Mid-scale looping waterfall setup

[MonosP]

Hi all,
I'm in the process of setting up a mid scale waterfall and am hoping I could get some pointers for setup and render optimisation.
I've not done anything like this with Phoenix before so don't have a benchmark for what reasonable sim and render times should be either but I'm aiming for something similar to these:

https://vimeo.com/130328238#t=20s
https://vimeo.com/141040510#t=2s

Or Ivaylo’s Phoenix waterfalls, https://vimeo.com/129956368 for example.

Here's a screengrab of my scene file (rocks in the final sim will be more detailed). The fluid emitters, one box for each side of the waterfall, are underneath the blue ledges, I thought doing this would allow the fluid to rise up within the blue container and settle to become less turbulent by the time it reaches the lip of the waterfall.



Currently only using 1 grid although I've done some simple cascade tests to get familiar with that feature incase it's a better solution.
Grid size is non adaptive, 50 x 52.5 x 75m, scene scale is 1 and cell size is 25cm, giving 12.6M cells. I used the default waterfall preset for the initial setup and most settings are still at their default values.
Does this seem like a reasonable starting point, in terms of grid and cell size, etc?

There're a few technical constraints I'm working to:

I need to render out a seamlessly looping sequence, around 10-15s. I’ve used the loop re-time feature with smoke already so assume it’s the same here? Also assuming foam/mist/spray are looped seamlessly too? I'm planning to run up pre-roll of 30s and then render out 30s-45s on the timeline.
I've been running tests at 24fps for quicker iterations but need to render the final version at 60fps. Am I correct in thinking the sim data needs to be at the target fps for looping? For example, I'd need to have the sim data at 60fps before I could loop it correctly, I couldn't retime a 24fps sim by changing the Play Speed value to 0.4 and loop it at the same time? I've tried that with smoke sims previously and had unexpected results.

I need to render out the waterfall in 2 passes; fluid surface (beauty, alpha, world normals) and foam/mist/spray (beauty, alpha). Is there an optimal way to achieve this from a single sim? Material swaps? State sets?

Thanks in advance for any help.

[Svetlin.Nikolov]

Hey,

The waterfall preset settings and your setup indeed are a good starting point.

Indeed looping and play speed change at the same time are not implemented currently. You would have to simulate at the final frame rate. You also need to make sure you have particle IDs and Ages exported for each and every particle system in the Output rollout, so looping would work correctly.

In order to separate the rendering of the simulator and the particle shaders, you can just make either not renderable. This would also greatly benefit the rendering speed, because raytracing of the particles across refracting and reflecting meshes such as the simulator's liquid is a very intensive calculation.

Hope this helps!

[MonosP]

Hey Svetlin,
Great, thanks for the quick reply and good info! I'll implement that and bear in mind the channel data that's needed when exporting.
Cheers!
Jon

[Svetlin.Nikolov]

Ah, and don't forget the velocity as well!

[MonosP]

Ah yeah, thanks for the reminder!

[MonosP]

Hey Svetlin,
When increasing the frame rate from 24fps to 60fps to run a new sim are there any other settings I should change to compensate and get a same/similar result?
I'm happy with the volume and shape of liquid in a 24fps test sim so I ran a sim at 60fps last night but it appears quite different, almost as though the fluid speed or emission is weaker and the liquid is trickling over the lip of the waterfall.

[Svetlin.Nikolov]

Sorry for the misunderstanding, I thought we agreed that you should setup the simulation at the final 60fps from the start. For fire/smoke you can retime it into the same shape and behavior using time-bend resimulation, but this won't work for liquids where the standard workflow is to just alter the play speed, which unfortunately won't work together with the looping.

[MonosP]

Sorry for the late reply Svetlin, I've been away for a few days. My previous message was probably poorly worded; I understand that to get the seamless looping with liquid I’d need to setup and run the sim at 60fps, I could be misunderstanding how liquid sims work at a fundamental level though.
Let’s say I setup and run a sim at 24fps which gives me the liquid shape and behaviour that I’m happy with. If I then only change the frame rate of the scene to 60fps, no other Phoenix or scene settings, and re-cache the sim at 60fps, I’d get a different looking liquid shape and behaviour? I originally assumed that I’d get the same or similar behaviour over the same time period, only with more discrete cache files with the 60fps sim, although I can see why behaviour might change with the smaller increments of a higher frame rate sim.

I most likely expected this as I did some initial tests with a simple smoke emission at 30fps and then 60fps and got near identical results. I've got a couple of short movie files which might help illustrate my (mis)understanding better, what's the best way to post them? File upload or links to youtube/vimeo?

[Svetlin.Nikolov]

This would be a dream come true, but unfortunately computations fluid simulations work in such a way, that changing the resolution or the time scale produces a completely different fluid shape and behavior. This is why in Phoenix we have the different resimulation modes - so you can upres or retime simulations by keeping their shape and behavior. Unfortunately, looping is still not supported together with retiming... Of course in some setups you can get lucky and get the same shape and behavior of the fluid after upres or retime and such setups are usually the kind where emission is steady and there aren't any moving obstacles - such as fire burning, or liquid pouring. However, more interesting sims usually turn into something completely different because of how the fluid voxel grid works...

[plsthatspls]

Ok then can I Resimulate, upres and retime liquid/splash/mist etc in the same same pass?

[Svetlin.Nikolov]

Hey,

There is no upscale for Liquid simulations, only for Fire/Smoke. Here are 3 folded sections with the 3 different Resimulation workflows for Liquids: https://docs.chaosgroup.com/display/...d+Resimulation

If you need to retime a Liquid simulation, you don't need Resimulation at all - just change the settings in the Input rollout the way you want and the particles will be retimed. If you are missing any channels from the simulation, Phoenix will display a warning and tell you that you should have them in the simulation - most important are IDs: https://docs.chaosgroup.com/display/...meBendControls

Please ping us if you have any difficulties

[plsthatspls]

Yes thankyou Svetlin, i had read those dropdowns.. and that all that was needed was to change the input rollout setting. I had already tried to loop.. cache origin 5, play length 180, overlap 80, hit render and didnt get good results so I was just unsure about whether i needed to actually resim (took 15 hours to find out it didnt work, and 4 minutes to loop perfectly in post)

So I sim at my desired res, then in input, choose my start frame and play length, say 120.. speed .5, then in vray render those 120 frames and ill get 240 images in that same time space?.. am i understanding this correctly?

Thankyou


Edit: Im on Maya btw

[Svetlin.Nikolov]

Yup, sounds right - you could run a very low-res sim (from the Grid rollout -> decrease resolution), and try out the Input settings in this sequence, as well as the Output settings regarding which particle channels to export, before going to the full high res sim.

[plsthatspls]

Great.. one more question that was asked earlier in this thread.. in a waterfall setup that has the shape i want what settings specifically would i change to upscale? foam and splash amount?.. lower affect liquid? when i upscaled 50% the splash and mist seem to run out before the bottom

[plsthatspls]

(Scene scale)