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POP Advect by Volumes dynamics node

A POP node that uses velocity volumes to move particles.

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Parameters
Inputs
Outputs
Locals
Examples

The Advect by Volumes POP is designed to make it easy to advect a particle system by a fluid simulation. Often the fluid simulation will be simulated as a separate pass and the velocity fields read off disk. However, the particles can be live-linked to an existing simulation.

This operator modifies the force, vel, and P attributes.

Parameters

Activation

Turns this node on and off. The node is only active if this value is greater than 0. This is useful to control the effect of this node with an expression.

Note

This is activation of the node as a whole. You can’t use this parameter to deactivate the node for certain particles.

Group

Only affect a group of points (created with, for example, a Group POP or Collision Detection POP) out of all the points in the current stream.

Parameters

Velocity Source

The source of the velocity volumes. The context geometry refer to the inputs of this DOP Network.

SOP

The SOP path which has three volumes to be interpreted as the x, y, and z components of the velocity field. These are often the result of importing a velocity field from a fluid simulation. You can directly link to a fluid simulation with something like /obj/AutoDopNetwork:smoke/vel.

DOP Object

The name of a DOP Object in the same simulation as this.

Field Name

The name of the velocity field in the DOP object.

Velocity Scale

A uniform scale to apply to the velocity field before any of the force applications.

Advection Type

There are several different ways to update a particle system in response to an external velocity field. They are broadly categorized as Force, Velocity, and Position updates.

The Update Force will adjust the acceleration of the particles according to the force scale, and the velocity field. This acts like the Force POP.

Update Velocity directly changes the velocity of the particles by blending in the velocity field, causing them to swiftly match the velocity field.

Update Position directly moves the particle positions. This is useful as it allows the particles own velocity to be considered independently of the bulk fluid velocity. Update Position is similar to what the Gas Advect DOP would perform.

Treat as Wind

Rather than treating the computed force as an amount of force to add to the particle’s velocity, treat it as a wind speed to be matched by the particle. This causes the particle to be dragged to the goal speed, avoiding overshoot.

Force Scale

Scales the force for force update.

Air Resistance

How strong of an influence to have on the particle. Higher values will cause it to match the wind velocity faster. This is also used to do a weighted average when competing winds are applied to the same particle.

Remap Force

Scale the force value by the force ramp, according to the forceramp variable in the VEXpression.

Force Ramp

The ramp that is evaluated according to each particle’s forceramp variable in the VEXpression. This value should be in the range 0-1.

Velocity Blend

In Update Velocity mode, the amount to mix in the field velocity every timestep.

Velocity Update

In Update Position mode it is possible to read in the velocity from the velocity field.

Note

The resulting velocity will be used by the particle integrator, so unless something special is done, this could result in double-moving the particle.

Advection Method

The method used for advecting the particles. The Single Step method takes only a single step in the velocity field, but the various Trace methods can take multiple steps within a voxel to avoid violating the CFL Condition. These methods are more accurate but take multiple velocity samples per step and are more expensive to evaluate.

CFL Condition

When tracing the particles, this controls how many voxels may be stepped in a single iteration. Higher values will result in faster tracing, and hence faster advection, but more errors.

Ignore Mass

Ignores any mass on the input particles.

Since forces are stored as force rather than accel (acceleration), this is done by multiplying the force by the mass attribute. This will then be canceled out by the solver.

airresist will also be similarly multiplied.

Ignoring mass ensures that small pieces of an RBD object move at the same speed as big pieces. This makes for a more controllable simulation.

Guide

Show Guide

Controls if the guide geometry for this node should be shown.

Note

This can also be disabled by using the Hidden flag on the DOP Node.

Plane

What axis to extract from the volume.

Offset

Where the plane should be positioned inside the volume. This is a relative coordinate, with -1..1 being the total range, so 0 means the center of the volume.

Resolution

The resolution of the visualization relative to the source velocity field.

Range

The speeds that correspond to the beginning and end of the visualization colors.

Bindings

Geometry

The name of the simulation data to apply the POP node to. This commonly is Geometry, but POP Networks can be designed to apply to different geometry if desired.

Evaluation Node Path

For nodes with local expressions, this controls where ch() style expressions in VEX are evaluated with respect to. By making this ., you can ensure relative references work. It is important to promote this if you are embedding a node inside an HDA you are also exporting the local expressions.

Inputs

First Input

This optional input has two purposes.

First, if it is wired to other POP nodes, they will be executed prior to this node executing. The chain of nodes will be processed in a top-down manner.

Second, if the input chain has a stream generator (such as POP Location, POP Source, or POP Stream), this node will only operate on the particles in that stream.

Outputs

First Output

The output of this node should be wired into a solver chain.

Merge nodes can be used to combine multiple solver chains.

The final wiring should go into one of the purple inputs of a full-solver, such as POP Solver or FLIP Solver.

Locals

channelname

This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).

Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.

This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:

dopfield($DOPNET, $OBJID, dataName, "Options", 0, channelname)

If the data does not already exist, then a value of zero or an empty string will be returned.

DATACT

This value is the simulation time (see variable ST) at which the current data was created. This value may not be the same as the current simulation time if this node is modifying existing data, rather than creating new data.

DATACF

This value is the simulation frame (see variable SF) at which the current data was created. This value may not be the same as the current simulation frame if this node is modifying existing data, rather than creating new data.

RELNAME