Hi @Ingo,
The issue here is that the main input file that needs to be provided to tckgen can change depending on the particular tracking algorithm being invoked; this is listed in the command’s help page.
So when you provide file %s_fod.mif to tckgen -algorithm fact, which is an image containing spherical harmonics, most likely of maximal harmonic order 8 (45 volumes), the FACT algorithm is mis-interpreting these data as 15 triplets of XYZ fibre directions in each voxel. This will inevitably lead to erroneous tracking (I’m guessing the images have been mis-labeled: the upper erroneous image is FACT, whereas the lower one is SD_Stream). tckgen is unable to detect the fact that you’ve provided the incorrect image since it actually satisfies the requirements for that algorithm (number of input volumes must be a multiple of 3).
To get tckgen -algorithm fact working, you can use either sh2peaks, or fod2fixel combined with fixel2voxel split_dir.
Cheers
Rob