Domain Simulation Settings
This page documents the domain settings for the FLIP Fluid Simulation panel. This is one of the panels you will find yourself visiting frequently as it contains important settings for setting up your simulation!
- Parameters
- Bake Operators
- Domain Simulation Grid
- Grid Info
- Simulation Method
- World Scale
- Domain Boundary Collisions
- Frame Rate and Time Settings
- More Bake Settings
All parameters in this panel must be set before baking for changes in the simulation to take effect.
Operators to control simulation baking. Tip: The FLIP Fluids Helper Sidebar menu also contains a copy of these operators as a convenient way to control baking without needing to select the domain.
| Bake | Start running a simulation. Notes: Blender will not be able to autosave your scene while the bake process is running. If you make any changes to your scene while the simulator is running, it is advised to save manually and not to rely on the Blender autosave feature. You are not required to save your scene in order to save the simulation progress. If you close your scene without saving, the simulator can be resumed from the last computed frame. |
| Stop / Pause | Stop running the current simulation. The simulation can be resumed at a later time with the Resume Baking operator. This operator is only available if a simulation is currently running. Notes: The simulator must wait until the current frame is finished computing to completely terminate the simulation process. The simulation process can be terminated immediately by exiting Blender. To ensure that the cache does not become corrupted, only terminate the simulation if the operator displays Safe to quit Blender. Avoid exiting Blender if the operator displays Do NOT quit Blender as there is a small chance that this could corrupt the simulation cache. |
| Resume Baking | Resume running a simulation from the last completed frame. If the Enable Savestates checkbox is enabled, you will be able to select the frame to resume the simulation from. This operator is only available if the last simulation was stopped or terminated after completing at least one frame. Notes: If Blender has crashed or was shut down during a simulation, baking can still be resumed from the last computed frame! |
| Reset | Reset the simulation and clear the simulation mesh and data cache. This operator is only available if the last simulation was stopped or terminated after completing at least one frame. |
See the More Bake Settings topic lower in this document for more settings to customize your simulation bake.
See this documentation topic for tips on how to optimize your simulation setup: Simulation baking is taking too long to compute!
It is possible to bake a simulation from the command line. See the Baking from the Command Line documentation for details.
The FLIP Fluids simulator is a grid based simulator where physics calculations are run and stored on a 3D grid of voxels. This group of settings defines the resolution and physical size of the simulation grid, and may be some of the most important settings in your simulation. See the Grid Info topic below for a more detailed explanation of the grid.
The resolution settings define how the domain grid is divided into voxels. These settings define how much 'physics' detail can be resolved in your simulation.
| Resolution | The domain grid resolution. This value specifies the number of voxels on the longest side of the domain bounding box. See the Debug Panel for grid visualization tools. Related Topic: How to size your domain to maximize grid detail and simulation performance. |
| Preview Resolution | The resolution to use for generating lower quality meshes for the preview fluid surface. Notes: If set too low, the preview mesh may not be detailed enough to capture any details of full resolution simulation and may not be a usable representation of the final mesh. In most cases, it is recommended to use a preview resolution that is half that of the full resolution. Increasing preview resolution will not take longer to simulate, but will increase cache size. |
| Use Recommended Preview Resolution | Automatically set preview resolution based on recommendations. |
| Lock Voxel Size | Lock the current voxel size. If the domain bounding box is scaled to a different size, the domain Resolution will be automatically updated to maintain the current voxel size when this option is enabled. TIP: enable this setting to maintain the same level of simulation detail when scaling the domain smaller/larger. |
Changing the simulation resolution can cause side effects that affect the simulation result. Changes in resolution will affect the level detail and accuracy of the physics calculations which can affect the result of the simulation. Changes in resolution can also affect the number of simulation substeps required which also affects calculation frequency and accuracy. For more information, see this topic: Scene Troubleshooting: Fluid behaviour changes when increasing resolution.
When resuming a bake from a paused state, the simulation can be upscaled to a higher resolution. Simply stop/pause your simulation, increase resolution, and then resume baking. The simulator will automatically upscale the simulation data to the new resolution.
