Liquid Simulation Animation - Liquid stretching/flickering effect

[jgreen3895]

Hello,

I'm hoping someone out there can help out with an issue I am having with a Phoenix FD simulation/animation that I've been working on. The animation (which can be previewed below), is a fairly simply setup of some strawberries falling into milk. This is really just a personal project to help myself get more familiar with Phoenix.

https://drive.google.com/file/d/1A3X...ew?usp=sharing

My issue at the moment, which can be seen in the video, is that there is a flickering effect on the fluid. This isn't like GI flickering. It's more like the fluid is tearing apart in a way that causes a flickering effect. This is extremely noticeable around the 4 and 6 second marks where the most dramatic movements are taking place. My initial thought is that this may have something to do with the fact that I am using "Mesh Smoothing" with the following values:

- Smoothness: 10
- Use Liquid Particles: Checked
- Particle Size 0.5

I am using mesh smoothing because otherwise the fluid looks very blocky/chunky. The smoothing effect evens out these issue, but the reason I think this might be the cause of it is because the smoothing also stretches the mesh and makes it thinner. The GRID by the way uses a "Cell Size" of 2 which is currently equating to 13,560,000 Total Cells. I can't really increase this by much due to hardware and file space (the cache files are already about 50mb in size).

If the "Mesh Smoothing" is indeed the issue (not sure it even is), I could render the animation using the "Isosurface" mode. It does take longer though and while the liquid does look a bit smoother (in terms of it's curvature) it also is a little thicker which I personally don't like as much.

One other thought I had was could this be caused by not having enough "Steps per Frame". Currently mine are only set to 1.

I am hoping someone can take a look at the current animation and let me know why the current artifacts are occurring and what I might be able to do to solve the issue.

Thanks!

[squintnic]

I would give a higher SPF a go first you might get away with 3 or 4. I think this will fix your issue (and triple the sim times)

[jgreen3895]

So I tried re-simulating using a SPF of 3, but I can tell from doing a viewport grab preview animation that the flickering effect is still apparent. It probably also doesn't help that the animation isn't quite in real-time. It's a bit of a slow-motion speed which makes any flickering even more noticeable.

I did some research and further reading of the documentation, specially about the "rendering" tab and found the following:

https://docs.chaosgroup.com/display/...quid+Rendering

The mesh smoothing uses two different methods. The first one simply pulls the vertices towards each other, but has two significant drawbacks: animation flickering and causing small formations in the mesh to shrink. The second method is more sophisticated and uses the Liquid particles from the simulation to overcome the first method's problems.

As I said, I has been using the following settings via "Mesh" Mode:

- Smoothness: 10
- Use Liquid Particles: Checked
- Particle Size 0.5

So, I am pretty confident this is the cause of the issue. The problem now is what can I do about it. The above method says the second method overcomes the regular smoothing problems, but I haven't really noticed that to be true. Plus it seems like the second option requires the first option to at least have a value of 1 on the "Smoothness". Without any smoothing, the fluid looks a bit too chunky/polygonal (although some of that gets hidden with motion blur). I thought about lowering the cell size, but I don't really think that'll help much either. It's already at over 13 million cells with a "Cell Size" of 2 and cache files at around 50mb and there are 300 total cache files so it's about 15GB of space total.

I could render using Isosurface mode, which takes significantly longer to render, but may fix the problem. I'd have to run some tests.

Any other thoughts would be helpful here.

Thanks!

[Gosho.Genchev]

Hey there!
Another option that may help as a last resort is the Channel Smoothing under the Input roll-out : https://docs.chaosgroup.com/display/...X/Liquid+Input

[Svetlin.Nikolov]

A few more notes - if the liquid in Isosurface mode looks thicker, you can use Isosurface Level to adjust it.

Can you tell us more about why the animation is not in real time? Did you use the Time Scale of Phoenix under Dynamics, or did you change the Play Speed in the Input rollout, or do you mean something else? This is very important, because flickering usually occurs when you retime the simulation after it's finished.

