[novice] Sims and Sub Steps - how many is enough?

[art3mis]

Hi

Creating a slow motion liquid sim at room scale (think aquarium or table top). Using a Scale Time value of 0.25 in my DOP network for the slow motion.

But confused by Sub Steps parameter.

How do you know you need them and how many is enough? Can anyone point me to a visual example?

Is it only to avoid any 'strobing' effects if either the camera or object movement is too fast?

I did a very quick test using 3 SubSteps instead of the default 1 and it seems to completely change the look of my sim(reduce the amount of splash). 

Should this be the case?

 

[catchyid]

Hi,

I don't have experience in liquid simulation, my experience is in rigidbodies...But I think the concepts for sub-stepping are the same! Anyways, I think it's best to start with default parameters, then if simulation looks jagged/chopped (usually due to fast moving objects) then you should use more substeps, rational: things/events happen so fast in your physical world (i.e. liquid interaction with objects in your scene) and your simulator cannot even capture these events (i.e the simulator ticks @ 1 second, then @ 2 second, but an event occurred @ 1.5 second), so you end up loosing important events and your simulation looks "wrong". When this happen just increase your substeps (I am not aware of rule of thumb on how much more you need to increase), but I think it's a balance of computing power, how much you can wait for simulation to finish,.... From my experience, If substeps = 1 and I have bad result, I would change it to 2, if still bad result, I would go with 4...

Also, note each solver has its own substepping parameters, so you need to decide if you want to increase the substepping for the entire simulation or only your liquid solver.

Also note that when you slow time to 0.25, you just get slower simulation, i.e. every thing slows down, so in 30 Frames (given 30FPS), you will have effectively 0.25 second (do you want that or you want solve the bad result thing? )

Another note (yes, last one), how fast a camera is moving is not related to simulation...

Just another, usually liquid/gas simulator has CFL parameter, that should be used to control substepping (as far as I remember, it's a hit for a simulator when to increase its sustepping, e.g. you could say I want max substeps to 10, but then the CFL parameter says: well at this time step you need only 4...anyways, I am not expert here, just remember it and wanted to share :))

Hope this helps

Edited October 23, 2016 by catchyid