The streamline tracking allows to set a region from which the seeding points are drawn, however, only an image of equal size as the respective tensor image is accepted. This restriction can be dropped and hence allow seeding from some greater mask (i.e. parcellation image).

Currently, the itk::StreamlineTrackingFilter::ThreadedGenerateData([...]) iterates over the seed image, but this can be iterate over the mask image and the seeding value is extracted from the arbitrarily-sized image via world-coordinates.