[Insight-developers] itk::ImageRegionIterator::operator--() [ ReverseIterators]

[Miller, James V (CRD)]

To complicate our lives further, I decided to add reverse iterators to itk to see how they look.
Please take a look at their use to see if we would like to keep them in itk. I modified
Testing/Code/Common/itkImageIteratorTest.cxx to use ImageRegionIterators and
ImageRegionReverseIterators exercising both operator++ and operator--

I checked in:

itkImageReverseIterator
itkImageRegionReverseIterator

(can't decide whether the latter should be ImageRegionReverseIterator or ImageReverseRegionIterator)

Anyhow, ImageIterators are bidirectional (see earlier message) and ImageReverseIterators are
bidirectional and they walk a region backwards. So ImageRegionReverseIterator::Begin() returns an
iterator to the last pixel in the region and ImageRegionReverseIterator::End() returns an iterator
that points to one pixel before the region.

To walk a region forward, use an ImageRegionIterator like:

itk::ImageRegionIterator<itk::PhysicalImage<itk::Vector<unsigned short, 5>, 3> > itt(o3,
region);

it = it.Begin();
for (; !it.IsAtEnd(); ++it)
{
// do something at each iterator position.
}

To walk are region backwards, use an ImageRegionReverseIterator like:

itk::ImageRegionReverseIterator<itk::PhysicalImage<itk::Vector<unsigned
short, 5>, 3> > reverseIt(o3,
region);

reverseIt = reverseIt.Begin();
for (; !reverseIt.IsAtEnd(); ++reverseIt)
{
// do something at each iterator position.
}

Notice the only difference in the above code is which iterator is created.

I would suggest algorithms that scan forward through the entire region, then backward use a
combination of Iterators and ReverseIterators.

If an algorithm needs to revisit the previous pixel while scanning a region, both iterators have
defined operator--.




-----Original Message-----
From: Miller, James V (CRD)
To: 'insight-developers@public.kitware.com '
Sent: 6/23/2001 9:31 PM
Subject: [Insight-developers] itk::ImageRegionIterator::operator--()

At long last, I added operator--() to the ImageRegionIterator class.

The main hurdle in adding operator--() was what would be the syntax for
walking backwards through a
region.  

Luis had proposed that we replace the Begin()/End() routines with
GotoBegin()/GotoEnd()

I hestitate to use GotoBegin()/GotoEnd() because it changes the meaning
of what End is.  In STL,
iterators walk half-open intervals. begin() points to the first element,
end() points one element
past the end.  To avoid confusion, I think we should stick with the
half-open interval concept.
Currently, our Begin() points to the beginning of a region and End()
points one pixel past the end of
the region.

To walk forward through a region, we use a loop like:

it = it.Begin();
for ( ; !it.IsAtEnd(); ++it)
{
// do something at each iterator position
}


To walk backward through a region, we should use a loop like:

it = it.End();
do
{
--it;

//do something at each iterator position

}
while (!it.IsAtBegin());

This last loop assumes the region is not empty.

Another option is add "reverse iterators" to itk.  A reverse iterator
for a region would have Begin()
return an iterator to the last pixel in the region and End() return an
iterator one pixel before the
first pixel in the region.  operator++ and operator-- would have the
reverse semantics.  To walk a
region backwards using reverse iterators, the code would look like:

itk::ImageReverseRegionIterator<itk::PhysicalImage<itk::Vector<unsigned
short, 5>, 3> > reverseIt(o3,
region);

reverseIt = reverseIt.Begin();
for (; !reverseIt.IsAtEnd(); ++reverseIt)
{
// do something at each iterator position.
}





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