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The Whitewater Source SOP generates an emission field that is used by the
Whitewater Solver for birthing particles. Values in
the emission field correspond to probabilities that whitewater will appear at
that point. One can approximate the amount and location of newly-created
whitewater in a frame by visualizing the generated
emit VDB: density of the
fog is directly related to the birth rate. This node is also able to source
whitewater particles using the same method as the
Whitewater Solver; these particles are colored based
on their emission criteria, enabling assessment of particle counts, as well as
tuning of emission controls with concrete visual feedback.
In contrast to the previous particle-based sourcing, the new system is scale-independent with respect to the underlying liquid simulation. Consequently, it is much easier to control the physical amount of whitewater, without having to consider particle counts. Additionally, the new system is designed to facilitate full-pipeline prototyping for fluid simulations. That is, the artist is free to work with low resolution settings to find a satisfactory overall look for both the liquid and whitewater simulations; the particle count can then be increased for the higher-fidelity final pass on both solvers, without needing to worry about significant changes in the amount of whitewater.
This operator expects
vel volumes in its input.
Surface and velocity volumes or VDBs from a liquid simulation.
Extra Source Points
Extra points to splat into the emission volume. The emission amount for each extra source point (indicated by the value of its Emission Attribute) is distributed trilinearly to its nearest voxels.
Mask to limit the emission regions.
Geometry containing the emission volume (
emit). If Output Volumes
is enabled, the geometry also contains liquid’s
fields. If Source Particles is enabled, the birthed whitewater
particles are also included.
Limit by Depth
When enabled, whitewater will only be emitted within the specified range of depths.
An attempt will be made to activate the signed distance VDB to ensure that accurate depth readings to the specified limiting values can be made. The Maximum Half-Width parameter can be used to limit the number of voxels that get activated, preventing exhaustion of computing resources.
This should be turned on when using Narrow Band, and off for dense SDFs such as oceans.
All regions of the liquid that have a speed greater than the minimum are eligible to emit whitewater. Regions moving at or above the maximum speed are considered fully emitting.
By default, the speed is mapped linearly to the (0, 1) range. This behavior can be changed by specifying a custom mapping.
The final emission probability can be optionally remapped with the Emission Ramp.
Emit from Curvature
Augment emission rates with the curvature of the surface SDF, which can be an effective criterion for detecting the leading edges of breaking waves.
High resolution fluid simulations tend to contain fine details that may result in spurious curvature-based sourcing. To eliminate this, consider smoothing out surface field using controls under Surface Filtering.
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.
Range of curvatures that gets mapped to the (0, 1) emission range.
By default, the curvature is mapped linearly to the (0, 1) range. This behaviour can be changed by specifying a custom mapping.
Emit from Acceleration
Augment emission rates with the magnitude of acceleration. Acceleration is effective at detecting areas where particles are rejoining existing parts of the fluid, resulting in trapped air that causes whitewater. Note that this uses Eulerian acceleration, which is the time-rate of change of fluid velocity at fixed locations in space.
Range of accelerations that gets mapped to the (0, 1) emission range.
By default, the acceleration is mapped linearly to the (0, 1) range. This behavior can be changed by specifying a custom mapping.
Emit from Vorticity
Augment emission rates based on the vorticity (or magnitude of rotation) within the velocity volume. This criterion can be effective in identifying churning areas of the fluid, which generally occur deeper below the surface.
Range of vorticities that gets mapped to the (0, 1) emission range.
By default, the vorticity is mapped linearly to the (0, 1) range. This behavior can be changed by specifying a custom mapping.
Add Extra Sources
Use points of the second input as additional sources.
Only points of the second input that are part of this group will be used for extra emission.
Name of the attribute that stores emission amount for each source point. If the specified attribute does not exist, all points are assumed to be fully emitting.
This parameter controls how extra sourcing is combined with the emission volume.
Extra emission is added to the original volume.
Final emission is the maximum of the original volume and extra emission.
Emission in the areas influenced by extra sources is replaced by their value.
When this option is enabled, the node also outputs the liquid’s signed
surface) and velocity (
Voxel size for the emission volume.
Physical size of the emission region; this option can be used to restrict emission to a certain subregion of the fluid simulation.
Center of the restricted emission region.
Name (or name pattern) of primitive to use as an emission mask.
The filtering operations in this section can be used to smooth out the surface field before it’s used for generating the emission field. This can be helpful for eliminating noise from curvature-based emission.
Dilation is performed first, followed by smoothing and, finally, erosion.
If Output Volumes is enabled, the filtered surface will be exported by this node. Be wary of letting the Whitewater Solver use it, however, as aggressive smoothing may give rise to liquid regions that suddenly disappear.
The number of voxels to expand the surface by.
Smooth the surface using the specified filter for the given number of iterations.
The number of voxels to shrink the surface by.
When this option is enabled, whitewater particles will be created using the
Whitewater Solver’s method. This can be used to get a concrete idea of
emission amount for the generated
The actual number of particles birthed by the solver may actually be substantially fewer than this approximation if Limit Emission is turned on.
Whitewater scale controls the number of particles that are created to fill a given volume; value of this parameter should match Whitewater Scale of the solver.
A modifier on the emission amount; value of this parameter should match Emission Amount of the solver.
Color of particles that are birthed as a result of curvature emission.
Color of particles that are birthed as a result of acceleration emission.
Color of particles that are birthed as a result of vorticity emission.
Color of particles that are birthed as a result of extra sourcing.