can't iterate over simple image

Beginner Questions
1

I have a 3d boolean image and would like to compute the number of non zero voxels.

I wrote this simple code that is supposed to iterate over the whole image and update a counter variable.

using BoolImageType = itk::Image<bool, 3U>;
double computeVolumeFromBinaryImage(const BoolImageType::Pointer& a_Image)
{
	itk::ImageConstIterator<BoolImageType> it(a_Image, a_Image->GetRequestedRegion());
	it.GoToBegin();
	std::size_t numVoxels = 0;
	while (!it.IsAtEnd())
	{
		if (it.Get())
			numVoxels++;
		++it;
	}
	auto size = a_Image->GetSpacing();
	double voxelVolume = size[0] * size[1] * size[2];
	return voxelVolume * numVoxels;
}

But I get the following compiler error:
error C2675: unary '++' : 'itk::ImageConstIterator<BoolImageType>' does not define this operator or a conversion to a type acceptable to the predefined operator
This error points to ++it;

I get the same error if I remove the custom typedef.
I’m using visual studio if that helps.

Any idea ?

2

OK so after reading the documentation more in details I found out that itk::ImageConstIterator does not provide any means of moving the iterator. Same goes for itk::ImageIterator.

I was confused by the first documentation page that pops in google when searching for itk image iterator.
https://itk.org/Doxygen/html/ImageIteratorsPage.html
The code samples there, even though they do not represent full programs somehow indicates the use of ImageIterator with the ++ operator.

replacing itk::ImageConstIterator with ImageRegionConstIterator solved my issue.

this piece of code works for me:

using BoolImageType = itk::Image<bool, 3U>;
double computeVolumeFromBinaryImage(const BoolImageType::Pointer& a_Image)
{
	itk::ImageRegionConstIterator<BoolImageType> it(a_Image, a_Image->GetLargestPossibleRegion());
	std::size_t numVoxels = 0;
	for (it = it.Begin(); !it.IsAtEnd(); ++it)
	{
		if (it.Get())
			numVoxels++;
	}
	auto size = a_Image->GetSpacing();
	double voxelVolume = size[0] * size[1] * size[2];
	return voxelVolume * numVoxels;
}
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3

Thanks for following up your post with the solution!

4

The only thing worse than an orphan open question is when the author comes back and say “nevermind, fixed it” and gives no details.

I'm doing my part

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