[Insight-users] Gradients of binary image pointing in wrong direction?

[Amardeep Singh]

Dear ITK Users

I am still having problems with the gradient vector flow and so I went a 
step back and looked at the gradients
of my image. At the moment, I am just dealing with a synthetic image of 
a square (see attachment).
I calculate the gradient magnitude with the 
itkGradientMagnitudeImageFilter and then the gradient with the
itkGradientImageFilter. I save the output of the gradient filter as a 
*.vtk image file and visualize it in Paraview.
I find that the vectors are pointing outwards, whereas I would expect 
them to point inwards, to the middle of
the edge.
Could anyone tell me whats going wrong, please? Or is everything correct 
and I misunderstood something?
Thank you for any help.

Best regards
Amar

P.S. I have attached my code below:

/*
 * gvfOnSyntheticImage.cc
 *
 *  Created on: 11-Feb-2009
 *    
 */



//ITK Headers
#include "itkImage.h"

#include "itkImageFileReader.h"
#include "itkImageFileWriter.h"
#include "itkCovariantVector.h"
#include "itkGradientVectorFlowImageFilter.h"
//#include "itkLaplacianImageFilter.h"
#include "itkGradientMagnitudeRecursiveGaussianImageFilter.h"
#include "itkGradientMagnitudeImageFilter.h"
#include "itkGradientImageFilter.h"
#include "itkCastImageFilter.h"

#include "vtkImageWriter.h"

#include <iostream>


int main( int argc, char *argv[] )
{

      typedef   float  InternalPixelType;
      typedef float OutputPixelType;
      typedef unsigned char BinaryPixelType;

      const     unsigned int    Dimension = 3;

      typedef itk::Image< InternalPixelType, Dimension >  InternalImageType;
      typedef itk::Image< OutputPixelType, Dimension > OutputImageType;
      typedef itk::Image< BinaryPixelType, Dimension > BinaryImageType;


      typedef itk::GradientMagnitudeImageFilter< InternalImageType, 
OutputImageType > GradientMagnitudeFilterType;
            GradientMagnitudeFilterType::Pointer gradientMagnFilter = 
GradientMagnitudeFilterType::New();
     // typedef 
itk::GradientMagnitudeRecursiveGaussianImageFilter<InternalImageType, 
OutputImageType> GradientGaussianFilterType;

      typedef  itk::ImageFileReader< InternalImageType > ReaderType;
      //typedef  itk::ImageFileReader< BinaryImageType > BinaryReaderType;
      typedef  itk::ImageFileWriter< OutputImageType > WriterType;
     // typedef  itk::ImageFileWriter< BinaryImageType > BinaryWriterType;
      ReaderType::Pointer imageReader = ReaderType::New();

      argv[1] = "/square.nii";

      imageReader -> SetFileName( argv[1] );

          typedef itk::CovariantVector< float, Dimension >  
GradientPixelType;
          typedef itk::Image< GradientPixelType, Dimension > 
GradientImageType;
          //typedef itk::GradientVectorFlowImageFilter< 
GradientImageType, GradientImageType > GVFFilterType;
          //typedef itk::LaplacianImageFilter<InternalImageType, 
InternalImageType> LaplacianFilterType;
          typedef itk::GradientImageFilter<InternalImageType, float, 
float> GradientImageFilterType;
          typedef itk::GradientMagnitudeImageFilter<InternalImageType, 
InternalImageType> GradientMagnitudeImageFilterType;

          GradientMagnitudeImageFilterType::Pointer gmFilter = 
GradientMagnitudeImageFilterType::New();
          gmFilter->SetInput(imageReader->GetOutput());

          //GradientGaussianFilterType::Pointer gaussianFilter = 
GradientGaussianFilterType::New();
          //gaussianFilter->SetSigma( 0.5 );
          //gaussianFilter->SetInput( imageReader->GetOutput() );

          GradientImageFilterType::Pointer gFilter = 
GradientImageFilterType::New();
          gFilter->SetInput(gmFilter->GetOutput());

          try
        {
             // gmFilter->Update();
              //gaussianFilter->Update();
             gFilter->Update();
        }
        catch( itk::ExceptionObject & excep )
        {
          std::cerr << "Exception caught !" << std::endl;
          std::cerr << excep << std::endl;
        }


          WriterType::Pointer gmWriter = WriterType::New();
          gmWriter->SetFileName("/gradientMagnitudeSquare.nii");
          gmWriter->SetInput(gmFilter->GetOutput());

          //WriterType::Pointer gaussianWriter = WriterType::New();
          
//gaussianWriter->SetFileName("/gradientMagnitudeGaussianSquare.nii");
          //gaussianWriter->SetInput(gaussianFilter->GetOutput());

          typedef itk::ImageFileWriter< GradientImageType > FieldWriterType;
          FieldWriterType::Pointer gradientWriter = FieldWriterType::New();
          gradientWriter->SetInput(gFilter->GetOutput());
          gradientWriter->SetFileName("/gradientSquare.vtk");
          try
        {
              gmWriter->Update();
            gradientWriter->Update();
        }
        catch( itk::ExceptionObject & excep )
        {
          std::cerr << "Exception caught !" << std::endl;
          std::cerr << excep << std::endl;
        }


}

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