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's geometries 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, using the same or similar emitters and colliders to set up a basic low-resolution simulation (see Create a Bifröst simulation). This approach avoids the memory and computational overhead of having two simulations in the same scene, and also allows you to regenerate the low-resolution cache from the original scene if needed.

   However, it is possible to reuse the same scene or even the same simulation if you prefer. In this case, be careful not to overwrite the cache files.

2. In the global guide properties, activate Enable in the Guided Simulation attribute group.
3. In the Input subgroup, activate Simulation mode.
4. 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.
5. 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.

   You should see a band of liquid at the top surface of the emitters, rather than to the full depth of the emitter.

6. Optionally, you can restrict the simulation to the region defined by a mesh's volume. To do this, select the mesh and either the main liquid container or shape, and then select Bifrost > (Add) Emission Region.
7. 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.