[ITK Community] [Insight-users] MultipleImageIterator

[Matt McCormick]

Hi Joel,

Sounds like an interesting topic for an Insight Journal article [1].

An challenge is to handle IsAtEnd().  In your example !it[0].IsAtEnd()
is applied, which may be appropriate for some cases.  In other cases,
the iterators may complete at different points. Something to think
about ...

Thanks,
Matt

[1] http://insight-journal.org/

On Thu, Feb 6, 2014 at 5:54 AM, Joël Schaerer <joel.schaerer at gmail.com> wrote:
> 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
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