POP Spin by Volumes dynamics node

A POP node that uses the vorticity of velocity volumes to spin particles.

On this page	Parameters Parameters Guide Bindings Inputs Outputs Locals
Since	13.5

The Spin by Volumes POP is designed to make it easy to add spin to a particle system from the vorticity of 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 torque, w, targetw, spinresist, and orient 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.

Vorticity Scale

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

Spin Type

There are several different ways to spin a particle system in response to vorticity from an external velocity field. They are broadly categorized as Torque, Angular Velocity, and Orientation updates.

Update Torque

Adjust the angular acceleration of the particles according to the torque scale, and the field vorticity. This acts like the Torque POP.

Update Angular Velocity

Directly change the angular velocity of the particles by blending in the field vorticity, causing them to swiftly match the velocity field.

Update Orientation

Directly update the particle orientations. This is useful as it allows the particles own angular velocity to be considered independently of the bulk fluid vorticity.

Treat as Wind

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

Torque Scale

Scales the torque for torque update.

Spin Resistance

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

Angular Velocity Blend

In Update Angular Velocity mode, the amount to mix the field vorticity into the particle angular velocity every timestep.

Angular Velocity Update

In Update Orientation mode the orientation of the particle is set directly from the field vorticity. The angular velocity is set according to this parameter:

No Change

The angular velocity is not updated.

Final Velocity

The angular velocity is set from the vorticity of the velocity field.

Note

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

Zero

The angular velocity is set to zero.

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.

Scale

Scale the guide geometry by this amount.

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 if 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