This is a helpful workflow feature to save time for certain simulation effects. Useful for if you have a simulation that takes some time to generate motion before you want to render the animation. For example, the initial motion and fill of a river could be simulated at a low resolution quickly, and then upscaled and continued at a higher final resolution.
upscaling_simulation_example.mp4
This section displays information about your simulation grid and contains tooltips to help explain the important aspects of the grid.
| Voxels 3D | The FLIP Fluids Domain is a 3D grid of cubes called voxels, or cells. This info displays the number of voxels on each of the X/Y/Z axis of the domain. The number of voxels on the longest side of the domain is controlled by the Resolution setting. Voxels in the 3D grid are similar to the 2D pixels in an image. Instead of storing color data, the voxels store physics data that are used to run the fluid simulation. |
| Dimensions 3D | Displays the physical size of the domain on the X/Y/Z axis in meters. The physical size is controlled by the World Scale settings. Setting an appropriate scale can be an important factor for realistic motion and speed of your simulated fluid. Liquid in a glass of water will ripple more quickly than compared to water in a swimming pool, for example. |
| Voxels Size | Displays the physical size of a single voxel. A voxel is always a 3D cube with equal width on each side. You can think of a voxel as being the 3D version of a 2D image pixel. In an image, the pixel size is the minimum amount of detail that can be resolved or seen in the picture. In the domain, the size of a voxel is the minimum amount of physics detail that can be resolved in the simulation, such as the smallest droplets and ripples, or the thinnest splashes. |
| Voxels Count | Displays the total number of voxels in the domain. The fluid physics are computed for each voxel and the total count can be a good measure for how much work your system will be doing. The resolution value is often not a good measure for the size of the simulation workload. For example, a 300 resolution cube domain (300x300x300=27M) will contain so many more voxels compared to a 300 resolution thin rectangle domain (300x100x25=0.75M). That's 36 times more voxels! In general, this is how we describe the workload size of a simulation: Small simulation - less than 2M (million) voxels Medium simulation - 2M to 10M voxels Large simulation - 10M to 40M voxels Very large simulation - over 80M voxels |
Related topic: How large should I make my domain object?. Setting a domain grid that fits tightly around your fluid effect maximizes performance and detail.
Select which simulation method to use, FLIP or APIC.
- Choose FLIP for high energy, noisy, and chaotic simulations. Generally better for large scale simulations where noisy splashes are desirable.
- Choose APIC for high vorticity, swirly, and stable simulations. Generally better for small scale simulations where reduced surface noise is desirable or for viscous simulations.
Refer to the Domain > Advanced > Simulation Method and FLIP vs APIC documentation topics for more info on these settings and for examples and comparisons.
The world scale settings define the physical size of the domain grid, which is an important factor for realistic and predictable results in your simulation. These settings help make sure that your liquids flow at realistic speeds. Refer to the World Size documentation for a detailed explanation for how these settings work.
Settings used to set open or closed boundaries for each side of the domain. By default, whitewater particles will use the same boundary collisions, but custom collisions can be set for each whitewater particle type (See Whitewater Boundary Collisions).
| Enabled Collisions | Enable or disable collisions for each side of the domain. For example, X- represents the side of the domain that is in the negative X direction and X+, the side of that is in the positive X direction. If enabled, this side of the domain will be closed and fluid will collide with this boundary. If disabled, this side of the domain will be open and fluid will flow out of this boundary similar to an outflow object. |
| Open Boundary Width | The distance (in number of voxels) from the domain boundary that fluid will be removed for open boundary sides (disabled collisions). If fluid is flowing out of the domain too slowly, try increasing this value up to 5. Note: This setting is currently for testing purposes and may be removed in a future update if not needed. |
Setting a correct frame rate ensures that your rendered simulation will run at the desired speed. Adjusting time scale controls the simulation speed and can help with creating nice slow motion effects.
| Frame Rate Mode | The frame rate that will be used in the simulation. Scene Use the frame rate defined in the Blender Render/Output properties. Custom Use a custom defined frame rate. |
| Frame Rate | Frame rate value to use if Frame Rate Mode is set to Custom. The frame rate is in frames per second |
| Time Scale Mode | Scale the frame timestep by this value. If set to less than 1.0, the simulation will appear in slow motion. If set to greater than 1.0, the simulation will appear sped up. A custom time scale value may be used or the simulation can match the time scale of another simulation type. Custom Use a custom defined time scale. Rigid Match the rigid body simulation time scale in the Rigid Body World. Soft Match the soft body simulation time scale of a soft body object. Cloth Match the cloth simulation time scale of a cloth object. Fluid Match the Blender Mantaflow fluid simulation time scale of a Mantaflow Domain. |
Changing the simulation frame rate and time scale settings between simulations can cause side effects that affect the simulation result. Changes in these settings will affect the frequency of physics calculations which affect the accuracy of the simulation. For more information, see this topic: Scene Troubleshooting: Fluid behavior changes when changing Frame Rate, Time Scale, or Substeps.