It's also very important to make sure you are using Phoenix 3.12. If you are on an older version, please do tell.

Cheers!

[jgreen3895]

Hey Svetlin,

I am using version 3.11. Where would I go to upgrade to 3.12? As for the speed of the animation, I have not changed anything in the Input rollout. I did lower the "Time Scale" under the Dynamics rollout to .75. I did this simply to get a slight slow motion effect.

[georgi.zhekov]

Just login with your account at chaosgroup.com and you should be able to download the 3.12 build.

[Svetlin.Nikolov]

I just went through the changelogs and it seems that the last major change to the Smoothing using particles was before 3.11, but please pick the latest build just in case. Did the isosurface level work our for you?

[jgreen3895]

Hey Svetlin,

I will grab the latest build first chance I get. I have no had a chance to try the isosurface level options yet, but I did render a sequence with the mode set to isosurface and the flickering was still occurring. I'm not sure if using isosurface mode with isosurface level options will fix the problem. I'm wondering if maybe I need to really crank up the SPF. Last time I tried 3. Maybe I need to consider 6 - 8. I don't know. Gonna have to experiment some more this weekend.

[Svetlin.Nikolov]

Yes, increasing the SPF will make the liquid follow a more continuous movement. If there are still issues, please send the scene over if possible - it would be good to know if it's a matter of settings or if there might be an issue we need to fix.

Cheers!

[jgreen3895]

I'll let you know.

[jgreen3895]

So I've been running some more test simulations and I think the problem is most likely being caused by the SPF value. My most recent simulation was done at 6 SPF and the animation does appear smoother. I've only done preview viewport grab sequences so far, but even on the prior sequences the flickering was noticeable on those. I just wasn't aware of it at the time, but looking back it's pretty clear. Had I noticed it I would not have rendered the sequence at that time.

Furthermore, I ran previews with and without smoothing on the liquid and on the simulation with 1 SPF value the flickering is noticeable regardless of the smoothing (again I just didn't realize prior). So even though the documentation says smoothing can cause flickering, I think in this case I can now rule that out and assume it was a low SPF issue.

The negative of this though is that raising the SPF from 1 to 6 has changed the liquid simulation, from a visual standpoint, pretty significantly. It's now less chaotic and a bit too plain for my liking. I suppose I have to play with the settings some more to get a similar look as before with a high SPF value. I wish there was an option where raising the SPF could not effect the look though. Almost seems odd that there isn't since most preview simulations will be done using a low SPF value (and a high cell size for that matter) and after an artist puts in a lot of trial and error to the point where they are happy with the results they then have to account for what increasing the SPF (or even lowering the cell size) might do to the look. It's almost comparable to doing a test render at a low resolution and then when it comes time to do a production render the overall look would change. That really wouldn't make any sense. Would love to get some opinions on this. Maybe there's something I am unaware of or maybe I simply need to better understand subtle effects of changing the Phoenix FD settings (even those that focus mostly on quality).

Lastly, Gosho mentioned that there were some Channel Smoothing options under the Input rollout. I experimented with this a bit and need to look at it some more, but can anyone tell me how this differs from the Smoothing under the Rendering rollout. The results don't seem much different from my initial point of view.

Thanks!

[Svetlin.Nikolov]

Hey,

Will update the docs because it's not clear currently - Using particles for smoothing can NOT cause flickering. This is a problem when NOT using particles for smoothing.

The difference between Channel smoothing and Mesh smoothing is that channel smoothing works on the raw grid channels and so can smooth smoke, temperature, speed, etc. Mesh smoothing works over the already constructed mesh.

Raising SPF inevitably will change the look of the simulation to some extent. You can check the Phoenix Explained section of the docs for more info. There you can find about more about cell size and how changing it cannot keep the shape of the simulation, which is why resimulation for fire/smoke was created in Phoenix for upres of simulations.

Cheers!