Is it true that the seeding strategies
-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.
-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.