Houdini 20.5 Nodes Dynamics nodes

POP Curve Force dynamics node

A POP node that creates forces generated from a curve.

On this page
Since 13.0

The POP Curve Force node applies a force to particles to steer them along, toward, or around a curve.

This operator modifies the force attribute.

Note

This node creates forces which can cause the particles to get pushed away from the curve and outside the Max Influence Radius. For curves with sharp turns or complex shapes, the Suction Force will need to be balanced with the other forces to keep the particles near the curve and under the influence of the Curve Forces.

Note

For convenience, this node creates an oncurve attribute whose value is 1 for particles on the curve and 0 for particles beyond the Max Influence Radius. This attribute can be used to influence other forces or to create particle groups.

Using Curve Force

  1. Create a curve using the Draw Curve shelf tool.

  2. Create a particle system along the curve using the Location or Source shelf tools.

  3. Click the Curve Force tool on the Particles tab.

  4. Select the curve you want to affect your particles and press Enter.

Tip

The particle system should be inside the diameter of the force tube.

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.

Curve Force

Geometry Source

The curve geometry to import.

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. When this checkbox is enabled, the Global Falloff From Curve will scale the air resistance.

Air Resistance

How much particles are to be influenced by this wind field.

Max Influence Radius

The maximum distance from the curve where forces will be applied to the particles.

Individual Forces

Follow Scale

The amount of force to apply to the particles which will push along the length of the curve, in the direction it was drawn. Negative values will reverse the direction.

Suction Scale

The amount of force to apply to the particles which will push them toward the curve itself. Negative values will cause the particles to be pushed away from the curve.

Orbit Scale

The amount of force to apply to the particles which will cause them to orbit around the curve. Negative values will cause the direction of the orbit to be reversed.

Inherit Velocity Scale

If the curve geometry has a velocity attribute, this parameter controls how much of the curve’s velocity will be transferred to the particles.

Scale Radius Along Length

This ramp is a scale on the Max Influence Radius and allows the user to vary the distance from the curve where forces will be applied along the length of the curve.

Follow Force Falloff

Follow Force Falloff From Curve

This ramp controls how the follow force falls off as it moves away from the curve, up to the Max Influence Radius.

Suction Force Falloff

Suction Force Falloff From Curve

This ramp controls how the suction force falls off as it moves away from the curve, up to the Max Influence Radius.

Orbit Force Falloff

Orbit Force Falloff From Curve

This ramp controls how the orbit force falls off as it moves away from the curve, up to the Max Influence Radius.

Velocity Force Falloff

Incoming Vel Force Falloff From Curve

This ramp controls how the inherit velocity force falls off as it moves away from the curve, up to the Max Influence Radius.

Global Forces

Global Force Falloff From Curve

This ramp controls the falloff of all forces from the curve up to the Max Influence Radius. When the Treat As Wind checkbox is enabled, this force can also be considered the air resistance.

Force Along Length

This ramp is a scale on all forces from the curve from the beginning of the curve until the end.

Shaping

Resample Curve

This checkbox enables resampling of the curve in order to allow the user to control the number of times the curve force is sampled along its length.

Max Segment Length

How often the curve should be sampled along its length.

Guides

Show Guide Geometry

This checkbox determines whether the curve force guide geometry will be shown in the viewport.

Guide Color

The color of the curve force guide geometry.

Guide Spacing

How often to divide the incoming curve when creating the guide geometry. Often, the guide geometry does not need to be as detailed as the curve which is used internally to generate the forces. Lower numbers will give more accurate results.

Show Curve Only

This checkbox will adjust the guide geometry to show only the incoming curve without any indication of the Max Influence Radius.

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

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to the name of the relationship to which the data is being attached.

RELOBJIDS

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affected Objects of the relationship to which the data is being attached.

RELOBJNAMES

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the names of all the Affected Objects of the relationship to which the data is being attached.

RELAFFOBJIDS

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affector Objects of the relationship to which the data is being attached.

RELAFFOBJNAMES

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the names of all the Affector Objects of the relationship to which the data is being attached.

ST

The simulation time for which the node is being evaluated.

Depending on the settings of the DOP Network Offset Time and Scale Time parameters, this value may not be equal to the current Houdini time represented by the variable T.

ST is guaranteed to have a value of zero at the start of a simulation, so when testing for the first timestep of a simulation, it is best to use a test like $ST == 0, rather than $T == 0 or $FF == 1.

SF

The simulation frame (or more accurately, the simulation time step number) for which the node is being evaluated.

Depending on the settings of the DOP Network parameters, this value may not be equal to the current Houdini frame number represented by the variable F. Instead, it is equal to the simulation time (ST) divided by the simulation timestep size (TIMESTEP).

TIMESTEP

The size of a simulation timestep. This value is useful for scaling values that are expressed in units per second, but are applied on each timestep.

SFPS

The inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.

SNOBJ

The number of objects in the simulation. For nodes that create objects such as the Empty Object DOP, SNOBJ increases for each object that is evaluated.

A good way to guarantee unique object names is to use an expression like object_$SNOBJ.

NOBJ

The number of objects that are evaluated by the current node during this timestep. This value is often different from SNOBJ, as many nodes do not process all the objects in a simulation.

NOBJ may return 0 if the node does not process each object sequentially (such as the Group DOP).

OBJ

The index of the specific object being processed by the node. This value always runs from zero to NOBJ-1 in a given timestep. It does not identify the current object within the simulation like OBJID or OBJNAME; it only identifies the object’s position in the current order of processing.

This value is useful for generating a random number for each object, or simply splitting the objects into two or more groups to be processed in different ways. This value is -1 if the node does not process objects sequentially (such as the Group DOP).

OBJID

The unique identifier for the object being processed. Every object is assigned an integer value that is unique among all objects in the simulation for all time. Even if an object is deleted, its identifier is never reused. This is very useful in situations where each object needs to be treated differently, for example, to produce a unique random number for each object.

This value is also the best way to look up information on an object using the dopfield expression function.

OBJID is -1 if the node does not process objects sequentially (such as the Group DOP).

ALLOBJIDS

This string contains a space-separated list of the unique object identifiers for every object being processed by the current node.

ALLOBJNAMES

This string contains a space-separated list of the names of every object being processed by the current node.

OBJCT

The simulation time (see variable ST) at which the current object was created.

To check if an object was created on the current timestep, the expression $ST == $OBJCT should always be used.

This value is zero if the node does not process objects sequentially (such as the Group DOP).

OBJCF

The simulation frame (see variable SF) at which the current object was created. It is equivalent to using the dopsttoframe expression on the OBJCT variable.

This value is zero if the node does not process objects sequentially (such as the Group DOP).

OBJNAME

A string value containing the name of the object being processed.

Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.

The object name can also be used to treat a number of similar objects (with the same name) as a virtual group. If there are 20 objects named “myobject”, specifying strcmp($OBJNAME, "myobject") == 0 in the activation field of a DOP will cause that DOP to operate on only those 20 objects.

This value is the empty string if the node does not process objects sequentially (such as the Group DOP).

DOPNET

A string value containing the full path of the current DOP network. This value is most useful in DOP subnet digital assets where you want to know the path to the DOP network that contains the node.

Note

Most dynamics nodes have local variables with the same names as the node’s parameters. For example, in a Position DOP, you could write the expression:

$tx + 0.1

…to make the object move 0.1 units along the X axis at each timestep.

Examples

CurveForce Example for POP Curve Force dynamics node

This example demonstrates the use of the POP Curve Force node to control the flow of a particle sim AND a flip fluid sim.

See also

Dynamics nodes