Can I down-sample a larger tractogram to mimic a smaller one?



Hello MRtrix experts,

I have a large set of tracks that has been generated by applying tckgen -select 100M .... Now I want to have another smaller set, say one that has 10M tracks in it. Could I just run tckedit 100M.tck -number 10M 10M.tck or should I run tckgen again (ie apply tckgen -select 10M...)?

Thanks much,


Assuming you’re using a fully random seeding mechanism, then that’s fine: all streamlines are generated from random locations, there should be no systematic difference between the first and last 10M streamlines (or indeed any other arbitrary selection of 10M streamlines).

On the other hand, if you used a deterministic seeding strategy (i.e. -seed_random_per_voxel or -seed_grid_per_voxel), then there will be a systematic difference between earlier and later streamlines. Not too sure how to address that with the current functionality offered in tckedit, unfortunately…


Thanks for your response.
Is it true that the seeding strategies -seed_gmwmi and -seed_dynamic are random?


I’m pretty sure they are, but @rsmith would be better placed to confirm this…