Houdini 16.5 Nodes Dynamics nodes

Whitewater Solver dynamics node

Sets and configures a Whitewater solver.

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The Whitewater solver simulates foam, spray, and bubbles from a source liquid simulation. The solver classifies particles in the simulation according to their depth within the surface field that is sourced from the input liquid simulation. Particles can freely transition between different particle types during their lifetime, and can be partially advected by the velocity field along with ballistic behavior. Individual particle types can be disabled, in which case particles classified as that type will be killed, allowing a simulation to be limited to only spray, for example.

This solver simulates three types of particles:

Setup

The Whitewater Source node can create the proper whitewater emission points and input fields to serve as input to this solver. A Whitewater Emitter node can effectively create a denser, volumetric particle cloud from single emission particles.

The Whitewater Object DOP creates an object with the proper data for this solver, which uses the following subdata:

  • A surface field that represents the extent of the input liquid surface as a Signed Distance field.

  • A vel vector field that represents the velocity of the input liquid simulation.

  • Geometry data that stores the particle geometry for each timestep.

Tips

  • The Whitewater shelf tool sets up a whitewater simulation from a source FLIP or Liquid simulation.

  • Visualize the surface field on the Whitewater Object to display the whitewater particles in context with the liquid surface.

  • Disable all but one of the particle types to augment the primary whitewater simulation with additional passes. For example, a spray pass with different emission settings can be layered on top of a full whitewater simulation.

  • Use a Whitewater Emitter DOP for volumetric emission of whitewater particles and to customize the whitewater simulation. Increase the Velocity Offset parameter on this node to move the emitted particles farther along the emission velocity vector, leading to more particles initially classified as spray. Increase the Radial Velocity to give more outward direction to spray particles.

Inputs

Objects to Solve

One or more Whitewater Object for which to solve. This object contains the required subdata and has options for visualizing the particle types and the source liquid simulation fields.

Particle Forces

Additional forces to apply to the particles.

Tip

The solver organizes the different particle types into point groups named foam, spray, and bubbles, making it easy to apply forces to specific types of particles.

Sources

Particle sources, usually including a Whitewater Emitter DOP to provide volumetric emission.

Parameters

Volume Source

The path of the SOP node providing the source volumes for the solver. The node should provide an SDF volume or VDB that represents the extent of the liquid surface and has a primitive name attribute set to surface. It should also provide a set of three volumes or one vector VDB containing the velocity field of the source simulation, with the three primitives having name attributes of vel.x, vel.y, and vel.z for volumes, or simply vel for a single vector VDB. The WhitewaterSource, Fluid Compress, and DOP Import nodes all output volumes with the proper names.

Import Volumes Into DOPs

Import the source volumes into DOP fields before using them for advection and depth testing against particles. If this option is disabled, those operations are performed using the original source volumes from SOPs, avoiding any resampling or duplication of volume data and saving processing time and memory.

Note

The field guides on the accompanying Whitewater Objects will not work if the data is not copied to DOPs first.

Foam

Depth

The distance to the top or bottom of the foam layer, measured from the middle of the foam layer. The total height of the foam layer will be twice this value.

Iso Surface

The middle of the foam layer will lie at this depth value on the surface field. Increasing this value will raise the entire foam layer, which can be effective in ensuring it stays above the liquid surface for rendering.

Enable Foam

Enable solving for foam particles. If foam particles are disabled, any particles that enter the foam layer depth will be killed.

Lifespan

Lifespan Attribute

The initial lifespan of foam particles can be mapped from a zero to one attribute on the foam particles. The lifespan will range from Min Lifespan to Max Lifespan, or just be set to Min Lifespan if this parameter is not set. Using the same attribute that originally controlled particle emission (typically the emit attribue) will give larger clumps of foam a longer initial lifespan.

Min Lifespan

The minimum lifespan for foam particles that map to the zero value of of the Lifespan Attribute, or for all particles if that attribute is not set.

Max Lifespan

The maximum lifespan for foam particles that map to the one value of the Lifespan Attribute.

Lifespan Variance

A random number of seconds within this range is added or subtracted from the foam lifespan.

Preserve Foam

Increase or decrease the lifespan of foam particles based on the local density of foam particles, which can help create clumps and streaks of foam on the liquid surface over time.

Min Foam Density

When the local foam density surrounding a foam particle lies in the range of this value down to zero, proportionally reduce its lifespan each timestep by the Reduction Rate.

Max Foam Density

When the local foam density surrounding a foam particle lies in the range of Min Foam Density up to this value, proportionally increase its lifespan each timestep by the Preservation Rate.

