The solver takes care of birthing new whitewater particles, using the emission volume created by the Whitewater Source node to determine the amount. When Limit Emission is enabled, the present distribution of whitewater particles is also considered, avoiding over-emission.

Aging and killing of particles is likewise handled by the solver. As simulation progresses, whitewater ages in an ordinary manner; however, particles do not have a prescribed lifetime. Instead, a death chance is dynamically calculated for each particle, taking into account its current condition. The following factors are considered when determining a particle’s dying probability:

• age - Older particles are more likely to be killed.

• depth - As depth varies, so does whitewater’s chance of dying, per values of the Aging Rates.

• density - When erosion is enabled, whitewater density in a particle’s neighborhood is also used to augment its death chance.

Advanced users can further manipulate particle lives by accessing their deathchance point attributes.

You can quickly generate a very simple whitewater simulation by pressing ⇥ Tab and putting down a FLIP Configure Beach Tank. This is a helpful example to look at when you first start, as it shows how FLIP Fluids are generated and how to apply whitewater to a FLIP simulation.

Another method of sourcing whitewater is deformation based emission. This method identifies stretching, squishing, and scaling of the fluid surface. Stretch corresponds to the expansion of the surface along a specific direction. Squish corresponds to the compression of the surface along a specific direction. Surface scale corresponds to how much the surface compresses (negative values) or expands (positive values) considering all directions. These options are found on the Deformation sources tab of the Whitewater Source SOP.

The Visualization tab on the Whitewater Source node lets you turn on visualization for all of the different emission types that are activated. The following example shows visualizations of the Curvature (red), Acceleration (green) and Deformation (blue). The displayed color is chosen based on the most dominant criteria. So in this example, areas that are green mean that acceleration based emission is dominant over curvature and deformation. This doesn’t mean that emission wouldn’t be activated by another criteria. It just displays the most dominant one at that frame.

• By default, emit, surface, and vel are all output through the first output of the node. However, you can choose to only output the emission field to cache it in isolation and save some disk space. To enable this option, turn off the Output Fluid Fields checkbox on the Fluid Fields tab.

• You can turn on the Extra Sources feature by connecting geometry to the fourth input. This is where you can connect another way of sourcing whitewater and it will convert those points into emission volume for the simulation.

• If you don’t see enough whitewater or if you see too much whitewater, you could try adjusting the sourcing. However, most of the time the better approach is to revisit the FLIP simulation, because the momentum and energy comes from the underlying simulation and is transferred to the secondary simulation.

• If the default ranges are not adequate for your specific scene, you can click the Compute Range From Data buttons beside the various sources to give you a better starting point. These values will still have to be refined, but should provide a better estimate of suitable Min and Max values.

• To preview the generated particles, turn on visualizations in the Visualization tab and use the fourth output. This will show visualization of the source types.

• You can also export raw attributes on the points by turning on the Output Emission Attributes checkbox. These are the raw attributes coming from the FLIP simulation.

• All sources on the Sources and Deformation Sources tabs are affected by the Masks tab. It does not affect Extra Sources.

Wind only affects spray particles in whitewater. When particles are lifted off the surface, they are carried by the wind that is defined on the Forces tab of the Whitewater Solver SOP. There are also Wind Shadow options, to make the spray behave more realistically. The Enable Collider Shadow checkbox lets you use collision geometry to block wind from affecting the spray. In this case, the wind is being occluded by the collider and the particles simply obey gravity. There is also an option to Enable Water Shadow, which uses the water surface (such as a wave) to block wind from affecting the spray behind it.

In this example, the spray is yellow, the foam is blue, and the bubbles are purple. When the particles go behind the wave, they stop being carried by the wind and fall linearly back to the surface.

For information on other forces, see the DOP Whitewater help page.

You can Enable Erosion of foam on the Foam tab of the Whitewater Solver SOP. This preserves foam in denser areas and erodes it in sparser regions. The parameters in this section allow you to set how fast the foam disappears and the density at which whitewater is protected from erosion. The following image shows the before and after of Erosion Strength at 0.5*100 and Preservation Strength at 0.5*0.

This feature is useful in simulations where a lot of whitewater is being emitted and starts to look like a big white patch with no details. Adding erosion can be used to get more of the particles to disappear faster and create more holes where you can see the surface of the water.

• You can use the Flatten Outside Bounding Box options on the Domain tab of the Whitewater Post Process SOP to flatten the whitewater particles to the water level offset by their depth attribute. This is useful when you have a fluid simulation that you want to blend with a procedural ocean simulation. Turning this on will prevent the whitewater from looking like it’s floating above the surface of the other particle fluid.