Pop Advect By Volume Problems

[GlennimusPrime]

With Pop Advect By Volume inside a POPnet, when particles reach the edge of the volume / the volume bounds, they lose all velocity and just stop.


I recall seeing a video somewhere of someone who builds a better Pop Advect By Volume node, by diving inside and setting the particle velocity to continue, rather than just hitting zero.
Does anyone know the video I'm talking about? I cannot find it anywhere.

Or does anyone have any ideas on how to implement this technique?

[tamagochy]

You can detect by volume sample is particles inside volume and advect them

[GlennimusPrime]

Unfortunately vops knowledge is fairly limited. I somewhat understand the process but would get definitely get stuck.

Would that entail first comparing whether the particle is inside or outside of the volume, then working out the vector the particle was last using while still inside the volume and lastly transferring that vector to the velocity of the particle if it were no longer inside the volume?

[GeneralLethal]

You can store the velocity pre-advection in a different attribute each frame, and copy it back to v post-advection if the volume sample turns out negative.

[bumper]

Hello I guess you are talking about this video from BlueZi : 

 

Edited August 2 by bumper

[tamagochy]

Well instead of converting VDB to polygons better use volume gradient

[roastedabc]

That's a fascinating observation, GlennimusPrime. I remember encountering a similar issue with particles losing velocity at volume bounds. Implementing a custom Pop Advect By Volume node to maintain particle velocity sounds like a promising solution. Unfortunately, I'm not familiar with the specific video you mentioned, but exploring this technique further could yield great results. Have you considered experimenting with custom VEX expressions to control particle behavior at the volume edges? It might offer more flexibility in achieving the desired effect.

[GlennimusPrime]

On 8/3/2025 at 4:37 AM, bumper said:

Hello I guess you are talking about this video from BlueZi : 

Apologies I missed your post here @bumper This is a great solution which will help me out when I need this again next time.

I'm still unable to find the original video, but I recall the guy unlocking the pop advect by volumes node and doing something very simple like comparing if the particle is outside of the smoke volume, then continuing it's velocity in the direction it was already travelling (something like that, but not entirely sure). Seems like very simple solution rather than converting the VDB, comparing if it's under a certain speed, then pushing it back toward the vel volume.
 

10 hours ago, roastedabc said:

That's a fascinating observation, GlennimusPrime. I remember encountering a similar issue with particles losing velocity at volume bounds. Implementing a custom Pop Advect By Volume node to maintain particle velocity sounds like a promising solution. Unfortunately, I'm not familiar with the specific video you mentioned, but exploring this technique further could yield great results. Have you considered experimenting with custom VEX expressions to control particle behavior at the volume edges? It might offer more flexibility in achieving the desired effect.

I would really prefer a VEX solution, as that is my usual go-to when wanting to control anything in Houdini. VEX just seems to click more for me than VOPs.

 

On 7/26/2025 at 7:20 AM, GeneralLethal said:

You can store the velocity pre-advection in a different attribute each frame, and copy it back to v post-advection if the volume sample turns out negative.

@GeneralLethal You pretty much hit the nail right on the head there. That's exactly what I was wanting to do. Although I prefer VEX over VOPs my knowledge is still a little limited in these areas.
I'm not currently in need of a solution to this at this time, but the need will undoubtedly arise again some time soon. I will see if Chat GPT can give me a hand writing some VEX to implement this.

Edited Wednesday at 08:49 PM by GlennimusPrime