Curve u bias to aid Vellum sims.
download: http://www.vusta.com.au/Houdini.htm
my page is being a biatch with updating the link....must be no one in the office....be patient.
For density usage, please see Masterclass tute from John Lynch:
https://www.sidefx.com/tutorials/vellum-constraint-features-john-lynch-h18-masterclass/