# NeighborhoodIterators1.cxx¶

Example usage:

./NeighborhoodIterators1 Input/ROI_QB_PAN_1.tif Output/NeighborhoodIterators1a.png


Example source code (NeighborhoodIterators1.cxx):

// This example uses the \doxygen{itk}{NeighborhoodIterator} to implement a simple
// Sobel edge detection algorithm \cite{Gonzalez1993}.  The algorithm uses the
// neighborhood iterator to iterate through an input image and calculate a
// series of finite difference derivatives.  Since the derivative results
// cannot be written back to the input image without affecting later
// calculations, they are written instead to a second, output image.  Most
// inputs.
//
// We begin by including the proper header files.  The
// \doxygen{itk}{ImageRegionIterator} will be used to write the results of
// computations to the output image.  A const version of the neighborhood
// iterator is used because the input image is read-only.

#include "otbImage.h"
#include "otbImageFileWriter.h"
#include "itkUnaryFunctorImageFilter.h"
#include "itkRescaleIntensityImageFilter.h"

#include "itkConstNeighborhoodIterator.h"
#include "itkImageRegionIterator.h"

int main(int argc, char* argv[])
{
if (argc < 3)
{
std::cerr << "Missing parameters. " << std::endl;
std::cerr << "Usage: " << std::endl;
std::cerr << argv[0] << " inputImageFile outputImageFile" << std::endl;
return -1;
}

// The finite difference calculations
// in this algorithm require floating point values.  Hence, we define the image
// pixel type to be \code{float} and the file reader will
// automatically cast fixed-point data to \code{float}.
//
// We declare the iterator types using the image type as
// the template parameter. The second template parameter of the
// neighborhood iterator, which specifies
// the boundary condition, has been omitted because the default condition is
// appropriate for this algorithm.

using PixelType  = float;
using ImageType  = otb::Image<PixelType, 2>;

using NeighborhoodIteratorType = itk::ConstNeighborhoodIterator<ImageType>;
using IteratorType             = itk::ImageRegionIterator<ImageType>;

// The following code creates and executes the OTB image reader.
// The \code{Update}
// call on the reader object is surrounded by the standard \code{try/catch}
// blocks to handle any exceptions that may be thrown by the reader.

try
{
}
catch (itk::ExceptionObject& err)
{
std::cout << "ExceptionObject caught !" << std::endl;
std::cout << err << std::endl;
return -1;
}

//  We can now create a neighborhood iterator to range over the output of the
//  reader. For Sobel edge-detection in 2D, we need a square iterator that
//  extends one pixel away from the neighborhood center in every dimension.

// The following code creates an output image and iterator.

ImageType::Pointer output = ImageType::New();
output->Allocate();

// Sobel edge detection uses weighted finite difference calculations to
// construct an edge magnitude image.  Normally the edge magnitude is the
// root sum of squares of partial derivatives in all directions, but for
// simplicity this example only calculates the $x$ component. The result is a
// derivative image biased toward maximally vertical edges.
//
// The finite differences are computed from pixels at six locations in the
// neighborhood.  In this example, we use the iterator \code{GetPixel()}
// method to query the values from their offsets in the neighborhood.
// The example in Section~\ref{sec:NeighborhoodExample2} uses convolution
// with a Sobel kernel instead.
//
// Six positions in the neighborhood are necessary for the finite difference
// calculations. These positions are recorded in \code{offset1} through
// \code{offset6}.

NeighborhoodIteratorType::OffsetType offset1 = {{-1, -1}};
NeighborhoodIteratorType::OffsetType offset2 = {{1, -1}};
NeighborhoodIteratorType::OffsetType offset3 = {{-1, 0}};
NeighborhoodIteratorType::OffsetType offset4 = {{1, 0}};
NeighborhoodIteratorType::OffsetType offset5 = {{-1, 1}};
NeighborhoodIteratorType::OffsetType offset6 = {{1, 1}};

// It is equivalent to use the six corresponding integer array indices instead.
// For example, the offsets \code{(-1, -1)} and \code{(1, -1)} are
// equivalent to the integer indices \code{0} and \code{2}, respectively.
//
// The calculations are done in a \code{for} loop that moves the input and
// output iterators synchronously across their respective images.  The
// \code{sum} variable is used to sum the results of the finite differences.

for (it.GoToBegin(), out.GoToBegin(); !it.IsAtEnd(); ++it, ++out)
{
float sum;
sum = it.GetPixel(offset2) - it.GetPixel(offset1);
sum += 2.0 * it.GetPixel(offset4) - 2.0 * it.GetPixel(offset3);
sum += it.GetPixel(offset6) - it.GetPixel(offset5);
out.Set(sum);
}

// The last step is to write the output buffer to an image file.  Writing is
// done inside a \code{try/catch} block to handle any exceptions.  The output
// is rescaled to intensity range $[0, 255]$ and cast to unsigned char so that
// it can be saved and visualized as a PNG image.

using WritePixelType = unsigned char;
using WriteImageType = otb::Image<WritePixelType, 2>;
using WriterType     = otb::ImageFileWriter<WriteImageType>;

using RescaleFilterType = itk::RescaleIntensityImageFilter<ImageType, WriteImageType>;

RescaleFilterType::Pointer rescaler = RescaleFilterType::New();

rescaler->SetOutputMinimum(0);
rescaler->SetOutputMaximum(255);
rescaler->SetInput(output);

WriterType::Pointer writer = WriterType::New();
writer->SetFileName(argv[2]);
writer->SetInput(rescaler->GetOutput());
try
{
writer->Update();
}
catch (itk::ExceptionObject& err)
{
std::cout << "ExceptionObject caught !" << std::endl;
std::cout << err << std::endl;
return -1;
}

// The center image of Figure~\ref{fig:NeighborhoodExamples1} shows the
// output of the Sobel algorithm applied to
// \code{Examples/Data/ROI\_QB\_PAN\_1.tif}.
//
// \begin{figure} \centering
// \includegraphics[width=0.45\textwidth]{ROI_QB_PAN_1.eps}
// \includegraphics[width=0.45\textwidth]{NeighborhoodIterators1a.eps}
// \itkcaption[Sobel edge detection results]{Applying the Sobel operator to an image (left) produces $x$ (right)  derivative image.}
// \protect\label{fig:NeighborhoodExamples1}
// \end{figure}

return EXIT_SUCCESS;
}