Particles with fixed number of particle

[archo_p]

Total noob to dynamics in Houdini.  I've been trying to wrap my head around Vellum as it seems designed for this need ( fixed number of dynamic elements ) but I really just want a particle system that starts with a fixed number of particles, no birth, no death, use forces with turbulence to get them to a collider and use the Stop collision behavior to stop them in their tracks upon collision.  I can do this with a simple emitting POP setup, but i need a fixed number of particles.  Vellum grains seems overly heavy for what I need ( don't care about per particle interaction right now) and I can't for the life of me find any good example of getting vellum grains to have their stopped attribute set to 1 ( POP collision detect doesn't seem to effect them?).  So primarily I'd love to just see how to setup a fixed number of POP particles, but as well wouldn't mind being shown how one could stop vellum grains from getting dynamics applied after hitting a collider.  Finally, in either case, how or can you specify one collider that allows a bounce, while another has a stop behavior.  In general I just haven't really gotten my head around DOPs so trying to understand.

 

For now just attaching my basic emitted particles that get stopped by using a POP collision detect.

 

Thank you!

 

P

stoppingParticles.hiplc

[Noobini]

if(@hitnum >0) @stopped = 1 ?

(or even @mass = 0 ?)

ps: ahh i just re read your requirements...you want different behaviours for different colliders....dunno.

Edited April 9, 2020 by Noobini

[Noobini]

here's a setup for you to dissect...thanks to @animfor telling me to unlock the vellumsolver the other day.

01) hit play, the grains will bounce on the grid, inside the solver i told them to stop, but they won't....however they WILL on hitting the ground!

the grid is a surface, while the ground as old school Geoff Wagner told me in a video is actually a 3D volume.

02) now connect the 2nd (not 1st) output of the collisionsource1 to vellum collision, taking over from the grid.

03) hit play, the grains fall right thru the box (they still stop on hitting the ground which is good)

04) allow editing of the vellumsolver, dive in, find dopnet, dive in, find static object, collision detection is currently on solver default , change it to use volume

05) hit play, voila now the grains stop dead on hitting the box !

ie. grains don't like surface collisions, except for the default ground cos that is actually a volume.

vu vellumgrainstest.hipnc

Edited April 9, 2020 by Noobini

[archo_p]

Cool thanks so much Noobini!  The step by step is great and worked.  That makes sense then to think more about volumes for collisions for the grains.  I'm still quite confused about how unlocking that vellum solver turns it into a completely different network from what was in there previous to unlocking, the collision behavior and null that were in there are completely gone after that move, i get how there might be hidden nodes, but totally don't get how unlocking the node seems to "remove" the popcollisionbehavior and force_output null, and re-matching the definition  brings them back.  curious

I guess I need to find a more thorough description of how dynamics in general work because it seems so unintuituitive compared to other workflows in houdini ( sops, cops, etc) but i bet if I just got a real rundown I could be better equipped to experiment, anyone know of a good place to start?  Just seems like there are so many different ways to setup DOPs.

As well I'm still interested in finding a way to just have a fixed number of simple POP particles, so will continue hunting there.

Thanks again!

p

[anim]

11 minutes ago, archo_p said:

  I'm still quite confused about how unlocking that vellum solver turns it into a completely different network

it doesn't, it will just ignore the Dive Target so instead of diving deep into particular node it will just dive one level at a time so you will see levels that were skipped before (personally I find it annoying that Dive Target is suddenly ignored)

pay attention to path bar on your Network View, you'll see that with solver locked you are diving in 3 levels

obviously unlocking is necessary only if you want to modify some nodes that are otherwise not editable

[anim]

19 minutes ago, archo_p said:

As well I'm still interested in finding a way to just have a fixed number of simple POP particles, so will continue hunting there.

emit only on a single frame and use All Points emission type

Edited April 9, 2020 by anim

[Noobini]

just an addendum:

 say you created a box for use as collision, you then search on the shelf in either vellum or collisions, they give you only surface collider

so you just make do...click it and then go in to DOP and change it to volume

but then....I found this....

go to the FEM shelf, there is a volume Collider....happy days.

[archo_p]

Thank you @anim now i see the network path and dive target field in Type Properties and did figure out putting if($F == 1, 1, 0) into the Impulse Activation parameter of POP Source to get one blast of particles.  I'm coming from TouchDesigner land, which doesn't have nearly as complex a way of doing external assets, so getting used to plethora of options and methods!