Is it true that the seeding strategies `-seed_gmwmi`

and `-seed_dynamic`

are random?

“Random” is not quite the right word to be using here. What’s requisite for the streamlines extracted via `tckedit -number`

to be a representative subset of the larger tractogram is for the streamlines to be *order-independent*.

This *is* the case for `-seed_gmwmi`

, since each streamline seed point is derived entirely independently of any other tractogram information.

While `-seed_dynamic`

contains stochasic elements, and hence could be described as “random”, it is *not* order-independent: the probability of seeding from each fixel constantly evolves over time as the tractogram is generated; initially, there is no track density information, and so seeding behaves comparably to the `-seed_image`

option, but as the tractogram gets increasingly dense, the probabilities of seeding in fixels densely reconstructed vs. sparsely reconstructed diverges. So the seed locations of streamlines in the latter part of the tractogram are *not* independent of those streamlines that were generated in the earlier part of the tractogram, and thus using `tckedit -number`

is not going to be an entirely representative subset. Combining it with the `-skip`

option to sample a batch of streamlines from later in the tractogram may be more representative, but there are no guarantees.