Scale - relevance, and ways around it

[StufishIT]

I'm still completely flummoxed by '1cm' as an enforced default basic unit. In our office everything is drawn and modelled real world size, some engineering style CAD communication is in mm, but everything else (particularly MAX) is in metres, (the international unit of measurement?).
So anything I set up in Phoenix is going to have models built in meters, with all units set in metres, and most importantly all the stuff x-reff'ed in for rendering in metres too.
Phoenix seems to work fine for large scale water (big ponds & stuff) in metres, smoke is sort of easy-ish to set up, but smaller scale liquid stuff seems impossible without massive adjustment to the 'scale' settings in Phoenix, which seems to throw many of the other settings out.
If your file is in meters & Phoenix scale is 100, what is gravity ? - and all which of all those settings are unit-related? I though I might get away with MAX having System Units in Metres & Display Units in cm, but that doesn't seem to work.

I'm old enough to have worked in feet and inches, I transitioned more than 40 years ago to metres (& mm when required) but what on earth is this 'cm' stuff about? - What is Phoenix Scene Scale actually affecting in terms of all the possible input fields ?

[Svetlin.Nikolov]

Hey,

Sounds like you've seen or read something which does not correctly describe what Phoenix needs.

In order for a Phoenix simulation to behave like the real thing, it needs to be in the scale of the real thing. So if you need to simulate a volcano, the simulator has to be something like a 100 meters tall. Or 10000 cm tall, or 330 feet - it's all the same to Phoenix how you choose to view the units. What matters is that the scale is right.

If you want to simulate a candle, the simulator should be around 10 cm tall, or 0.1 meters, or 4 inches - again Phoenix does not really care how you choose to view the units in Max or Maya.

If someone needs to work with models which are not the correct scale, then there is the Scene Scale option in the Grid rollout of the Phoenix Simulator, which allows you to cheat the scale. In that case, please watch the dimensions for the X/Y/Z axes shown in the Grid rollout - these are the dimensions corrected by the Scene Scale cheat, which the solver would actually see when it simulates.

And finally, as for small scale simulations, they would move more rapidly and more chaotically, because 1 step of gravity has a much stronger relative effect on those sims than on a massive scene. For 1 second the gravity can move all the fluid out of a small scale simulator, while it could hardly make a visible change to a huge simulation. So when you need to work with such sims, you often need to use more Steps per Frame so the simulation can catch up to the effects of gravity. And often people cheat in order to get a more cinematic effect and counter the effect of gravity by simulating their small scale stuff in simulators 2-3-5 times larger than the actual effect - I don't advertise this, but it certainly is something that happens often

I hope this makes sense!

Can you link the source of the information you got? If it's somewhere where we can change it and make it better, it would be great.

Cheers!

[StufishIT]

Thanks Svetlin, I suppose my starting point is that I am constrained to always working in 1 unit = 1 m, but many online tutorials tell me I have to start by setting my scene units to 1 unit = 1 cm. - without saying why, just saying it won't work if I don't. And it generally doesn't , even if the objects & stuff I create are exactly the same real life size - if it says 15cm, I make it 0.15m.

What I need to know is, how do I make any use of that tutorial's information if my scene HAS to be 1 unit = 1m. OK, do I set Phoenix scene scale to 100 ? and then just use all the settings in Phoenix as suggested in the tutorial,(without knowing which have dimensional relevance)? - because that doesn't seem to work!

[Svetlin.Nikolov]

Ah, I see. So setting the scene scale to 100 will not be enough to fix it. Many other options are in units, units/sec, or units/sec^2, so all of them need to be adjusted as well. For example - foam falling and rising speed, various distances for birth volumes, etc. So all such options which are shown in the user interface in some unit scale need to be adjusted as well. I was thinking recently about adding a quick way to rescale the entire simulation like the toolbar presets do, maybe this would be useful in cases such as yours...

[StufishIT]

Thanks Svetlin, I've been poking at settings, but so many of those dozens of settings for dynamics & foam etc are NOT labelled in any obvious unit, or clear whether they are/not unit related , and the detail on all the 'help' documentation gives no clue either !
I've been trying to set up a 'simple' liquid pour - bottle into a container which overflows & falls onto a surface (cascade setup & foam). Nothing I try will get the overflow to behave hitting the surface (3rd sim in the setup) when the pour settings give a good effect in the first two. takes overnight each time to generate the first two sims, impossible time waste!

[Svetlin.Nikolov]

Hey,

Sorry if I'm stating the obvious, but I need to make sure we are on the same page - by default 3ds Max does not show any of the units for the settings. You gotta go to the units settings in 3ds Max and choose to show the units.

Once you do that, all Phoenix settings which are in units will show up as such. If they don't show in any units, then they are simply not in units. They might be in voxels, or they might be dimensionless. If the docs don't say what units an option is and you need to know, please do ask specifically and if we missed it, we will add it


BUT if you are focusing on such stuff as the units, this makes me think you've dug in too deep in the wrong direction. Let's roll back to the beginning.

Please show me how your setup looks in the viewport, a video of what is the issue you are having, and the settings of the simulators (you can drag the modify panel and expand it to the left so you can screenshot all simulator options at once).

Such a simulation should not take one night to complete, so sounds like there are some redundant options we can help you adjust But please help us see the bigger picture so we can direct you.

Cheers!

[luckett_jason@hotmail.com]

Hi guys,

I realise this thread is old but I notice it's relevant to my problem.

So download this https://docs.chaos.com/display/PHX4MAX/Template+Scenes Liquid Cascade Simulation, very useful for cascade waterfalls in large architectural projects.

Please notice the system units is inches and display units are cms, fine but still a little bit weird.

Now take this simple simulation and merge it into another 3dsmax file with the units of that new file set in meters for both system and display. Now see what happens to the sim, you'll notice a massive scale issue amongst other things.... is there a way around this?

Thanks in advance.

Jason

[Svetlin.Nikolov]

Yup, start by making sure to adjust your Scene Scale option so the grid dimensions shown in the Grid rollout are the same lengths as the ones in the original scene - e.g. if the original scene had the simulator 100 cm wide, then make sure that in the merged scene you adjust the Scene Scale option so the Grid rollout shows the simulator as 1 meter wide.