Thanks, Mark, for starting this conversation.

One thing that I think would be very useful would be support for a sparse data representation within the data part of NIfTI files - even if it's just using simple (location, value) tuples. Many images are moderately sparse, particularly when they represent the result of some kind of segmentation, and some are very sparse (FSL-TBSS skeletons are a good example). Even if the data are stored in a sparse array for processing, the intermediate step of reading in the full image data can put unnecessary demands on system memory, all for the sake of reading in and then disregarding huge numbers of zeroes.

I'd be interested in others' thoughts on whether this would be useful, too.

All the best,
Jon