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The Whitewater Source SOP generates emission particles and surface
and velocity volumes to be used as input to the
It calculates emission probabilities by analyzing the input particles and
volumes according to several criteria, choosing the maximum probability for
each candidate particle. The probability of each particle emitting
whitewater is stored in its
emit attribute. All particles with
non-zero emission probability are put in the
emitters group, and
non-emitting particles are deleted.
Input to this SOP will often be the output of a FLIP simulation, including
particles, a Signed Distance volume named
surface that represents the
liquid, and three velocity volumes named
containing the velocity values from the input fluid simulation.
Input can also consist of only particles from a Particle Fluid simulation or another simulation package, in which case this SOP can generate compatible volumes. Conversely, it can take surface and velocity volumes from the Liquid Solver or another volume-based liquid simulation and generate emission particles.
If the source fluid simulation is cached to disk or memory, this SOP allows quick preview and tuning of whitewater emission particles throughout the frame range of the source simulation. Choosing Hide Other Objects in the Scene Viewer can ensure no simulations solve in the background when scrubbing frames.
If the input particles have been packed using the Fluid Compress or Pack Points SOPs and are being read from disk files, ensure that Delay Load Geometry is enabled on the File or File Cache SOP used to read them in. With this setup, particles won’t actually load from disk until needed for emission, saving load time if using Distribution to divide emission into regions.
Create swirls of bubbles underneath the surface
Turn on Reseed Compressed Fluid on the Whitewater Source SOP. This will create new particles under the specified particle bandwidth to use as potential whitewater emitters.
Only emit whitewater at the surface
Turn on Limit By Depth on the Whitewater Source SOP. This will limit whitewater emission to particles with a certain depth range within the surface SDF. Alternatively, because FLIP compression already culls particles deep below the surface, it can often be effective (and faster) to simply turn off Reseed Compressed Fluid.
Only emit whitewater in a particular region
Change the Division Type to Specified Box on the Divisions tab of the Whitewater Source SOP. This will only load whitewater in the specific slice.
When fine-tuning emission criteria or generating whitewater for very large liquid simulations, it is important to understand the multiple stages of this node’s operation:
Culling the input particles based on Min Speed and Distribution settings.
Sampling the emission criteria from the input volumes into attributes on the culled points.
Mapping the sampled attribute values onto their emission values, with the final emission probability being the maximum of all emission values.
Visualization of the attribute or emission values for any criterion.
Grouping emitting particles into the
emittersgroup and culling any non-emitting particles.
Enabling parameters such as Keep Curvature Attributes allows inspecting the sampled attribute values directly in the Geometry Spreadsheet to determine good ranges for mapping to emission probability.
The Cache tab allows caching the candidate emission points to file after the Culling and Sampling stages. At this point there are typically far fewer candidate points and the input volumes no longer need to be sampled, so fine-tuning only Mapping stage parameters such as Min Accleration will be much faster.
Particles and / or surface and velocity volumes or VDBs from a liquid simulation.
Liquid Simulation Volumes
Surface and velocity volumes or VDBS from a liquid simulation.
Particles will only emit whitewater when they have sufficient speed and, optionally, are within a specified surface depth range. The maximum value from all other criteria is taken, then the particle’s emission probability is fit to a 0-1 range based on its speed.
The minimum speed at which a particle can emit whitewater.
The maximum speed at which a particle can emit whitewater.
Reseed Compressed Fluid
If the input fluid simulation has been compressed with the Fluid Compress SOP, automatically reseed potential emitter points below its specified particle Cull Bandwidth. Disabling this option will speed up emission and avoid creating emitters deep within the liquid, which is sometimes preferable. The density of the reseeded particles is controlled by the Particle Separation parameter on the Sources tab.
Limit By Depth
Limit whitewater emission to particles with a certain depth range within the surface SDF.
The minimum depth at which a particle can emit whitewater.
The maximum depth at which a particle can emit whitewater.
Output Points Only
Do not include input volumes in the output of this node.
Keep Non-Emitting Points
Do not cull points with zero emission probability. This is mostly useful for inspecting sampled attribute values.
Remap the speed curve from a linear mapping to a specific curve.
The ramp for remapping speed.
Apply a global ramp to the emission probability after all other criteria have been applied.
The ramp for remapping emission.
Visualize the emission probability of each source particle as color.
Emission Color Ramp
The ramp for mapping emission probability to color.
Emit From Curvature
Create emission particles based on the curvature of the surface SDF, which can be an effective criteria for detecting the leading edge of breaking waves.
Keep Curvature Attributes
sampled when Emit From Curvature is enabled. This option is useful for
inspecting the raw curvature values to determine good settings for
the mapping parameters below.
The minimum surface curvature at which to emit whitewater.
The maximum surface curvature at which to emit whitewater.
