• 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.
Guide with Low-Resolution Simulation
• It is best to start a new scene, using the same or similar emitters and colliders to set up a basic low-resolution 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.
• In the global guide properties, activate Enable in the Guided Simulation attribute group.
• In the Input subgroup, activate Simulation mode.
• 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_particle cache files.
• 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.
• 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.
• Play back, and adjust the settings as necessary. Use a low resolution at first (high Master Voxel Size), and then increase the resolution once it's working satisfactorily.
Note
• If the guide mesh has a color set named bifrostVelocity, it will be used as the guiding velocity input. If it does not, the velocity is calculated from the deforming point
positions, so in these cases the mesh topology should not change.
• For best results, the "waves" in the guide mesh should move in a manner that's not too different from real waves. If the waves are extremely fast, slow, or big, then the simulation may have artifacts or produce other unexpected results.
• For realistic ocean waves, the motion should be horizontal as well as vertical.
• The animation should last as long as the number of frames that you want to
simulate. This is a particular consideration when using cached geometry as a guide.
Note
• You can use a collider mesh as an emission region, or you can use a separate mesh to define a volume.
• If you are using a collider mesh, then set Thickness in the mesh's emission region properties so that there is a band of liquid around the solid shape (because particles won't be emitted
inside a collider).
• If you are using a separate mesh, it should contain the volume below the guide to at least the depth that you want to simulate (that is, to at least Min Simulation Depth in the global guide properties). It should also contain sufficient space for splashes above the guide.
• The emission regions can be animated and deforming — as the regions move, new particles are seeded in formerly empty areas, and particles outside the regions die after the time
specified by Death Age in the emission region properties.
• The emission regions should not extend horizontally beyond the guide meshes.
Note
• Min Simulation Depth in the global guide properties controls the depth of the liquid in world space, and may need to be adjusted depending on the scale to which the
scene was modeled. Alternatively if you just want surface splashes with no depth, you can set Min Simulation Depth to 0.0 and use Surface Layer to control the height of a
thin band of liquid on top of the surface. You can also use both.
• Use Blend with Guide on the guide mesh properties to control the blend from the
simulated velocities at the center of the emission region to the input guide velocities at the boundary of the region. This helps to provide a smooth transition from the
simulation to the guide mesh.
• For best results in general, make sure that all particle density settings are equal.
This includes Particle Density in the Particle Reseed group of the global guide
properties, as well as Interior Particle Density and Surface Particle Density in the Emission group of the liquid properties.
Note
• If the simulation does not hold its shape, you can try turning off Compute Guide from High Res Liquid in the global guide properties.
• If the simulation seems "overactive", for example, if there are explosive bursts, then there may be an issue with air pockets developing between the liquid and guides. One way to try to address this is to reduce
Interface Distance. However, this value should always be greater than or equal to 1.5 times the Master Voxel Size to avoid noise at the surface.
• Another way to try to eliminate air pockets is to increase Guide Overlap and Liquid Overlap. However, increasing these values too much can also cause problems.
• You may need to use additional influences to tone down an overly energetic simulation. For example, this may be the case especially when using a polygon mesh as a guide, if the mesh's "waves" move in a way
that's very different from real waves. For example, if the guiding mesh changes shape too quickly then it may impart extreme accelerations, which in turn cause the simulation to overshoot — in this case, try damping
the velocity with a motion field.
• To improve wakes behind boats, try increasing Min Simulation Depth in the global guide properties. Larger values allow waves with longer wavelengths to propagate.