Riverbed flooding animation with debris

[ecarlsen]

Hello! Trying to figure out how best to tackle an animation with muddy silty water flooding a dry riverbed, and carrying with it lots of branches, debris, and sediment. Something like around 3:20 in this video, but much smaller scale with less debris in the water: https://www.youtube.com/watch?v=oWHvi_tW-rg

I'm using PFD 3.x for Maya and don't do fluid sims that often, so am hoping there's a simple-ish approach. What I'm stumped by is how to get debris to move with the muddy water and keep pace with it, like how in the linked video the debris and water seem to be one single entity. An initial thought was to set up a MASH bullet sim for a bunch of debris getting pushed through the riverbed channel, make an alembic out of that, then run a PFD water sim through the same channel, having it collide with the alembic bullet sim. Or, if the bullet sim isn't working well, I'd hand animate a bunch of debris so it's not as dense and I have more control. But it will be hard to match the pace, and I'm stumped on how to get the PFD sim's material to not look too clean/smooth.

Any thoughts on this, or other ideas/approaches I should consider? Are there ways to "attach" things to an existing PFD water sim, particularly a material with image textures (i.e. diffuse, normal, displacement, etc.) that moves and deforms with the sim?

Thanks in advance for your help!

[georgi.zhekov]

Hey,

For such shots we have added the Active Bodies in Phoenix 4. You can make objects float and be carried away by flooding waters. Here we prepped a simple guide on you can make a ship float - https://docs.chaosgroup.com/display/...e+Bodies+Guide
The workflow for floating debris should be exactly the same
We have also added support for TexUVW channel which can transport the uvs with the mesh so that you can have sticky textures over the mesh. You can find more about the TexUVW here - https://docs.chaosgroup.com/display/...-TexUVWControl

My suggestion is to get a Phoenix 4 trial and give these a shot.

Sadly there is no easy way to that with Phoenix 3. You can set your mesh type to Maya mesh from the Rendering rollout > Mesh options of the simulator and then use this mesh to instance some objects over it with MASH (though as the liquid mesh has changing number of vertices the mash instances would flicker each frame).
The other option would be to use nParticles. What you can do is go to the Outliner and under the Display options disable the checkbox for DAG objects only. Then you will be able to see the Phoenix particle groups. You can select the one for the liquid particles - it would be called something like PG_PhoenixFDSimulator1_Liquid_0. With it selected you can turn on Create nParticles from this system. This will create nparticles from your liquid particles and now you can instance objects over them, though this is not really flexible approach.

For the muddy waters you can try this workflow here - https://phoenixfd.cgrecord.net/2018/...hoenix-fd.html
The article shows how to do it in 3ds Max but it should work exactly the same in Maya.

Hope this helps

[ecarlsen]

Thanks so much, Georgi! Both for the Phoenix 4 info and for sharing some possible workarounds in the old version that we have. You guys are doing amazing work - the Active Bodies and TexUVW features seem perfect for what we're trying to do. I have a few follow-up questions that I'm hoping you can help with:

1. Does the Active Bodies approach allow for other collisions of the active bodies, beyond just reacting to the liquid? For example, in the floating ship example you shared, if the ship was surrounded with static rocks and a lighthouse, would the ship collide with those just as the fluid would? In our scene we're sending a bunch of debris-filled mud (i.e. sticks, branches, bones, plants) down a dry riverbed (which has uneven terrain and various protruding rocks), and I'd want to be sure that the debris objects would not just be carried with the water, but would also bounce off of one another and off of the riverbed terrain.
2. Are there limitations for Active Bodies, like with number of objects or poly counts? If so, are there workarounds with say V-Ray proxies to speed things up? My ideal scenario would be to send dozens of debris objects along with the water (again things like sticks, branches, bones, plants). Maybe some combination of that with the TexUVW control for added detail?
3. Alas we're on V-Ray 3.6. Would we need to upgrade V-Ray to use Phoenix 4, or could we render Phoenix 4 content with V-Ray 3.6?

Thanks again!

Eric

[georgi.zhekov]

Yes, the active bodies can collide with one another. In Phoenix 4.40 we have implemetend mutual collisions using the Bullet solver. You can check how this works in the video here - https://www.youtube.com/watch?v=qmgdwv39jVM
It is for 3ds Max but, the workflow and the controls are exactly the same in Maya.

We also have this guide on how you can make a ship float over the ocean waves here - https://docs.chaosgroup.com/display/...e+Bodies+Guide
If you substitute the ship with some branches and you adjust the mass/density it should work just as well for your case.

When using the Active Bodies - the more objects and the more polys the geometry has - the slower the sim will be.

As V-Ray 3 is quite old already Phoenix 4 doesn't have official support for it. Though we have nightly builds accessible for all the people who purchased Phoenix, where you can still get builds for V-Ray 3.

Meanwhile here is the scene that I set up the other day with MASH, the issues that I mentioned in my previous post are still there, but you still might find it youseful somehow.

Cheers!

[ecarlsen]

Thanks so much for your response and for the sample scene. It's cool to know about the feature of attaching MASH objects to a fluid, and also I can totally see what you mean about the flickering issue!

Does Active Bodies work with instanced geometry? For example, I may want to generate a large amount of branches/debris using instances from a few models, then have a flood of water pick all of them up using Active Bodies as it fills up a dry riverbed.

And if instanced geometry can be used, will Active Bodies work with both MASH instances (using a Distribute node) and regular Maya instances (i.e. Edit>Duplicate Special with geometry type set to "instance")?

[georgi.zhekov]

Hey,

Yes, Active bodies are working with instances( the Duplicate special ones) and will collide properly between each other. The MASH instances will work though they will be treated as a single object so it's best to use the duplicate special method.
Note that when using instanced geometry you can't have different mass/density or friction options for each instance. As the geometry is instanced it will share the properties of the original object. If you need objects with different mass, then it's best to use proper copies of the geometry.

[ecarlsen]

Ok that's great to hear, thanks Georgi! All of your input has been super helpful.