random vellum rest length in houdini core

 To randomise the stretch rest length value on initialisation (not dynamic, it can't be animated like this). make a primitive wrangle between the vellum hair constraint and the solver.

I've used a connectivity node set to Primitives and changed the "class" attrib to "hairID". Here we affect the restlength value by multiplying it by some remapped randomness.

f@restlength*=fit01(chramp("remap",rand(@hairID)),chf("min"),chf("max"));

un pinning vellum constraints

in a pop wrangle, inside the vellum solver, do something like this

 if(@Frame>1175){i@stopped=0;}

as far as i know it works on Pin constraints, set to "stopped".

this vid from Sidefx explains it towards the end, sort of.

https://youtu.be/t_lyBo1U1t8

vellum hair thickness

To randomise vellum hair thickness, you use pscale values. To get Vellum to recognise these values, UNTICK the Thickness box in the Vellum Constraints node.