itk Get Image Region Methods

* LargestPossibleRegion is the total size of the image

* BufferedRegion is the portion of the image that is
                  currently loaded in memory
* RequestedRegion is the portion that the pipeline
                  request from a filter at the moment
                  of execution.


If you are writing a filter, you should only generate the
RequestedRegion.

If you are planning to visit pixels with Image Iterators you should use
*at most* the BufferedRegion

If you are computing features from the entire image geometry, then you
should use the LargestPossibleRegion.