vtkImageData.ToBitmap() produces uncompatible bitmaps

Hi, I love VTK but im kind of struggling right now:

        double[] viewport=viewportRenderer.GetViewport();
        vtkWindowToImageFilter imageFilter = vtkWindowToImageFilter.New();
        vtkImageData imageData = imageFilter.GetOutput();
        Bitmap result = imageData.ToBitmap();
        vtkBMPReader reader = vtkBMPReader.New();	  
        int i = reader.CanReadFile(@"C:\test\viewportsbitmaps\test3.bmp");

CanReadFile() returns 0. I cant figure out how to use ToBitmap() in a way, that i can use result again in vtk using Activiz with vtk5

Ok, the functionality isnt implemented

public static vtkImageData FromImage(Image img, int numberOfScalarComponents)
  Bitmap bitmap = new Bitmap(img);
  return vtkImageData.New();