Guide a Bifröst liquid with a low-resolution simulation

You can use the top surface layer of a low-resolution liquid simulation to guide a high-resolution simulation. This gives a detailed simulation of the surface, with breaking waves and so on, without the need to perform a high-resolution simulation to the full depth of the liquid. You can preview the bulk movement of liquid at low resolution, and when you are satisfied, use it to guide a high-resolution simulation for finer details.

Cache a low-resolution simulation

1. Set up a low-resolution liquid simulation in the usual way. Make sure that the colliders are fairly thick (at least 4 times the Master Voxel Scale), including that of the object used to hold the pool of liquid in place.
2. Create a cache of the low-resolution simulation. See Work with Bifrost user caches.

Set up a guided simulation

1. It is best to start a new scene. This approach avoids the memory and computational overhead of having two simulations in the same scene, while also allowing you to regenerate the low-resolution cache from the original scene if you need to make changes later. In this case, it can be useful to import the geometry used for the emitters, colliders, and so on from the original scene to serve as a visual reference for where the liquid is supposed to be in the new scene.

   Alternatively, you can re-use the original scene for the guided simulation. In this case, it's a good idea to turn off the Enable checkbox in the original simulation's liquid properties to minimize overhead. In addition, you need to be careful to adjust the settings of the correct simulation.

2. With nothing selected, choose Bifrost Fluids > Liquid.
3. In the global guide properties, activate Enable in the Guided Simulation attribute group.
4. In the Input subgroup, activate Simulation mode.
5. In addition, activate Mesh mode. Although this checkbox needs to be on, you do not need to add a guide mesh to the simulation.
   Note: If you do add a guide mesh, it will act as an additional guide influence on the simulation.
6. Click the folder icon next to Sim Cache Name, then browse to the directory where the low-resolution cache is stored and select any one of the voxel_liquid_volume cache files.
7. Optionally, adjust Min Simulation Depth. This controls the depth of the liquid that gets simulated in world space, and may need to be adjusted depending on the scale to which the scene was modeled.
8. Create one or more meshes to define the regions of the liquid volume to simulate. For best results, the meshes should not extend horizontally beyond the boundary of the cached liquid guide, nor too close to the bottom. However, they should extend vertically below the liquid surface to at least the Min Simulation Depth, and far enough above the surface to contain room for splashes.
9. Select the meshes and either the main liquid container or shape, and then select Bifrost Fluids > (Add) Emission Region.

   You should see a band of liquid inside the emission region.

   Note: If desired, repeat this step to add additional emission regions. Each set of meshes is connected to the simulation with a separate emission region property. To connect one or more meshes using an existing property, select the meshes and the property node (instead of the simulation container or shape) and then select Bifrost Fluids > (Add) Emission Region.
10. Play back, and adjust the settings as necessary (see Adjust a Bifrost guided simulation). Use a low resolution at first (high Master Voxel Size), and then increase the resolution once it's working satisfactorily.