VTKPNGWriter printing out black images?

Question

I am doing some image processing using ITK and then using VTK to print the results in a .png format however, the output image is always black.

Currently, I am converting itk::Image to vtk::vtkImageData using the itk::ImagetoVTKImageFilter(typedeffed to ITKtoVTKFilterType in my code).

ITKtoVTKFilterType::Pointer itk2vtkGray = ITKtoVTKFilterType::New();
itk2vtkGray->SetInput(grayBinary);  //grayBinary is of type itk::Image<unsigned short, 2>
itk2vtkGray->Update();

vtkSmartPointer<vtkImageData> grayVTK = vtkSmartPointer<vtkImageData>::New();
grayVTK->SetExtent(extent);
grayVTK->SetSpacing(m_spacing);
grayVTK->SetScalarTypeToUnsignedShort();
grayVTK->SetNumberOfScalarComponents(1);
grayVTK->AllocateScalars();
grayVTK->DeepCopy(static_cast<vtkImageData*>(itk2vtkGray->GetOutput()));
//grayVTK = itk2vtkGray->GetOutput();

I have even confirmed that my VTK ImageData contains values of either 255 or 0 using the following code.

int *dims = grayVTK->GetDimensions();

std::cout << "Dims: " << " x: " << dims[0] << " y: " << dims[1] << " z: " << dims[2] << std::endl;
std::cout << "Number of points: " << grayVTK->GetNumberOfPoints() << std::endl;
std::cout << "Number of cells: " << grayVTK->GetNumberOfCells() << std::endl;

for (int y = 0; y < dims[1]; y++)
{        
     for (int x = 0; x < dims[0]; x++)
     {
         unsigned short *pixel = static_cast<unsigned short*>(grayVTK->GetScalarPointer(x,y,0));
         std::cout << "PIXEL LOC/VAL "<< y*dims[0] + x << " " << pixel[0] <<std::endl;
     }
     std::cout << std::endl;
 }

I then go on to do an ImageCast to ensure the type of the data is unsignedShort.

vtkSmartPointer<vtkImageCast> cast2 = vtkSmartPointer<vtkImageCast>::New();
cast2->SetInput(grayVTK);
cast2->SetOutputScalarTypeToUnsignedShort();
cast2->ClampOverflowOn();
cast2->Update();

Then finally I use vtkPNGwriter to output the .png files. Notice that I have tried to output both the actual vtkImageData as well as output from the ImageCastFilter.

vtkSmartPointer<vtkPNGWriter> writer =
vtkSmartPointer<vtkPNGWriter>::New();
writer->SetFileName(filename.toStdString().c_str());
writer->SetInputConnection(cast2->GetOutputPort());
//writer->SetInput(grayVTK); I have tried to method as well but to no success
writer->Write();

However, the .png output is always black. Does anyone know what I am doing wrong.

Answer 1

For future reference it seems that many PNG readers do not display 16 bit data. Hence the casting I was doing to unsigned short at the end should have rather been to char.

vtkSmartPointer<vtkImageCast> cast2 = vtkSmartPointer<vtkImageCast>::New();
cast2->SetInput(grayVTK);
cast2->SetOutputScalarTypeToChar();
cast2->ClampOverflowOn();
cast2->Update();