[Insight-developers] How to add an index-only iterator?

[M Stauffer -V-]

Hi,

I'm working on the v4 metrics. Currently they use and Image object to
store virtual domain information, and to provide mapping methods such as
TransformPhysicalPointToIndex(). But this isn't very safe because I
don't allocate memory for the image in order to safe memory, and thus
the user can cause a bad access by inappropriately accessing the image
memory in some way.

I'd like to switch the virtual domain representation to use ImageBase.
But it's complicated by the fact that the metrics iterate over the
virtual domain in different ways (region iterator, random iterator and
neighborhood iterator in different cases). In all cases the iteration is
only performed to get virtual indecies, which are transformed into
physical points in fixed and moving domains, and then transformed into
indecies again to access valid memory in either fixed or moving data.

1) One option is to create new iterators that don't allow any image
memory access. So for example a new class
ImageRegionConstIteratorWithOnlyIndex. Mainly because the iterators rely
on being templated over an itkImage (they need the pixel type), I think
it's easisest and cleanest to create wrapper iterators for index-only
access. Such a wrapper would contain a separate iterator (not be derived
from it) and provide a limited interface that only allows memory-safe
operations.

Otherwise a parallel tree of index-only iterators would need to be
created, which seems overly redundant.

2) Another option is a new VirtualImage type derived from ImageBase that
has a pixel type to satisfy the iterators, but no memory and doesn't
have memory-access mehtods like GetPixel. It'd be tricky to work with
the current iterators w/out changing them in ways that would decrease
efficiency, because they directly de-ref the image memory buffer. Maybe
the iterator methods that directly or indirectly access memory could be
template-specialized to have do-nothing methods when used with a
VirtualImage type? Or they could throw an exception to alert the
programmer. But generally a compile-time error would be better if the
progammer tried to use the iterator incorreclty, which is what option 1
provides.

Any thoughts or techniques I'm missing? Thanks.

-M