Looping simulations

[TL4500]

Hi.

I have been trying to loop a smoke simulation but wherever I set the play start frame the loop always runs from the start of my simulation to the end. Am I misunderstanding the idea behind this? I am expecting a loop starting from a point that I want to specify. Is this not the case? Does loop simply mean looping throughout the entire animation timeline but using the specified set of frames?

I am trying to use and existing set of aur frames. Ideally what I want to achieve is run the sim from smoke starting at zero and rising. Then the smoke continues looping for a long period of time (~5000 frames). Then I would like to end with a third section which is the first section in reverse (is this possible?).

I have searched all tutorials and looked at the YouTube tutorial page but cannot find a tutorial on looping simulations. Do any exist?

Many thanks,

Terry

[Ivaylo Katev]

the loop option can't produce result that starts and ends with different sequence, it actually loops a set of frames, that's all. the play start parameter just introduces an offset, that's all.
your goal is not directly achievabe, you have to render in different passes the start and the end of the sequence. in order to have smoth transition, the point must be the starting frame of the loop sequence. the ending part is not quite clear, how you intend to avoid the sudden jump in the smoke movement?

[TL4500]

Hi.

So, just to clarify. Ignore for now the start and end.

If I have a timeline of 5000 frames and I have an existing smoke simulation of 400 frames, is it possible to loop the 400 frames between two points, say start at frame 2000 and end at frame 4000?

Thanks,

Terry

[Ivaylo Katev]

you mean an option that sets limits for the looped rendering? no, there is no such option, in general the cases when a looped simulation must exist only in part of the animation are very rare cases, the usual cases are fireplaces, fountains, waterfalls.
you have to render the looped part , then disable the rendering of the simulator and then render the rest of the animation.

[TL4500]

Thanks Ivaylo.

That clarifies the looping. I was thinking that maybe you could set a start and end but now it is clear you cannot.

My next question is:

What do I do in my situation? I need to show smoke starting (appearing from nothing) at a certain point in the animation and dieing off at another point. Both of these points are not the start and end of the animation timeline but somewhere inside it. As my animation is nearly 7000 frames long, I am assuming that looping is the option to create smoke that lasts a few thousand frames rather than simulating it all? The question is:

Can I "seamlessly" create a short section of smoke that is then followed by a loop, that is then followed by a short section of smoke that has a definite end (e.g. emission stops at a point and the smoke drifts to a natural end).

I think you have started to answer this with your first reply to this thread.

So, I first create a smoke simulation, let's say 900 frames. I use frames 0 to say 300 as my "start" section. I then create a loop using say frames 300 to 600 and when I render this part I just render the frames for the section of animation using the loop that I require. I then use frames 600 to 900 as the last section so that the smoke stops "naturally". (At this point just as an aside - I am rendering these atmospherics off as VRayAtmosphere render elements from a separate copy of the Max file where all other scene objects are mattes. I have several repeats of this exercise to do in order to create different smokes and want them all in separate layers to composite in After Effects so they are rendered as elements one at a time).

The question then is: will my three sections fit together "seamlessly" by using say frame 300 as the last frame of section 1 and the first frame of the loop, and then frame 600 for the first frame of section 3 and as the last frame of the loop?

Or, what can I do to ensure that they do fit together?

Many thanks,

Terry

[Ivaylo Katev]

to ensure smooth transition you have to render the leading sequence up to the start of the loop+the loop overlap.
for example if the looping sequence starts at frame 200, and the loop overlap is 20, you have to render the leading sequence up to frame 220, then you can switch to the looped sequence.

[TL4500]

Thanks Ivaylo.

Does that also apply to the end sequence? e.g. I render the loops as required, then render the last section including 20 frames before (which are also the last 20 of the loop)? Can that be done?

Thanks,

Terry

[Ivaylo Katev]

if the end sequence is the reversed start sequence then yes.

[TL4500]

Thanks Ivaylo.

How do I reverse playback. Do I set play (in Input) to -1?

Thanks,

Terry

[Ivaylo Katev]

yes, this is the way.

[TL4500]

Hi Ivaylo.

I have now simulated a sequence for 800 frames. I want to use 0-350 as the "start sequence" and then seamlessly add a loop to this using the remaining frames. I am also using a Playback speed of 2 for both.

Based on what we said, could you please confirm that these are the correct settings for the two simulators:

Start up. Cache start = 0. Play start = 0. Length = 200 (as 2 x speed). e.g. a sequence of 400 (start + overlap) divided by play speed of 2.
Loop. Cache start = 350. Play start = 350. Length = 400. Loop overlap = 50.

Many thanks,

Terry

[Svetlin.Nikolov]

Hello,

Some values must be corrected:

Start up. Cache start = 0. Play start = 0. Length = 175 (the starting sequence is 350 frames, div by 2)
Loop. Cache start = 350 (cache index). Play start = 175 (timeline frame). Length = 200 (in timeline frames). Loop overlap = 50.

The problem with that setup is that the loop overlap will kick in straight from frame 175 and there won't be a smooth transition between the two simulators.

Another problem is that the first simulator will still be visible after frame 175.

We'll try to figure something out in time for tomorrow's nightly build, stay tuned

[TL4500]

Thanks Ivaylo.

I set the numbers as you advised and if I take a still of both emitters at frame 175 the smoke looks different. Should I not be seeing the same smoke position for frame 175 for both emitters?

When you say "The problem with that setup is that the loop overlap will kick in straight from frame 175 and there won't be a smooth transition between the two simulators." do you mean the problem with the numbers I originally sent?

It is not a big deal for the emitter to be still visible as I will render the start sequence and then change emitters before I render the loop.

Thanks,

Terry

[Svetlin.Nikolov]

The problem is that the loop mode always blends the end of the cycle with the beginning, that's why you get an abrupt difference at frame 175 - then you are actually looking at the end frame of the loop cycle.

You can make it work if you drop in at the middle of the loop when there is no overlap, e.g. at frame 200:

Start up. Cache start = 0. Play start = 0. Length = 200
Loop. Cache start = 350. Play start = 175. Length = 200. Loop overlap = 25 (that's in timeline frames, so must be 50/2)