Preservation Rate

The proportional amount to increase the lifespan of a foam particle with sufficiently high local foam density. This value is the number of seconds to add per second, so if greater than one can effectively preserve high density foam clumps from every being killed.

Reduction Rate

The proportional amount to decrease the lifespan of a foam particle with sufficiently low local foam density. This value is the number of seconds to reduce the lifespan per second.

Behavior

Advection Strength

The strength of the advection force that carries the foam with the velocity field from the source liquid simulation. Very high values will cause the foam to follow the liquid surface very closely and lead to fine, streaky foam. Lower values give more diffuse, spread-out foam.

Falloff Bias

The rate at which the foam advection force falls off away from the middle of the foam layer. Low values representing a slow falloff mean more foam particles have strong advection forces and produce tighter streaks of foam. With a higher falloff the foam gradually gets smeared across the surface over time.

Behavior At Limits

Specifies the foam particle behavior when hitting the Volume Limits.

Collide at Closed

Collide with any closed boundaries. At open boundaries this is equivalent to Kill.

Make Ballistic

Make any foam particles ballistic when outside the Volume Limits.

Kill

Kill any foam particles type when outside the Volume Limits.

Volumetric Foam

Adds additional advection velocity towards a unique depth within the foam layer for each particle, giving more control over the volume of the resulting foam and keeping the foam particles closer to the foam Iso Surface value over time.

Depth Scale

A multiplier for the the foam Depth when choosing unique depth values for each foam particle. Setting this parameter below 1 acts similar to surface tension, where the foam will be attracted inwards toward the fluid surface over time.

Depth Scale Variance

A random variance added to the Depth Scale parameter. Higher values for this parameter increase the "fuzziness" of the foam around the edges. Lower values give harder edges to the foam.

Move Amount

The amount each foam particle will move towards its goal depth on each timestep. Higher values give a stronger volumetric force, but can cause snapping artifacts as bubble and spray particles transition to foam.

Seed

A random seed for controlling the goal depth and variance for each form particle.

Depth Scale Lifetime Ramp

A ramp that multiplies the Depth Scale over the lifetime of the foam particle. A decreasing ramp creates foam that starts out more volumetric then attracts inwards over time as the foam begins to dissipate.

Spray

Enable Spray

Enable solving for spray particles. If spray particles are disabled, any particles that are emitted or leave the surface as spray will be killed.

Lifespan

The default initial lifespan of spray particles.

Lifespan Variance

A random number of seconds within this range is added or subtracted from the spray lifespan.

Gravity

The gravity force to apply to spray particles.

Drag

The amount of drag apply to the spray particles.

Speed Limit

The maximum allowed speed of the spray particles.

Behavior At Limits

Specifies the spray particle behavior when hitting the Volume Limits.

Collide at Closed

Collide with any closed boundaries. At open boundaries this is equivalent to Kill.

Make Ballistic

Make any spray particles ballistic when outside the Volume Limits.

Kill

Kill any spray particles when outside the Volume Limits.

Bubbles

Enable Bubbles

Enable solving for bubble particles. If bubble particles are disabled, any particles that are emitted or dip below the surface as bubbles will be killed.

Lifespan

The default initial lifespan of bubble particles.

Lifespan Variance

A random number of seconds within this range is added or subtracted from the bubbles lifespan.

Advection Strength

The strength of the advection force that carries the bubbles with the velocity field from the source liquid simulation.

Buoyancy

The buoyancy force to apply to bubble particles, which will move the bubbles upwards towards the surface.

Behavior At Limits

Specifies the bubble particle behavior when hitting the Volume Limits.

Collide at Closed

Collide with any closed boundaries. At open boundaries this is equivalent to Kill.

Make Ballistic

Make any bubble particles ballistic when outside the Volume Limits.

Kill

Kill any bubble particles when outside the Volume Limits.

Solver

Timescale

An overall scale applied to the actual timestep. This parameter can be animated.

Closed Boundaries

The volume limits can be clamped to prevent any particles from entering or leaving the box. Any particles that leave open boundaries will be killed.

X, Y, Z

When closed boundaries is set, these select which sides will be closed.

Visualize Limits

Display the guide showing the maximum limits of the simulation.

Volume Limits Size

The size of the maximum volume.

Volume Limits Center

The center of the maximum volume.

Delete Internal Attributes

To save memory, the solver deletes several particle attributes used internally during the solving process. Some of these can be useful as debugging aids or for further processing, in which case they should be removed from this list.

foamamount

The 0-1 amount of foam forces applied to the particle.

foamdensity

The local foam density when Preserve Foam is enabled.

depth

The surface depth.

emit

The original emission probability for the particle.

See also

Dynamics nodes