vtkResampleToImage.h>
@@ -21,7 +22,8 @@ std::int32_t main(std::int32_t argc, char** argv)
 
   std::cout << "Reading unstructured grid.\n";
   reader   ->SetFileName           (filepath.c_str());
-  reader   ->SetInputArrayToProcess(0, 0, 0, vtkDataObject::FIELD_ASSOCIATION_CELLS, "Q");
+  auto selection = reader->GetCellDataArraySelection();
+  selection->AddArray              ("Q");
   reader   ->Update                ();
   auto input_data  = reader->GetOutput();
 
@@ -35,7 +37,6 @@ std::int32_t main(std::int32_t argc, char** argv)
   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;