vtk中图像数据类型转换

51CTO讲解地址：
http://book.51cto.com/art/201504/474109.htm
demo地址：
https://lorensen.github.io/VTKExamples/site/Cxx/ImageProcessing/RescaleAnImage/
关键代码：

  vtkSmartPointer<vtkImageShiftScale> shiftScaleFilter =
    vtkSmartPointer<vtkImageShiftScale>::New();
  shiftScaleFilter->SetOutputScalarTypeToUnsignedChar();
  shiftScaleFilter->SetInputData(image);
  shiftScaleFilter->SetShift(-1.0f * image->GetScalarRange()[0]); // brings the lower bound to 0
  float oldRange = image->GetScalarRange()[1] - image->GetScalarRange()[0];
  std::cout << "Old range: [" << image->GetScalarRange()[0] << ", " << image->GetScalarRange()[1] << "]" << std::endl;
  std::cout << "Old range magnitude: " << oldRange << std::endl;
  float newRange = 255; // We want the output [0,255]

  shiftScaleFilter->SetScale(newRange/oldRange);
  shiftScaleFilter->Update();

2.使用vtkImageCast对读入数据进行转换
官方demo：https://lorensen.github.io/VTKExamples/site/Cxx/Images/Cast/
关键代码：

  vtkSmartPointer<vtkImageCast> castFilter =
    vtkSmartPointer<vtkImageCast>::New();
  castFilter->SetInputConnection(source->GetOutputPort());
  castFilter->SetOutputScalarTypeToUnsignedChar();
  castFilter->Update();