deleting/killing particles by bounding box, but with threshold and/or falloff

Forums Technical Discussion deleting/killing particles by bounding box, but with threshold and/or falloff

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druitre: Member; 63 posts; Joined: May 2006; Offline

Aug. 18, 2017 7:30 a.m.

Hi,

as the title says… I'm coming from Softimage, where a ‘delete by volume’ node has options for a falloff range. How do I set up a ‘delete by bounding box’ in Houdini so that the deletion will fall off gradually?

Thanks,
Jasper


Enivob: Member; 2534 posts; Joined: June 2008; Offline

Aug. 18, 2017 9:49 a.m.

Particle deletion is a boolean state. You can't have something partially deleted. The particle either exists or it does not. So falloff is irrelevant. You may want to look into scaling a particle to 0 to implement some kind of falloff.

Using Houdini Indie 20.0
Windows 11 64GB Ryzen 16 core.
nVidia 3050RTX 8BG RAM.


patar: Member; 280 posts; Joined:; Offline

Aug. 18, 2017 11:10 a.m.

i think he means like when the particle entered a region it is not dead but entered a group….and from that group particles is selected randomly to die…

i think its very simple to do in houdini

like this?

Edited by patar - Aug. 18, 2017 11:10:44

Attachments:
randomKill.JPG (23.5 KB)
BoundingWithRandomKill.hip (222.3 KB)


druitre: Member; 63 posts; Joined: May 2006; Offline

Aug. 18, 2017 11:12 a.m.

Maybe the terminology is different, its a percentage thing. 100% of the particles get deleted within the bounding box, 0% a given distance from that. Halfway, 50%. Etc.


druitre: Member; 63 posts; Joined: May 2006; Offline

Aug. 18, 2017 11:14 a.m.

patar
i think he means like when the particle entered a region it is not dead but entered a group….and from that group particles is selected randomly to die…

i think its very simple to do in houdini

like this?

That might do it, looks like it's a deletion over time?

Thanks!
Jasper


patar: Member; 280 posts; Joined:; Offline

Aug. 18, 2017 11:14 a.m.

not over time once it enter a region it belongs to a group…
and every frames 12.5% of each particle in that group gets deleted…
or rather each particle in that group has a chance 12.5% of dying..

please check the hip file

Edited by patar - Aug. 18, 2017 11:18:28


druitre: Member; 63 posts; Joined: May 2006; Offline

Aug. 18, 2017 12:08 p.m.

Yep, I checked, it happens each frame, so over time. What I'm looking for is probably not a ‘kill’ but a ‘delete’ - it has to work with a cached input. Not as a DOP, but I guess as a SOP (new to Houdini, so perhaps not using the right terminology)

Edited by druitre - Aug. 18, 2017 12:14:49

Attachments:
ScreenShot001.jpg (218.6 KB)


old_school: Staff; 2540 posts; Joined: July 2005; Offline

Aug. 18, 2017 12:23 p.m.

But you could have an attribute that accumulates over time based on the density of the volume. Then cull when that accumulated attribute reaches 1. Is that kinda what delete by volume node does in Soft?

Does the attached file do what you want? I created a couple POP wrangle nodes that can be wrapped up in to an HDA if you wish.

Attachments:
reap_particles_by_volume_weight.hip (605.9 KB)

There's at least one school like the old school!


old_school: Staff; 2540 posts; Joined: July 2005; Offline

Aug. 18, 2017 12:28 p.m.

Just saw your post with the ramp weight snapshot in Softimage. Yes you can add a ramp parameter to the density accumulator scalar as well. I just set it to 0.1 I think.

Nothing stopping you from querying the density from the volume and using a transfer function (ramp) fed by a fit01() to remap the accumulator multiplier.

I chose to use a blur on the volume itself to set the volume weight blur. It's the most lightweight method and done all the time to have volumes as masks. Lots of volume tools to modify the weights as well.

Nice to have Houdini support volumes natively.

There's at least one school like the old school!


old_school: Staff; 2540 posts; Joined: July 2005; Offline

Aug. 18, 2017 12:30 p.m.

Could also add probability as well depending on the weight if you want this to happen on a specific frame. A rand() function with a weight bias would work nicely to fuzzy up setting of the dead == 1 line.

There's at least one school like the old school!


patar: Member; 280 posts; Joined:; Offline

Aug. 18, 2017 12:39 p.m.

try this one

Attachments:
boundingFalloff.JPG (158.6 KB)
BoundingWithFalloff.hip (91.6 KB)


druitre: Member; 63 posts; Joined: May 2006; Offline

Aug. 18, 2017 12:53 p.m.

patar
try this one

Ha! Yes, that's it. I'm gonna have to scrutinize over this one to figure out what you've done but it does what the Softimage equivalent does, and then some.

Very cool, thanks a lot!
-Jasper


patar: Member; 280 posts; Joined:; Offline

Aug. 18, 2017 12:54 p.m.

you are welcome…


druitre: Member; 63 posts; Joined: May 2006; Offline

Aug. 18, 2017 1:04 p.m.

@Jeff, I looked at your example and I think that one also works only as a simulation (so over time) whereas I want it to be unsimulated/instant. Thanks though, as I can learn from it.

Just to elaborate, here's another snapshot, from the delete by volume node in Soft - how it works inside. I think some of it corresponds to what Patar did. Am I right to assume that in H, you have to use volumes/VDBs to do this type of thing?:

Attachments:
ScreenShot002.jpg (1.0 MB)


patar: Member; 280 posts; Joined:; Offline

Aug. 18, 2017 1:06 p.m.

nah vdb is just faster ways of doing volume…but you can also use houdini standard volume..


druitre: Member; 63 posts; Joined: May 2006; Offline

Aug. 18, 2017 1:33 p.m.

Hi Patar, when i translate the box in your scene, the effect disappears. Why is that, and how do i fix it? I'm trying to find out but no success so far…

- I can move the VDB, but not the input object - for a lot of purposes, it would be very useful if it could be the object that moves.

Edited by druitre - Aug. 18, 2017 13:39:22


patar: Member; 280 posts; Joined:; Offline

Aug. 18, 2017 1:40 p.m.

to transform it you have to do it the right way

Attachments:
Capture.JPG (22.3 KB)


druitre: Member; 63 posts; Joined: May 2006; Offline

Aug. 18, 2017 1:55 p.m.

Nope. Did that. The area of effect grows/shrinks when I move, but no movement of the area.


patar: Member; 280 posts; Joined:; Offline

Aug. 18, 2017 1:57 p.m.

this? hit play

Edited by patar - Aug. 18, 2017 13:58:02

Attachments:
BoundingWithFalloff2.hip (94.7 KB)


druitre: Member; 63 posts; Joined: May 2006; Offline

Aug. 18, 2017 2:10 p.m.

I see what's happening - my scene is on a vastly bigger scale, I'm working with a couple of kilometers. If I scale up the box x 1000, the falloff range diminishes to near nothing. If I scale up the voxelsize x 1000, I keep the falloffrange but then when i move the box, it falls in between voxels so disappears. (If that is clear).

I'd have to increase the falloff range x 1000 as well to have it work, Band doesn't help either. I'll try some more.

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