Houdini 16.5 Nodes Dynamics nodes

POP Attract dynamics node

A POP node that attracts particles to positions and geometry.

On this page

The POP attract node applies a force to particles to steer them towards a target location.

This operator modifies the force attribute.

Using Point/Curve Attract

  1. Create a particle system using the Location or Source shelf tools.

  2. Select the particle system you want to be affected.

  3. Click the Point Attract or Curve Attract tool on the Particles tab.

  4. Select the object or curve you want to affect your particles.

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.

Goal

Attraction Type

Each particle is attracted to a goal position. This determines how that position is computed for each point.

Position

The Goal parameter is used.

Particles

A subset of particles inside this simulation is used. The Match Method is used to determine how they are targeted. This can be used to have some particles chase lead particles.

Points

A subset of points in an external geometry is used. The Match Method determines how they are targeted.

Surface Points

Points on the surface of an external geometry are targeted. This is useful if you want to target a point along a curve, for example.

Goal

The desired goal location. This is used in Position mode, and can be referred to in the other modes using Local Expressions.

Geometry Source

Specifies the geometry to use.

Use Parameter Values

Use the SOP specified in the SOP Path parameter.

Use DOP Object

Use the named DOP object in this DOP network.

Use First Context Geometry

Use the SOP connected to the DOP network’s first input.

Use Second Context Geometry

Use the SOP connected to the DOP network’s second input.

Use Third Context Geometry

Use the SOP connected to the DOP network’s third input.

Use Fourth Context Geometry

Use the SOP connected to the DOP network’s fourth input.

SOP Path

Path to the SOP (when Geometry Source is set to Use Parameter Values).

DOP Objects

Name of DOP Objects (When Geometry Source is set to Use DOP Object).

Point Group

The subset of points to use for targeting. This is not restricted to the current stream when in Particles mode.

Match Method

How the cloud of target points are assigned to each of the particles.

Average Position

The positions of all the target points are averaged and that becomes the target. If Number of Clusters is greater than 1, the cloud is divided into different regions using K-Means clustering and points are assigned to their closest cluster.

Point per Particle

Each particle is assigned to one point to follow.

Number of Clusters

In Average Position mode, how many clusters are formed. It is considerably slower to have more than one cluster.

Particle ID

When matching points and particles, this integer attribute is used to determine the match number on the particle. If the attribute doesn’t exist, the point number is used.

Goal ID

When matching points and particles, this integer attribute is used to determine the match on the goal geometry. If the attribute doesn’t exist, the point number is used. If the point number is used, Particle IDs greater than the number of destination points will wrap around.

If a point attribute is used and the Particle IDs cannot be found in the point attributes, attraction is disabled.

Primitive

In Surface Points, this controls which primitive whose surface point will be the goal.

UVW

The parameteric position on the surface. This is NOT a texture uv coordinate.

Force

Force Method

Accelerating

A force is applied in the direction of the goal. If it is within the reversal distance, a force outwards is applied.

Follow

The velocity of the next frame is towards the goal with the speed set to be equal to the leader.

Predict Intercept

Particles will try to predict the leader’s direction on the next frame and head it off.

Force Scale

The applied force is set to the normalized difference between the particle’s position and the goal position. It is then scaled by this force scale.

Reversal Distance

Particles within this distance will experience a repulsive, rather than attractive, force. At exactly the reversal distance the force is zero, growing until it reaches an additional Peak Force Distance.

Peak Force Distance

As particles are farther from their target, the force will keep increasing. This marks the distance at which they stop increasing based on distance. At this distance their force will be set by the forcescale.

This distance is added to the Reversal Distance.

Minimum Distance

Particles closer than this value to the goal will be slowed to a stop.

Maximum Distance

Particles farther than this value from their goal point will receive no forces.

Ambient Speed

If the goal speed is less than this value, this speed will be used instead as the goal speed.

Speed Scale

Scaling factor for the target speed.

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.

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

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 the data 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

This value is the simulation time for which the node is being evaluated.

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

This value 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

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

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

TIMESTEP

This value is the size of a simulation timestep. This value is useful to scale values that are expressed in units per second, but are applied on each timestep.

SFPS

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

SNOBJ

This is the number of objects in the simulation. For nodes that create objects such as the Empty Object node, this value will increase for each object that is evaluated.

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

NOBJ

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

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

OBJ

This value is the index of the specific object being processed by the node. This value will always run from zero to NOBJ-1 in a given timestep. This value does not identify the current object within the simulation like OBJID or OBJNAME, just 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 will be -1 if the node does not process objects sequentially (such as the Group DOP).

OBJID

This is the unique object 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.

The object identifier can always be used to uniquely identify a given object. This makes this variable very useful in situations where each object needs to be treated differently. It can be used to produce a unique random number for each object, for example.

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

This value is the simulation time (see variable ST) at which the current object was created.

Therefore, to check if an object was created on the current timestep, the expression $ST == $OBJCT should always be used. This value will be zero if the node does not process objects sequentially (such as the Group DOP).

OBJCF

This value is the simulation frame (see variable SF) at which the current object was created.

This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).

OBJNAME

This is 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 only on those 20 objects. This value will be the empty string if the node does not process objects sequentially (such as the Group DOP).

DOPNET

This is 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 node, you could write the expression:

$tx + 0.1

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

See also

Dynamics nodes