[Insight-users] MultipleImageIterator

[Joël Schaerer]

Hi all,

Recently I've had to iterate over many images at the same time. Instead 
of mixing the tedious iterator housekeeping code with the algorithm, I 
thought it could be a good idea to have a "multiple image iterator" for 
that. Since I didn't find one in ITK, I started writing a simple one 
myself. Its main limitation is that all iterators and thus all images 
must be of the same type:

namespace itk {
template<typename TIterator>
class MultipleImageIterator {
public:
   typedef MultipleImageIterator Self;
   typedef TIterator IteratorType;
   typedef typename IteratorType::ImageType ImageType;
   IteratorType& operator[](const int i) {return m_iterators[i];}
   void AddIterator(const IteratorType& it) {m_iterators.push_back(it);}
   Self& operator++() {
     for (typename std::vector<IteratorType>::iterator it = 
m_iterators.begin();
       it != m_iterators.end(); ++it) {
       ++(*it);
     }
   }
   void GoToBegin() {
     for (typename std::vector<IteratorType>::iterator it = 
m_iterators.begin();
       it != m_iterators.end(); ++it) {
       it->GoToBegin();
     }
   }
   unsigned int Size () const { return m_iterators.size(); }
protected:
   std::vector<IteratorType> m_iterators;

};
}

Here is a pretty straightforward usage example, which prints the values 
of 4 images side by side:

int main()
{
   typedef itk::Image<float,3> ImageType;
   typedef itk::ImageFileReader<ImageType> ReaderType;

   typedef itk::ImageRegionIterator<ImageType> IteratorType;
   itk::MultipleImageIterator<IteratorType> it;

   std::string filenames[] = 
{"originalT1.mha","csf_reg.nii.gz","grey_reg.nii.gz","white_reg.nii.gz"};
   std::vector<ImageType::Pointer> images; // Need to keep a reference 
as iterators only have weak references
   ReaderType::Pointer r = ReaderType::New();
   for (unsigned int i=0; i<4;++i) {
     r->SetFileName(filenames[i]);
     r->Update();
     ImageType::Pointer im = r->GetOutput();
     im->DisconnectPipeline();
     images.push_back(im);
it.AddIterator(itk::ImageRegionIterator<ImageType>(im,im->GetLargestPossibleRegion()));
   }

   for (it.GoToBegin(); !it[0].IsAtEnd(); ++it) {
     if ((it[1].Get() != 0) && ((float)std::rand()) / RAND_MAX < 0.01) {
       for (unsigned int i=0; i<it.Size(); ++i) {
         std::cout << it[i].Get() << ";";
       }
       std::cout << std::endl;
     }
   }
}


Any comments? What is the usual way of doing this with ITK? If there is 
any interest, this could be improved and potentially included in the 
library.

joel