It’s really difficult to estimate a timeline for the fixel ROI capability, since it’s one component of a larger set of changes that’s just one of many development branches I have going at once. I wouldn’t be holding your breath…
If you wanted to code something up to get yourself by, the best place to start would be the code in
cmd/fixel2tsf.cpp. You’d need to modify this to select streamlines based on the input fixel data file, and output the selected streamlines to a track file (there’s a class in
src/dwi/tractography/file.h for performing the output).