51CTO详解：http://book.51cto.com/art/201504/474123.htm

1.vtkImageGradientMagnitude提取边缘
https://lorensen.github.io/VTKExamples/site/Cxx/Images/ImageGradientMagnitude/
关键代码：

  vtkSmartPointer<vtkImageGradientMagnitude> gradientMagnitudeFilter = 
      vtkSmartPointer<vtkImageGradientMagnitude>::New();
  gradientMagnitudeFilter->SetInputConnection(source->GetOutputPort());
  gradientMagnitudeFilter->Update();

  vtkSmartPointer<vtkImageCast> gradientMagnitudeCastFilter =
    vtkSmartPointer<vtkImageCast>::New();
  gradientMagnitudeCastFilter->SetInputConnection(gradientMagnitudeFilter->GetOutputPort());
  gradientMagnitudeCastFilter->SetOutputScalarTypeToUnsignedChar();
  gradientMagnitudeCastFilter->Update();

2.vtkImageSobel2D提取边缘
https://lorensen.github.io/VTKExamples/site/Cxx/Images/ImageSobel2D/
关键代码：

  // Find the x and y gradients using a sobel filter
  vtkSmartPointer<vtkImageSobel2D> sobelFilter = 
    vtkSmartPointer<vtkImageSobel2D>::New();
  sobelFilter->SetInputConnection(source->GetOutputPort());
  sobelFilter->Update();