Example usage:

./SIFTDensityExample Input/suburb2.jpeg Output/SIFTDensityOutput.tif Output/PrettySIFTDensityOutput.png 3 3 7

Example source code (SIFTDensityExample.cxx):

#include "itkPointSet.h"
#include "itkVariableLengthVector.h"
#include "otbImage.h"
#include "otbImageFileReader.h"
#include "otbImageFileWriter.h"
#include "itkUnaryFunctorImageFilter.h"
#include "itkRescaleIntensityImageFilter.h"

// This example illustrates the use of the
// \doxygen{otb}{KeyPointDensityImageFilter}.
// This filter computes a local density of keypoints (SIFT or SURF,
// for instance) on an image and can
// be useful to detect man made objects or urban areas, for
// instance. The filter has been implemented in a generic way, so that
// the way the keypoints are detected can be chosen by the user.
// The first step required to use this filter is to include its header file.

#include "otbKeyPointDensityImageFilter.h"
#include "otbImageToSIFTKeyPointSetFilter.h"

int main(int itkNotUsed(argc), char* argv[])
  const char*        infname     = argv[1];
  const char*        outfname    = argv[2];
  const char*        prettyfname = argv[3];
  const unsigned int scales      = atoi(argv[4]);
  const unsigned int octaves     = atoi(argv[5]);
  const unsigned int radius      = atoi(argv[6]);

  const unsigned int Dimension = 2;
  using PixelType              = float;

  // As usual, we start by defining the types for the images, the reader
  // and the writer.

  using ImageType  = otb::Image<PixelType, Dimension>;
  using ReaderType = otb::ImageFileReader<ImageType>;
  using WriterType = otb::ImageFileWriter<ImageType>;
  // We define now the type for the keypoint detection. The keypoints
  // will be stored in vector form (they may contain many descriptors)
  // into a point set. The filter for detecting the SIFT is templated
  // over the input image type and the output pointset type.

  using RealVectorType = itk::VariableLengthVector<PixelType>;
  using PointSetType   = itk::PointSet<RealVectorType, Dimension>;
  using DetectorType   = otb::ImageToSIFTKeyPointSetFilter<ImageType, PointSetType>;
  // We can now define the filter which will compute the SIFT
  // density. It will be templated over the input and output image
  // types and the SIFT detector.

  using FilterType = otb::KeyPointDensityImageFilter<ImageType, ImageType, DetectorType>;
  // We can instantiate the reader and the writer as wella s the
  // filter and the detector. The detector needs to be instantiated in
  // order to set its parameters.

  ReaderType::Pointer   reader   = ReaderType::New();
  WriterType::Pointer   writer   = WriterType::New();
  FilterType::Pointer   filter   = FilterType::New();
  DetectorType::Pointer detector = DetectorType::New();
  // We set the file names for the input and the output images.

  // We set the parameters for the SIFT detector (the number of
  // octaves and the number of scales per octave).

  // And we pass the detector to the filter and we set the radius for
  // the density estimation.

  // We plug the pipeline.

  // We trigger the execution by calling th \code{Update()} method on
  // the writer, but before that we run the
  // \code{GenerateOutputInformation()} on the reader so the filter
  // gets the information about the image size (needed for the SIFT
  // computation).


  // Figure~\ref{fig:SIFTDENSITY_FILTER} shows the result of applying
  // the key point density filter to an image using the SIFT
  // detector. The main difference with respect to figure
  // \ref{fig:EDGEDENSITY_FILTER} is that for SIFTS, individual trees
  // contribute to the density.
  // \begin{figure}
  // \center
  // \includegraphics[width=0.25\textwidth]{suburb2.eps}
  // \includegraphics[width=0.25\textwidth]{PrettySIFTDensityOutput.eps}
  // \itkcaption[SIFT Density Filter]{Result of applying the
  // \doxygen{otb}{KeypointDensityImageFilter} to an image. From left
  // to right :
  // original image, SIF density.}
  // \label{fig:SIFTDENSITY_FILTER}
  // \end{figure}

  /************* Image for printing **************/

  using OutputImageType = otb::Image<unsigned char, 2>;

  using RescalerType = itk::RescaleIntensityImageFilter<ImageType, OutputImageType>;

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



  using OutputWriterType              = otb::ImageFileWriter<OutputImageType>;
  OutputWriterType::Pointer outwriter = OutputWriterType::New();


  return EXIT_SUCCESS;