Max Velocity Angle
The angle between the velocity of the particle and the surface normal must be lower than this value for whitewater emission. This limit helps ensure that whitewater is only created on the leading edges of waves.
Remap the curvature emission probability from a linear mapping to a specific curve.
Visualize the sampled attribute values or emission probability from curvature.
The ramp for remapping curvature.
Emit From Acceleration
Create emission particles based on the magnitude of the acceleration occurring at a particle’s position within the velocity volumes. This value is calculated by comparing the particle velocity to the velocity at that point in the velocity volumes time-shifted by one previous frame. Acceleration is effective at detecting areas when particles are rejoining existing parts of the fluid, resulting in trapped air that causes whitewater.
Keep Acceleration Attributes
accelerationemit attributes sampled
when Emit From Acceleration is enabled. This option is useful for
inspecting the raw acceleration values to determine good settings for
the mapping parameters below.
The minimum acceleration magnitude at which to emit whitewater.
The maximum acceleration magnitude at which to emit whitewater.
Remap the acceleration emission probability from a linear mapping to a specific curve.
The ramp for remapping acceleration.
Visualize the sampled attribute values or emission probability from acceleration.
Emit From Vorticity
Create emission particles based on the the vorticity, or magnitude of rotation, within the velocity volumes. This criterion can be effective in identifying churning areas of the fluid, which generally occur deeper below the surface.
Keep Vorticity Attributes
vorticityemit attributes sampled
when Emit From Vorticity is enabled. This option is useful for
inspecting the raw vorticity values to determine good settings for
the mapping parameters below.
The minimum vorticity at which to emit whitewater.
The maximum vorticity at which to emit whitewater.
Remap the vorticity emission probability from a linear mapping to a specific curve.
The ramp for remapping vorticity.
Visualize the sampled attribute values or emission probability from vorticity.
This SOP can modify the input sources before generating emission particles or generate potential emission points or volumes from the input.
Particles and Volumes
The input contains particles to use as possible emission points and surface and velocity volumes.
The input contains only particles from which to generate surface and velocity volumes.
The input contains only surface and velocity volumes. Candidate emission points will be generated inside the surface volume.
Particles and VDBs
The input contains particles to use as possible emission points, a surface VDB, and a velocity VDB.
The input contains only surface and velocity VDBs. Candidate emission points will be generated inside the surface VDB.
Volume Division Size
The voxel size for generated volumes.
Volume Limit Size
The maximum size of generated volumes.
The distance outside the surface volume to extrapolate valid depth values. This value should be greater than the sum of the foam Depth and Iso Surface parameters of the Whitewater Solver.
The smallest distance between any two of the generated points in the initial configuration.
Particles Radius Scale
Scales the Particle Separation value to calculate a radius
stored in the
pscale attribute of generated particles.
The amount of jitter to apply to the positional values of the generated candidate points.
The random seed for the jitter operation.
Rebuild Surface SDF
Rebuild the depth values in the surface SDF, up to the distance specified by Max Extrapolation. This operation can be useful if the input surface volume only has valid depth values for a very narrow bandwidth around the zero iso-surface, but the Whitewater Solver needs depth values farther away from the surface to create a thick foam layer.
Delete all specified attributes on the input geometry.
Divide the input points into multiple regions, allowing the resulting source particles to be distributed across multiple machines. When divisions are active the bounding area of the Current Region will be displayed in this node’s guide geometry.
Turn off divisions.
Divide the box specified by the Box Size and Box Center into regions according to the Divisions parameter.
Press Enter in the Viewport to display a handle for sizing and positioning this box.
Surface Bounding Box
Divide the the bounding box of the input surface field into regions according to the Divisions parameter.
Divide the frustum specified by the Camera and its window parameters into regions according to the Divisions parameter.
The size of the domain to be divided into regions.
The center of the domain to be divided into regions.
Restrict emission to the specified camera’s frustum. If the input particles are packed geometry and loaded with Delay Load Geometry, no particles will be loaded from disk outside this region.
The distance from the camera to start emitting whitewater.
The distance from the camera to stop emitting whitewater.
Use Camera’s Window
If set, the camera’s window scale, offset, and crop parameters will be used in addition to the following Window X/Y settings.
The min/max portions of the camera’s view for emission. This allows you to add padding to ensure good boundary conditions or focus into a key area of the scene.
The number of regions along each axis.
Specifies the region to evaluate. If distributing simulations across
machines, this parameter will often be set to a variable like
represents the region for the current job.
Show the polygonized surface volume as guide geometry to help visualize the whitewater emission in the context of the source fluid simulation.
Controls the sample rate of the polygon conversion for the guide surface volume.
These parameters allow caching the candidate points after sampling the volumes into attribute values, but before mapping to final emission probability.
Cache Points Only
Only save points to the cache file.
Load Sampled Points From Disk
Use the cache to load sampled points.
Save to Disk
Sampled Output File
Create Intermediate Directories
Save in Background