Implicit surface jagged walls, extreme foam rendertime, poor volume conservation.

[LudvikKoutny]

Hi,

so I've been using PhoenixFD demo to evaluate if Phoenix would be a good buy for my future jobs. To test it out, I took one job that involved fluid simulation. Pouring a coffee liquid into an invisible cup to match exact length in audio track provided by client. I would never assume something so trivial could be so difficult in Phoenix.

It has taken me 3 days of simulation tests just to get exact amount of liquid into the cup after exact given amount of time. The volume conservation of Phoenix seems to be the problem there. I wanted to do low res grid preview to get it right, and then up-res for final sim, but even changing grid resolution itself makes a difference between if the cup is almost empty or full. Even 10 steps per frame did not make it any more reliable. Sharpness parameter seems to actually have most impact on how much liquid ends up in the cup, where as for example discharge rate shows very little difference between value of 0.1 and 10. If I pour 100 times more liquid in, I would expect 100 times more liquid in the cup, and vice versa, yet that was not the case. But after 3 days of pain and cringe, I finally managed to get sim right.

Unfortunately now I am unable it to render. I tried both mesh mode and implicit surface mode, but I can not seem to be able to cut liquid volume by invisible cup in a way that leaves no jagged artifacts.

Lastly, I am also not able to render foam in reasonable time. Even with Fixed sampler set to 4 subdivs and MSR at 1, foam still takes ridiculous amount of time to render.

I am absolutely desperate here, because this seems to be a first job ever that I won't be able to deliver.

This is what I've got so far:


This is my reference, excluding the cup, just the liquid inside:


And here is the scene (without cached sim, as it takes about 5GB).
https://drive.google.com/file/d/0Bwn...ew?usp=sharing


I would be more than thankful for any help with the scene setup. All I want is to have liquid with sharp clean surface and foam that is cut exactly in line with that liquid, and does not take so extremely long time to render. When it comes to foam, i tried everything, different scattering modes, enabling LC, disabling LC, different rendering types, nothing helped.

Thanks in advance.

[Ivaylo Katev]

ok, if you already have the sim, i will not discuss how the volume conservation can be improved, so to the rendering
the steps on the wall - the last official release uses same core for fire and liquids, it's grid based (we have pure liquid core now, but is still accessible only in the nightly builds), this means the liquid=geometry contact is always 100% step-like, but there is a well working technique dealing with this, the gizmo. you have to use the glass as gizmo, like it is shown in the beer demo, it makes the liquid surface perfectly smooth
about the render times, didn't see your scene, but in all the cases the foam rendering is much slower than the geometry objects. in the best cases you have to wait minutes per frame, but the most users with no experience in the FX area do not hit these happy settings and are rendering in 20-60 min per frame.
the main factor for the slow down is the lighting, you have to keep it as simple as possible, the fastest setup is directional light+ambient

[LudvikKoutny]

That result above is is done with identical technique to one shown in the beer tutorial pt. 2 with identical settings, yet it still produces those jagged edges.

[Ivaylo Katev]

well, have no explanation then, we have to see the scene

[LudvikKoutny]

Okay, thanks. Scene is attached in the first post.

[Svetlin.Nikolov]

Hey,

In your screenshot the Object voxels are set to Inscribed, but in the scene they are set to Circumscribed. Please set them to Inscribed and simulate again - you should have no issues then.

As Ivaylo mentioned, there have been many changes since the latest official version came out and thus the latest demo. There is a brand new FLIP liquid solver which is designed especially to deal with such simulation oddities like the one you fought against.

Will check how you could improve your render speed and get back to you.

Cheers!

[LudvikKoutny]

Hey,

In your screenshot the Object voxels are set to Inscribed, but in the scene they are set to Circumscribed. Please set them to Inscribed and simulate again - you should have no issues then.

As Ivaylo mentioned, there have been many changes since the latest official version came out and thus the latest demo. There is a brand new FLIP liquid solver which is designed especially to deal with such simulation oddities like the one you fought against.

Will check how you could improve your render speed and get back to you.

Cheers!

Hey, I've been experimenting with all the different modes, so yes, in the scene I've attached, I may have left it at circumscribed, but I had mode set to inscribed at the time I was doing test render. But you said "simulate again". Does this mean I have to re-run the entire simulation every time i change that mode? Because if so, then that would definitely explain where is my problem. I may have accidentally ran sim in circumscribed mode.

Also, It's great to know there is FLIP solver in paid version, but the point of trying out demo was to evaluate if Phoenix would be worth buying, and so far I have rather mixed feelings, so especially now it would not be a smart step to buy full version blindly.

[Ivaylo Katev]

the voxelizaton mode does not affect the rendering directly, it affects the simulation, changing it after the simulation is done produces no effect

[LudvikKoutny]

the voxelizaton mode does not affect the rendering directly, it affects the simulation, changing it after the simulation is done produces no effect

I see, now that explains it. I thought it's something that happens after sim at rendertime. That explains my problem i guess.

[joconnell]

Nightlies are awesome by the way - fluid is much much nicer!

[Svetlin.Nikolov]

Getting back on the render speed issue - the foam renders slowly because of the BF/BF GI and the motion blur. Also, because I think if your setup allows it, you could change the foam render mode to Point or Fog - they are faster than Bubbles or Splashes. There are also many speed improvements in the nightlies compared to the official build, although there is more to be desired for sure.

Cheers!

[LudvikKoutny]

Thanks for taking a look into it. Although even disabling GI altogether does not seem to improve render speed much. Point mode seems to be a good compromise i guess.

[Ivaylo Katev]

if you find the point mode result good enough, point mode is really faster and is better to be used

[LudvikKoutny]

if you find the point mode result good enough, point mode is really faster and is better to be used

I've rendered the final with point mode. It rendered in reasonable time, but unfortunately, points were a lot more transparent so they did not look like a foam, more like fog. I wonder if there is any way to increase opacity/density of the points without affecting their size.

By the way, looking at the installer download page of PhoenixFD builds, version 2.20 was released on May 8th 2014, that's nearly two years ago. I am wondering why is new version of Phoenix in beta for almost 2 years? Did something go really wrong and took such a long time to get fixed?

[Ivaylo Katev]

you can make them thicker by the ior parameter, initially the foam shader was designed only with splash and bubbles mode, and this parameter controls the ior of the "liquid", the bigger the ior is, the bigger is the alpha. unfortunately it's not obvious that it leads to high density in point mode.