I'm new to Houdini (Apprentice - though I used Prisms a bit many years ago) and I'm trying to get the Resistance Pop to work in a simple car-driving-thru-snow/rain system, but I can't figure out how to point it to the geometry/SOP to use for the calculations/object surface.

Can anybody point me to a tutorial or tell me how to do this?
Can I wire some geometry into the Pop from inside the popnet?

TIA,
- Ron

As I recall, the Resistance POP is used for calculating resistance based on objects instanced on the particle. For example, if we had falling leaves, we could make the leaves realistically fall to the ground by taking into account the air resistance against the leaves via the Resistance POP. The way the geometry is specified on the particles depends on having an Instance POP upstream to create the instance attribute first.

For your example, you might want to try using the Collision POP. Something like this:
- Append a Divide SOP to your geometry. This will by default triangulate your geometry which will allow it to be used with all of the Collision POP's collision methods. For performance, you will likely want to put it somewhere before your geometry is animating though so that it won't recook every frame.
- Append to this, a POP Network SOP. (Wire the output of the Divide SOP into the first input of your POP Network.)
- Dive down into the POP Network. (Click on it and hit Enter)
- Set up your particles here (or copy/paste the pops out of the other popnet you have).
- Append to your particle system now, a Collision POP. Set the Hint parameter to something appropriate. In this case, I would say, “Deforming Geometry”. I'm not quite sure anymore, but I think “Translating Geometry” might also work but try Deforming first anyways as that covers the most situations. On the Behaviour tab of the parameters, change the Behaviour to Slide on Collision.
- Put your display flag on the last pop, hit play. Tweak, go back to frame 1, play again.

For control for the “clinginess” of the sliding particles, you can append to the Collide, a Property POP and on the Misc tab, turn on Cling. Delete the channel and try upping its value.

Hope that helps.

Aha, thanks for the info. Too bad, I was hoping that was geometry based resistance for the particles. Oh well.

The collision isn't the look I need, I'm looking for the aerodynamic swirls around the body of a vehicle (and hopefully the turbulence in the wake). I'll have to figure out a more advanced system. Maybe the Attractor set to negative to repel? - Or a area based “switch” to a different set of forces?

Anyway, I'll keep playing. Thanks again for the info.

- Ron

Use a point sop to establish point forces on the surface you wish the particles to travel across. You can group points and assign specific forces to different areas of your surface.
Leading edge flow and trailing edge turbulence, etc.