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Resimulation is a process that uses existing cache files (with exported Grid Velocity) as a base for a new simulation pass. It works differently for Fire/Smoke and Liquid simulations.

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You You can resimulate over the already resimulated cache files. By default Phoenix reads from a cache sequence and creates another resimulated cache sequence. But you can also use the same path as Resimulation Input and Ouput and overwrite the simulated cache sequence each time you run a new resimulation. For example, this way you can keep increasing the resolution every time you run a new resimulation, and repeat this many times.

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Due to the difference between the compression algorithms used by Phoenix's AUR format and the OpenVDB format, the resimulation process may produce a different result when using exported VDB caches as opposed to Aura caches if the Storage Quality option in the Output is not the maximum 20 (i.e. is not Lossless).

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titleFire/Smoke Resimulation WorkflowsWorkflow
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Fire/Smoke Resimulation uses existing cache files (with exported Grid Velocity) as a base for a new simulation pass, to do any of the following:

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  • Increase resolution and enhance details, while preserving the shape and behavior of the base simulation. For example, it's possible to add Wavelet turbulence to increase detail, or simply amplify the resolution.
  • Slow down or speed up the animation, as well as animate the time scale using Phoenix's Time Bend controls. See Slowing Down a Simulation, Animating the Time-Scale, etc. Note that for Liquids, changing the Time Bend settings in the Input rollout is enough, and no resimulation is needed.
  • Keep the grid channels such as Smoke, Temperature, etc., unchanged while modifying the dynamics or number of Drag particles, or vice versa.

Rendering and simulation cache paths can be set manually. For information on how to do this, see the Tips and Tricks page regarding simulation paths and resimulation paths.

When performing a resimulation, it is recommended that you leave the Dynamics and Liquid settings  settings unchanged between running a base simulation and resimulation, unless more advanced behaviors are specifically needed.

Additionally, the scene must be intact between running the base simulation and the resimulation. Please do not remove any Sources and their emitters, obstacles or forces that interact with the simulation. 


The resimulation workflow for Fire/Smoke is as follows:

 

 

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Enable Velocity in the Output rollout. If you are going to add wavelet turbulence as well, also enable the export of the Wavelet channel.

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Start the original (base) simulation and let it run through.

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Enable Particle Resimulation or Grid Resimulation or Particle Resimulation. Adjust the resimulation options in this rollout if needed.

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Start the simulation again - this time the Resimulation will take place. You can switch between preview and render of the base and resimulated caches by enabling/disabling the Resimulate checkbox.

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titleLiquid Resimulation Workflows
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Liquid Resimulation uses an already simulated cache sequence and allows you to add, delete or modify the Splash, Mist, Foam or Wetmap particle systems, while preserving the Liquid and other particle systems that you are happy with.

The particle resimulation will affect only the particle systems which are checked in this rollout. If a particle system is not checked, it will remain unchanged. The Liquid always remains unchanged. While the Fire/Smoke Resimulation creates new cache files during the simulation process, the Liquid Resimulation overwrites the existing cache files.

Using Resimulation to just modify a particle system is practical only in case you have a large number of secondary particles such as Foam, Splash, etc. in relation to the number of Liquid particles. Otherwise, if you have mostly Liquid particles and very few other particles, it will be faster to just simulate over again.

 

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titleModifying an existing particle system
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Select Resimulation → Affect → Particles and enable the respective particle system below (ie. Splash and Mist, Foam or Wetting).

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Modify the particle system's settings - e.g. for Foam, go to the Foam rollout and tweak the parameters there.

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Hit Start to run the simulation again - repeat this process until you are happy with the result.

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titleAdding a particle system that did not exist prior to resimulating
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Select Resimulation → Affect → Particles and enable the respective particle system below (i.e. Splash and Mist, Foam or Wetting).

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Make sure the particle system's simulation is Enabled, e.g. for Foam open the Foam rollout → select the Enable Foam option.

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Hit Start to run the simulation again - the particle system will be added to the cache sequence.

