> you could decide that for each ROI definition the first voxel index corresponds to the center voxel.
What if the "intended center" has been pruned?
> Well, these sparse masks are binary, and therefore in the current proposal we don't store the data associated with the voxels (why store only ones). Of course you could extend the idea to allow for storing data as well, but I'm afraid that will complicate things further.
My thought was that the data that describes the mask (i.e. the list of sparse coordinates) can be thought of as "data attached to the voxel". What I meant was that a program with a different purpose could use the format and store data that isn't a mask.
> Yes it would be, however why add this extra step if we can simply store this number at the beginning of the file?
The idea was to generalize the storage of any inhomogeneous data that is attached to voxel in a manner that is more general than the specific application of storing masks. Programs could easily reformat/extract spatial regions etc without understanding anything about what's actually represented in the data attached to each voxel. For example, it could also allow masked fMRI time-series data to be stored more efficiently without compression with each voxel's entire time series contiguously located in memory/disk.