slow-motion DOP smoke

[Waasha]

Hi all,

I'm trying to achieve a slow motion effect, where an object is animated already in slow motion (so I don't have the real time anim) and the smoke is emiting (also in slow motion) from a part of this object.

The Scale Time parameter of DOP network doesn't work correctly (if it's on 0.1, the smoke almost doesn't move) and the Time Warp/ Time Shift/ Time Blend nodes are of no use, because I have the object animated in slow motion..

Is there some other aproach than setting the buoyancy and other parameters to very low (slow-mo like) values?

Thank you for any response!

Edited August 14, 2013 by Waasha

[old school]

Pretty light on the details...

Time Scale is the main control on the Pyro and Smoke Solvers.

Fuel Speed is also used when combustion is enabled on gas advect type micro-solvers in the Pyro and Smoke Solvers. Fuel Speed is only activated when you enable combustion, which if you are just doing a smoke sim is not used. So Time Scale is the main way to slow the smoke down. This "will" change the smoke result.

Are you inspecting your velocity field? Given that this is what is moving your smoke?

How slow is slow? Does your emitter have some initial velocity? Example file?

I find that changing the Time Scale with a decent emitter does slow down the effect along with changing the result. You will find that you have to adjust the emitter velocity scale and perhaps the shaping controls as you adjust the Time Scale to taste.

Inspecting the velocity field is very important. Changing timescale definitely does change the velocity field in my quick test as all the shaping nodes are modulated by the Time Scale. It's interesting to see as time scale is reduced, the velocities present increase quite substantially as the simulation progresses.

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Changing Time Scale will change the effect.

Do you want the same/similar smoke result but slowed down? If so you may have to change your approach and take over the trailing velocity field.

Some ideas if this is what you want:

Take over the Velocity field Technique

- run the sim at full speed with a fast emitter, cache the velocity fields out to disk. Use the cached out velocity fields in a second pyro simulation and use Timescale to blend extend the velocity fields and use these as a pump on your much slower moving emitter and disable buoyancy and use time scale to suit.

- Take over the bulk behaviour using geometry with velocity (particles, trailed deformed geometry, etc). Then use this as a pump, same as the cached velocity fields to disk approach above. Essentially these are both the same. Heck use the above cached velocity fields to advect particles and use these as a pump on your smoke emitter.

Whatever goes but the basic idea is to own the velocity field and not let any of the shaping controls dominate the look but simply augment the look.

Cache Out Sub-Frames Technique

Just run your sim at full speed with the look you want but in the ROP Output driver for both your emitter and smoke fields, render out the sub-frame data. Set your frame range but use a fraction for your increment value to render out subframes (which in itself may change the simulation results). This may change the simulation so best to set the DOP sample rate to your sub-frame increment value as you test.

You can use $N (current frame being rendered) instead of $F (which will give you floating decimal values for frames = not good) in the name of your file. See the Help Card's Local Variables for the Geometry Output ROP.

Read in the sequence from disk to see things slowed down with little change in the look.

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Here's a synopsis as to how Time Scale and Fuel Speed are referenced inside the Pyro Solver:

Fuel Speed and Time Scale on the Pyro Solver are referenced by all the internal Gas Advect Field DOPs' timescale parameter that motivate the density, temperature, fuel, burn, etc, in the simulation:

advection_rate = time_scale * fuel_speed

Time Scale is used by all of the shaping type DOPs (Gas Shread, Gas Turbulence, Gas Vortex Confinement, etc.) to affect their magnitude which drives the velocity.

Time Scale is used by the local sharpening and smoke dissipation as expected so slower smoke will make it hang around longer and decrease sharpening over time.

Good Luck.

[Waasha]

Wow, old school, thank You VERY much for such a detailed response! I didn't expect someone to reply in such a thorough and detailed manner

You guessed right that there is no fire/combustion in the effect and that I am trying to get a same/very similar effect like in real time, but slowed down.. But the problem is, that I don't have the animations for this particuliar shots in real time.. There are shots, which are in real time and the smoke has the desired behavior and character.. And then I have shots, that are in slow motion (around 8 times slower), and I want the same behavior of the smoke, but slower... The problem is - I don't have the animation of the objects in real time.. they are animated in slow motion..

So I guess the only way is just to set the new simulation and play with the settings (like VERY small buoyancy, etc..), to look similar like on the shots that are in real time..

But I learned a LOT from your response, thank You very, very much!

Bye all!

[JaydenDP]

Wow, old school, thank You VERY much for such a detailed response! I didn't expect someone to reply in such a thorough and detailed manner

You guessed right that there is no fire/combustion in the effect and that I am trying to get a same/very similar effect like in real time, but slowed down.. But the problem is, that I don't have the animations for this particuliar shots in real time.. There are shots, which are in real time and the smoke has the desired behavior and character.. And then I have shots, that are in slow motion (around 8 times slower), and I want the same behavior of the smoke, but slower... The problem is - I don't have the animation of the objects in real time.. they are animated in slow motion..

So I guess the only way is just to set the new simulation and play with the settings (like VERY small buoyancy, etc..), to look similar like on the shots that are in real time..

But I learned a LOT from your response, thank You very, very much!

Bye all!

Why not timeshift the slowmotion animation 10x faster. Sim your smoke with 10 substeps and then timeshift the smoke back down before you write to disk? Timeshift the smoke with $F*0.1

[kleer001]

Why not timeshift the slowmotion animation 10x faster. Sim your smoke with 10 substeps and then timeshift the smoke back down before you write to disk? Timeshift the smoke with $F*0.1

You might have to use $FF for the floating point frame, but that sounds viable.

[JaydenDP]

You might have to use $FF for the floating point frame, but that sounds viable.

Nah, usually works for me, but yeah $FF is good practice!

Forgot to add, be sure to switch off Integer option on the timeshift