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titleDeleting an existing particle system
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Select Resimulation → Affect → Particles and enable the respective particle system below (i.e. Splash and Mist, Foam or Wetting).

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Make sure the particle system's simulation is Disabled, e.g. for Foam open the Foam rollout → deselect the Enable Foam option.

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Hit Start to run the simulation again - the particle system will be removed from the cache sequence.

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UI Path: ||Select PhoenixFDSim|| > Attribute Editor > Resimulation rollout


Parameters

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Help – Opens up the help documents for the Resimulation.

Use Simulation Start Frame | rs_start_from_sim - When enabled, the Resimulation will

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run from the first frame of the base simulation

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 to the simulation End Frame or the Custom Stop Frame, depending on the options.

  • Custom Start Frame |

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  • rsStartFrame – Explicitly sets the Start frame of the Resimulation. This can also be a negative number.

Use Simulation Stop Frame | rs_stop_from_

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sim - When enabled, the Resimulation will run from the base simulation Start Frame or the Custom Start

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Frame to

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the last frame of the base simulation.

  • Custom Stop Frame |

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  • rsStopFrame – Explicitly sets the End frame of the Resimulation. This can also be a negative number.

Affect | rsAffect – Specifies which part of the simulator to be affected during the resimulation.

Grid – Simulates only the grid content. If there are existing particle groups in the input cache, they will be directly copied to the new cache.
Particles – Simulates only particle groups (i.e. foam/splashes/drag particles). The grid will be directly reused from the input cache.
Both – Simulate both grid and particles.

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Particle Resimulation

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Particle Resimulation | resim_resimulate_particles – Enables/disables the Particle Resimulation. Use this option to modify the look of Drag particles. The resimulation produces the exact same result as the base simulation only with Steps per Frame = 1 in the Dynamics rollout. With a higher number of steps, the intermediate steps cannot be reconstructed, so full simulation is needed.

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Grid Resimulation

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Grid Resimulation | resim_resimulate_grid – Enables/disables the Grid Resimulation. Use this option when the grid detail has to be improved by increasing the resolution, by eventually adding wavelet turbulence, or if you need to re-time your simulation using Time Bend effects. The resimulation will produce the exact same result as the base simulation only with Steps per Frame = 1 in the Dynamics rollout. With a higher number of steps, the intermediate steps cannot be reconstructed, so full simulation is needed.

Amplify Resolution | rsResAmplify – Magnifies the grid resolution. A value of 0.0 means no change, 1.0 means doubling the voxel amount in all axes (2*2*2=8 times the resolution), 2.0 means tripling (3*3*3=27 times the resolution), etc. Non-integer values can also be entered, however, they are slightly slower to calculate. The resolution can also be decreased by using a negative value.

Amplify Method

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rsResAmplifyMethod – When the grid resolution is increased, this parameter specifies which method to use to sample read the velocity from the base cache files.

Interpolate

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– Use interpolation. The velocity channel is stretched from the lower resolution to the increased amplified one.

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Wavelet Fast – Use – Use wavelet turbulence to synthesize create fine detail. Needs a Grid Wavelet channel exported from the to be base simulation cache files. This is a slightly faster approximation method.
Wavelet Nice

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– Use wavelet turbulence to synthesize fine to create fine detail. Needs a Wavelet channel exported from the a Grid Wavelet channel exported to be base simulation cache files.

Wavelet Strength |

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 waveletStrength – Specifies the strength of the wavelet turbulence.

Wavelet Cutoff |

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 waveletCutoff – Turbulence with smaller amplitude will be cut off to reduce calculation time.

No-Export Channels |

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 rsTempChannels – Specifies the channels that are used just for resimulation, which will not be exported to the resimulation output cache. For example, for wavelet turbulence, removing the UVW/Wavelet and Velocity channels from the final simulation will reduce the cache size.

Splash and Mist | rsPartSplash – Specifies that the splash and mist will be recalculated. 

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The Affect Liquid parameter on the Splash and Mist rollout will be ignored when resimulating Splash, because the liquid is already simulated and cannot be changed without a full simulation.

