WikiExamples/ImageProcessing/ManuallyRemovingLabels.cxx

#include "itkImage.h"
#include "itkImageFileWriter.h"
#include "itkImageRegionIterator.h"
#include "itkBinaryImageToLabelMapFilter.h"
 
using ImageType = itk::Image<unsigned char, 2>;
static void CreateImage(ImageType::Pointer image);
 
int main(int, char *[])
{
  ImageType::Pointer image = ImageType::New();
  CreateImage(image);
 
  using BinaryImageToLabelMapFilterType = itk::BinaryImageToLabelMapFilter<ImageType>;
  BinaryImageToLabelMapFilterType::Pointer binaryImageToLabelMapFilter = BinaryImageToLabelMapFilterType::New();
  binaryImageToLabelMapFilter->SetInput(image);
  binaryImageToLabelMapFilter->Update();
 
  // The output of this filter is an itk::LabelMap, which contains itk::LabelObject's
  std::cout << "There are " << binaryImageToLabelMapFilter->GetOutput()->GetNumberOfLabelObjects() << " objects." << std::endl;

  std::vector<unsigned long> labelsToRemove;
  
  std::cout << "There are originally " << binaryImageToLabelMapFilter->GetOutput()->GetNumberOfLabelObjects() << " objects." << std::endl;
  
  // Note: do NOT remove the labels inside the loop! The IDs are stored such that they will change when one is deleted.
  // Instead, mark all of the labels to be removed first and then remove them all together.
  for(unsigned int i = 0; i < binaryImageToLabelMapFilter->GetOutput()->GetNumberOfLabelObjects(); i++)
    {
    // Get the ith region
    BinaryImageToLabelMapFilterType::OutputImageType::LabelObjectType* labelObject = binaryImageToLabelMapFilter->GetOutput()->GetNthLabelObject(i);
    //std::cout << "Region " << i << " has " << labelObject->Size() << " pixels." << std::endl;
  
    // Mark every other label to be removed
    if(i%2 == 0)
      {
      labelsToRemove.push_back(labelObject->GetLabel());
      }
    }
    
  std::cout << "Removing " << labelsToRemove.size() << " objects." << std::endl;
  // Remove all regions that were marked for removal.
  for(unsigned long i : labelsToRemove)
    {
    binaryImageToLabelMapFilter->GetOutput()->RemoveLabel(i);
    }

  std::cout << "There are " << binaryImageToLabelMapFilter->GetOutput()->GetNumberOfLabelObjects() 
            << " objects remaining." << std::endl;
  
 
  return EXIT_SUCCESS;
}
 
void CreateImage(ImageType::Pointer image)
{
  // Create a black image with a white square
  ImageType::IndexType start;
  start.Fill(0);
 
  ImageType::SizeType size;
  size.Fill(20);
 
  ImageType::RegionType region(start,size);
  image->SetRegions(region);
  image->Allocate();
  image->FillBuffer(0);

  // Make line of non-touching diagonal pixels
  for(unsigned int i = 0; i < 20; i+=2)
    {
    itk::Index<2> pixel;
    pixel[0] = i;
    pixel[1] = i;
    image->SetPixel(pixel, 255);
    }
}