Scalars

#include <vtkSmartPointer.h>
#include <vtkPoints.h>
#include <vtkPolyData.h>
#include <vtkPointData.h>
#include <vtkDoubleArray.h>
#include <vtkFloatArray.h>

void TypeSpecific();
void NonTypeSpecific();

int main(int, char *[])
{
  TypeSpecific();
  NonTypeSpecific();

  return EXIT_SUCCESS;
}

void TypeSpecific()
{
  // Create a point set
  vtkSmartPointer<vtkPoints> points =
    vtkSmartPointer<vtkPoints>::New();
  points->InsertNextPoint(0,0,0);
  points->InsertNextPoint(1,0,0);

  vtkSmartPointer<vtkPolyData> polydata =
    vtkSmartPointer<vtkPolyData>::New();
  polydata->SetPoints(points);

  vtkSmartPointer<vtkDoubleArray> weights =
    vtkSmartPointer<vtkDoubleArray>::New();
  weights->SetNumberOfValues(2);
  weights->SetValue(0, 1);
  weights->SetValue(1, 2);

  polydata->GetPointData()->SetScalars(weights);

  double weight = vtkDoubleArray::SafeDownCast(polydata->GetPointData()->GetScalars())->GetValue(0);
  std::cout << "double weight: " << weight << std::endl;

  /*
  // This causes a crash because the array is not a vtkFloatArray
  double weightf = vtkFloatArray::SafeDownCast(polydata->GetPointData()->GetScalars())->GetValue(0);
  std::cout << "float weight: " << weightf << std::endl;
  */
}

void NonTypeSpecific()
{
  // Create a point set
  vtkSmartPointer<vtkPoints> points =
    vtkSmartPointer<vtkPoints>::New();
  points->InsertNextPoint(0,0,0);
  points->InsertNextPoint(1,0,0);

  vtkSmartPointer<vtkPolyData> polydata =
    vtkSmartPointer<vtkPolyData>::New();
  polydata->SetPoints(points);

  vtkSmartPointer<vtkDoubleArray> weights =
    vtkSmartPointer<vtkDoubleArray>::New();
  weights->SetNumberOfValues(2);
  weights->SetValue(0, 1);
  weights->SetValue(1, 2);

  polydata->GetPointData()->SetScalars(weights);

  double weight = polydata->GetPointData()->GetScalars()->GetComponent(0,0);
  std::cout << "double weight: " << weight << std::endl;

  double weightf = polydata->GetPointData()->GetScalars()->GetComponent(0,0);
  std::cout << "float weight: " << weightf << std::endl;

}

cmake_minimum_required(VERSION 3.3 FATAL_ERROR)

project(Scalars)

find_package(VTK COMPONENTS 
  vtkCommonCore
  vtkCommonDataModel QUIET)
if (NOT VTK_FOUND)
  message("Skipping Scalars: ${VTK_NOT_FOUND_MESSAGE}")
  return ()
endif()
message (STATUS "VTK_VERSION: ${VTK_VERSION}")
if (VTK_VERSION VERSION_LESS "8.90.0")
  # old system
  include(${VTK_USE_FILE})
  add_executable(Scalars MACOSX_BUNDLE Scalars.cxx )
  target_link_libraries(Scalars PRIVATE ${VTK_LIBRARIES})
else ()
  # include all components
  add_executable(Scalars MACOSX_BUNDLE Scalars.cxx )
  target_link_libraries(Scalars PRIVATE ${VTK_LIBRARIES})
  # vtk_module_autoinit is needed
  vtk_module_autoinit(
    TARGETS Scalars
    MODULES ${VTK_LIBRARIES}
    )
endif () 

cd Scalars/build 

cmake ..

cmake -DVTK_DIR:PATH=/home/me/vtk_build ..

make

./Scalars