Control POP Advect by Volume "Air Resitance" Values by Volume Density... VEXpression Maybe?

[hayden martin]

A noob grains upres question here…

 

I created a massive fracturing grains sim with Vellum at a low-ish resolution, and I'm using the grains' velocities to drive smaller dumber particles.

 

When using the POP Advect by Volume, the particles are pushed around correctly as expected, but when they exist the Velocity field they slow down dramatically almost to a halt. This is because I am using "Treat as Wind" with very high "Air Resistance" values, to counteract the Gravity force.

How can I sample the Density of the same volume, and use that to influence the "Air Resistance"? This way the core of the volume will stick to the Velocity, and as we get further away from the core, the Velocity will have less and less influence, with gravity taking over...

 

I spent hours on this yesterday, trying to find a way to bring the volume Density into the DOPNet as a sort of falloff for the POP Advect by Volume node…

 

I even tried transferring the volume density to the Grain source points in SOPs as a float value, hoping I could hook strength data out of them that way, but again, just not luck at all!

 

I’m aware this must be incredibly simple to do, but I just cannot figure it out at all. There doesn’t seem to be any kind of masking options available in the nodes I’m using.

One kind Facebook user on the Houdini Artists group told me this VEXpression could be pasted into the POP Advect by Volume node:

@airresistance *= fit(length(volumevelocity), 0, value, 0, 1);

value = the speed value where you want the resistance to be full.

 

 

However, this is not referencing density, and so far, isn't working anyway - but I get the idea.... Can anyone advise on the best steps to do this?

Edited April 24 by hayden martin

[hayden martin]

I've created a HIP that reproduces the problem in a more manageable way

UPRES.hip

[hayden martin]