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Resimulating the Foam on Hit parameter on the Splash and Mist rollout requires Splash and Mist to be resimulated.

Foam | rsPartFoam – Specifies that the foam will be recalculated.

Wetting | rsPartWet – Specifies that a WetMap will be recalculated.

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The Sticky Effect setting for Wetting in the Liquid rollout will be ignored when resimulating wetting. This is because the liquid is already simulated and cannot be changed without a full simulation.

 

 

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Example: Resimulation with Wavelet Turbulence

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The example below shows the difference between a base Fire/Smoke simulation and the Resimulation result when using Wavelet Turbulence.

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Base Simulation

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Resimulated with Amplify Resolution = 1
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Time Bend Resimulation

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Phoenix has many tools for changing the animation timing of a cache sequence after it is simulated. Resimulation with enabled Time Bend Controls can produce smoother playback in cases where the Input Time Bend controls are insufficient. For  For more information on Resimulating with Time Bend, see the Tips and Tricks topic on How to slow down a simulation, animate the time scale, etc.

 

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Use Time Bend Controls | rsGridTimeBend 

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– Change the time scale of the base simulation according to the Time Bend Controls in the Input rollout. You need to re-scale the animation of all other interacting objects and sources in the scene as well.

Time Bend Method | rsGridTimeBendMethod

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 – A method to use when scaling the time of the resimulation. Note that both methods work best

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when Steps per

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Frame on the Dynamics rollout is set to 1. Note that speeds below 0.2 may produce unsatisfactory results when using either method.

General-

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Purpose – Works for all play speeds including backwards playback. It resimulates more quickly, but might produce flickering or jittering in the movement of the fluid. Sources, obstacles, or forces do not need to be present in the scene for this method. However, this method may not produce satisfactory results with animated sources.
Slow Down

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 – Works only for play speeds between 0 and 1.

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 It will produce smooth playback without flicker or jitter but will add more dissipation in the long run, so it's better to combine it with Multi-Pass Advection.

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 This method requires all used sources, obstacles and forces to be present in the scene. Any animation in the scene will need to be adjusted accordingly and slowed down to the play speed of the resimulation.

 

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Example: Time Bend Resimulation

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The example below shows the difference between the Input rollout → Grid Blend Interpolation used to slow down without resimulating, the Resimulation → Slow Down Time Bend Method and the Resimulation → General-Purpose Time Bend Method.


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Input Play Speed = 0.2, Interpolation Blend

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Slow Down method + Wavelet

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General-Purpose method + Wavelet

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Resimulation Cache

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During resimulation, Phoenix reads cache files from the Base Cache Files Path, performs resimulation operations, and then saves a new cache sequence in the Output and Render Path.

For information on changing the default paths for resimulation, see the Tips and Tricks  pagepage.

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Base Cache Files Path | rsInput

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 – Specifies the base simulation caches which will be read as base for resimulation. $(same_as_output) by default. The field below shows the resolved full path.

Output and Render Path | rsOutput

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 – Specifies the path for writing the output cache files from Fire/Smoke resimulation. Note that Liquid resimulation overwrites the files in the Base Cache Files Path and also uses that path for preview and rendering. See

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the Simulation Save Path parameter in

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the Output Rollout

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 for more details on the $(...) macros you can use. The field below shows the resolved full path.

Click the "..." button to open a menu with the following options:

Browse - Opens a dialog where you can select where the resimulation caches will be written to. The filename you type in must contain # signs so each resimulation frame will be written to a differently numbered file. Also, you can choose between two file formats - AUR and VDB cache files. 

Delete Cache Files - Clears the resimulation files.

Reset to Default - Resets the resimulation path to default.

Help - Opens this

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You can use the same Output and Render Path as the Base Cache Files Path, and this way Phoenix will not create a separate cache sequence for resimulation, but will overwrite the base simulation caches instead. It's valid to do so if you need a more complicated setup or when not enough disk space is available. This way you can increase the resolution more and more each time you start a resimulation.

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