Question around Particles and VDB

[Kirosana]

Hi everyone, i'm pretty new to Houdini and I'm a bit stuck right now.

I want to do a simulation of someone walking the snow using the PBD feature.

 

I have a simulation with 2.8 Millions particles (Upres from 300K particles) and I want to get rid of the "Particles" effec and get a render like this : https://vimeo.com/115714070

 

I understood that I need to use VDB to transform my particles, but I don't find the proper way to get this result. 

 

If anyone can guide me a bit or give some hints 

Thank you

I put the render i actually have, and the hip file, just in case.

Snow_Walk.hip

Edited July 23, 2015 by Kirosana

[ChristopherC]

You can convert particles to VDB using the `VDB From Particles` node.

 

But I'm a newbie myself and am not too sure what's the point of converting these particles into volumes? Is it used to blur the result?

[Kirosana]

Yes I know. I get the point of using Vdb From Particles while doing Flip Simulation, to have a mesh for the water etc.


But here I'm trying to get a volume from the particles, by using VDB From Particles, then Convert to with the volume option. But from there, i'm stuck and can't get anything on the render :/

I tried with Volume Rasterized Point with my Sim and the Convert To Volume fo the Base Volume Input, but still nothing.

Edited July 23, 2015 by Kirosana

[ChristopherC]

Apologies, I didn't check your scene before replying—you indeed already used this node!

 

Mantra do support rendering VDBs since Houdini 12.5 so it shouldn't be required to convert them to volume before rendering. This would anyways defeat a bit the purpose of using VDBs in the first place.

 

I can't play with your scene as I'm still using Houdini 13 and can't load the sand solver from your scene, so I made up a new scene instead that seems to render fine using only the `VDB From Particles` node. Feel free to check it out, but I believe that the only thing you're missing is to check `Fog VDB` in your `VDB From Particles` node instead of `Surface VDB`. `Fog` outputs a density field, while `Surface` is for level sets (SDF).

 

render_vdb.hipnc

[Kirosana]

Ok i got the point with your file. Thanks.

In mine, of course it doesn't work. I guess that the Voxel Size Attribut needs to be smaller than my particles PScale ( 0.016 ). ( I tried on a new scene with a smaller setup )

 

But If my voxel size is not 0.001, nothing appears in the viewport, and my Ram is exploding.

And in any case, nothing appears on the render (I put the same setup as your scene)

 

Really don"t know what i'm doing wrong 

Edited July 23, 2015 by Kirosana

[ChristopherC]

Shall I conclude that you now see something in the viewport with the help of the `Volume Visualization` node? If so, maybe try creating a Mantra PBR render node and assigning a basic volume shader to see what happens. That being said, I really don't know anything about rendering but these are things that I've done in my scene and that don't seem to be in yours

 

And to speed things up, maybe you can try deleting the bottom layers of particles by selecting the ones that have no velocity and that are located below a certain Y value?

[Kirosana]

Yep, I can see the volumein the viewport even without the Volume Visualisation (maybe a H14 thing) but nothing in the rendering (Mantra PBR or even RayTrace)

 

Everythning works perfectly with the basic PopSource as a source of particles. I don"t even need to put a volume shader, I can see the Fog VDB like this.

 

But when i'm using Grain particles and solver, and doing the same process. Nothing work. I can see something in the viewport but the rendering view is empty.




Edit : 

After tweaking some stuff here and here, I manage to visualize the VDB From Particle in the Render View (Mantra PBR).
I just had to low down the particle séparation from the grain solver.

Particle Separation : 0.012 ( Pscale of 0.006)
Voxel Size : 0.002



Apparently if the Particle Separation is too smal, the VDB node can't compute and don't use any particles since they're too small fore him. That way, i don't know how to use it with like half a million particle or more with an Upres simulation.

Edited July 23, 2015 by Kirosana

[Diego A Grimaldi]

I think the point of using VDB is to get a Mesh, not a Volume. Snow oftentimes looks like a clump of particles which is usually easier represented as mesh, at least it works in my mind, I haven't tested it yet.

 

Maybe try going that direction, that's how I would do it probably.