vtkMarchingCubes export nifti surfaces to wavefront OBJ - vtk

Question

I want to run vtkMarchingCubes on a nifti label set. Regions of voxels, for which I want to produce surfaces all share the same value. I have two problems. First, I seem to be setting up the algorithm incorrectly because the resulting vtkPolyData apparently has no vertices. Secondly, it is not clear to me from the vtkOBJExporter documentation how to export the vtkPolyData as a wavefront .OBJ file. If anyone sees any issues with the code below or can tell me how to export the vtkPolyData as an OBJ, I would be grateful.
//Read The Nifti Label File
string input_path = "/MyPath/labels.nii";
nifti_image *im = nifti_image_read(input_path.c_str(),true);
cout<<im->nx<<","<<im->ny<<","<<im->nz<<endl; //Confirms Read Works
// Set up vtk image data
vtkImageImport* importer = vtkImageImport::New();
importer->SetImportVoidPointer((void*)im->data);
importer->SetDataScalarTypeToFloat();
importer->SetDataExtent(0, im->nx-1, 0, im->ny-1, 0, im->nz-1);
importer->SetWholeExtent(0, im->nx-1, 0, im->ny-1, 0, im->nz-1);
vtkImageData* point_cloud = importer->GetOutput();
point_cloud->SetScalarTypeToFloat();
point_cloud->SetExtent(0, im->nx-1, 0, im->ny-1, 0, im->nz-1);
point_cloud->SetSpacing(im->dx, im->dy, im->dz);
//Apply Threshold To Cut Out Other Data
//Is this needed or will Marching Cubes properly identify the region
vtkImageThreshold* threshold = vtkImageThreshold::New();
threshold->ThresholdBetween(label_number,label_number);
threshold->SetInValue(255);
threshold->SetOutValue(0);
threshold->SetInput(point_cloud);
//Apply the Marching Cubes algorithm
vtkMarchingCubes* marching_cubes = vtkMarchingCubes::New();
marching_cubes->SetValue(0, 127.0f);
marching_cubes->SetInput(threshold->GetOutput()); //(vtkDataObject*)point_cloud);
vtkPolyData* surface = marching_cubes->GetOutput();
marching_cubes->Update();
//See That Marching Cubes Worked
cout<<"# Vertices: "<< surface->GetNumberOfVerts()<<endl;
cout<<"# Cells: "<< surface->GetNumberOfCells()<<endl;
//Export (How is this done properly?)
vtkOBJExporter* exporter = vtkOBJExporter::New();
exporter->SetInput(vtkRenderWindow *renWin); //I don't want a render window, I want at file
exporter->SetFilePrefix("/MyPath/surface");
exporter->Write();

Answer 1

You can use this class https://github.com/daviddoria/vtkOBJWriter to write the obj file in the way you would expect (like every other VTK writer). Unfortunately the vtkOBJExporter wants to also write additional information that I never have.