The frame rate and time scale will also affect the speed at which objects are moving within the simulation. For example if an objects animation remains the same while the frame rate is increased, this will result in a higher movement speed within the simulation. If lowering the time scale, this will result in a higher speed within the simulation. When adjusting framerate or time scale, you may want to also adjust the animation speed of moving objects for consistent speeds. Use the FLIP Fluids Sidebar > Measure Object Speed Tool to measure the speed of your moving objects within the simulation.
This section will describe the settings under the expandable More Bake Settings section.
| Frame Range Mode | Frame range to use for baking/caching the simulation. Timeline Use the start and end frame range from the timeline. Custom Use a custom start and end frame range. |
| Start Frame | First frame of the simulation cache. |
| End Frame | Final frame of the simulation cache. |
Before a simulation starts running, the addon exports all simulation settings and mesh data into files that the fluid engine can use to run the simulation. This section can be used to finer control which objects are exported, and which objects to skip if they have already been exported.
| Update Settings on Resume | Update Simulation settings and meshes when resuming a bake. If enabled, the addon will re-export all simulation settings and mesh data. If disabled, the simulator will use the original settings and meshes from when the bake was last started. |
In this section, you can filter simulation objects by motion type and contains an overview of which objects will be skipped, or forced to re-export on next bake. The options to skip or force export can also be found in the individual FLIP Fluid Object settings
- Static - Objects that do not move
- Keyframed - Objects with simple keyframed location/rotation/scale or f-curves
- Animated - Objects with complex motion such as parented animation or armatures. These are objects whose simulation properties have the Export Animated Mesh option enabled.
| Export Animated Mesh | Export the simulation object mesh as an animated one. TIP: Animated objects can be slower to export, so use this option only if necessary, such as for armatures or parented objects. Objects with keyframed position/rotation/scale do not require this option to be enabled. |
| Skip re-export | If enabled, the addon will skip re-exporting this mesh when starting or resuming a bake. If this mesh has not been exported or is missing files, the addon will automatically export the required files. Notes: Animated mesh export can slow down the export stage if the mesh is complex and contains a lot of geometry. Skipping mesh export for animated meshes will speed up export and help your simulation start quicker. If your animated mesh geometry or motion has changed, you will need to disable this option or force the object to export so that the addon will re-export the mesh geometry. |
| Force Re-Export On Next Bake | When enabled, this option will force the addon to re-export the object’s mesh data the next time that the user bakes or resumes the simulation. This option overrides the Skip Re-Export option. After the object has been re-exported, the force export option will be automatically disabled so that subsequent bakes will continue skipping the mesh for export. This option is only applicable if the Skip Re-Export option is enabled. TIP: Useful for if you have a complex animation or object with a lot of geometry with the Skip Re-Export option enabled to save time during export. Use this option if you make an change to your mesh geometry and want to easily mark the object to update the next time you bake/resume the simulation. |
A Savestate is a set of files that stores the current state of a simulation at the end of a frame. Savestate files are used inside the addon to resume a simulation instead of starting from the beginning. The addon will always store the savestate for the last computed frame. This is how the addon is able resume a simulation where you last left off.
The savestates feature can really help speed up your workflow when you want to see how the effects of changing settings will affect the result in the middle of the simulation. By using this workflow feature effectively and in appropriate situations, you can save a lot of time by avoid having to re-simulate from the beginning of the simulation. This feature is also helpful for correcting errors in the middle of a simulation, such as if you forgot to enable an object as an obstacle for example.
| Enable Savestates | Generate savestates/checkpoints as the simulation progresses. Savestates will allow you to rollback the simulation to an earlier point so that you can re-simulate from a previous frame. |
| Savestate Interval | The number of frames between each generated savestate. Notes: The size of a savestate file is proportional to the amount of fluid/whitewater particles. Saving state every frame can create a large cache if there is a lot of fluid. Specifically, each million fluid particles requires 24MB to store, and each million whitewater particles requires 30MB to store. The number of fluid/whitewater particles can be found in the Domain Stats Panel. |
| Delete Outdated Savestates on Resume | When resuming a simulation from a previous frame, delete all savestates that are ahead in the timeline. |
| Delete Outdated Meshes on Resume | When resuming a simulation from a previous frame, delete all cached meshes that are ahead in the timeline. |