Houdini 17.5 Nodes Dynamics nodes

POP Wrangle dynamics node

Runs a VEX snippet to modify particles.

On this page


This is a very powerful, low-level node that lets experts who are familiar with VEX tweak particles using code.

This node corresponds to the POP VOP DOP, but uses a textual VEX snippet instead of a VOP network.


This node requires that you understand the vex language. It is very easy to write incorrect code using this node.

This node runs the snippet on the detail or every point in the input geometry. The snippet can edit the input geometry by changing attributes. It can access information from other geometry using attributes and VEX functions.

  • Press on the node to see any error output from the snippet.

  • You can use the VEX function ch to evaluate parameters. The path is relative to this node (ch("parm") will evaluate the parameter parm on this node). This evaluation will be done at the current time.

  • Unlike the Attrib Create SOP, this does not use local variables. Further, all backtick expressions and $F variables will be evaluated at frame 1, not the current time. Use @Frame, @Time, or @TimeInc instead.


This node runs the snippet in the CVEX context.


The VEX snippet parameter lets you enter a snippet of VEX code to run on the input geometry. See VEX snippets for information on special syntax in the snippet parameter. See the VEX chapter for information on the VEX language.



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.


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


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.



A snippet of VEX code that will manipulate the point attributes. You can use @variable_name syntax to access geometry attributes.

Attributes to Create

Only create attributes if their names match this pattern. The default pattern allows any attribute to be created.

You can restrict the created attributes by replacing the * with a list of allowed names.

Bound attributes, such as vtxnum, cannot be created and will be ignored.

Enforce Prototypes

Automatic binding with the @ syntax can be convenient, but if your system becomes more complex there is the risk that a typo in an @ binding will silently just bind a non-existent attribute.

This option forces all @ bindings to be explicitly declared as prototypes before use.

Use Timestep

Determines if the current solver timestep will be used to apply this node.

If set, the current timestep size will be multiplied by the scale and used for the time increment for this operation. Otherwise, the time scale will specify an absolute fictitious time to integrate by.

By disabling the link between the actual real time and the microsolver time, you can perform operations in a separate, fictitious, time.

Time Scale

The timestep used for this microsolver will be scaled by this amount. This allows one to achieve non-realistic effects, such as parts of the simulation operating at different speeds than other parts.

Similarly, it is useful if a solver needs to be evaluated independently of the main timestep.

Data Bindings


The list of the geometry on the object to process.

Evaluation Node Path

VEX functions like ch() usually evaluate with respect to this node. Providing a path here can override where the path search starts from. This is useful for embedding in a digital asset where you would like the top level digital asset to be the search root.

Export Parameters

When a VEX parameter is exported, the bound attribute will be created if it doesn’t exist. This pattern can be used to override the export option on the VEX shader to avoid writing to or creating certain attributes. The pattern matches the VEX parameter, not the bound attribute. The attribute will still be bound for reading.

Autobind by Name

Will use the name of the attribute to determine which attribute binds with which parameter.

Integer attributes will bind to integer parameters. Float attributes will bind to float, vector, point, matrix, or matrix4 depending on their tuple size. String attributes will bind to strings.

Attribute Name, VEX Parameter

Manually specifies the bindings of each attribute.

Field Name, VEX Parameter

Optionally binds Scalar, Vector, Matrix, or Index fields from this object to the CVEX function. They will be sampled at the current P location of each element.

You can then access these with the @ syntax referring to the given Parameter.

Autobind Groups by Name

Automatically bind any groups to the integer parameter prefixed with group_.

Group Name, VEX Parameter

Manually specifies the bindings of each group.

Update Normals If Displaced

If points are being run over, and the P attribute is written to, but the N attribute is not written to, any incoming normals will become out of date. When this option is set, vertex and point normals will be updated when this occurs.


Input 1, 2, 3, 4

These control the four virtual inputs accessible inside of VOPs.

They can be accessed with the OpInput1-4 wires from the VOP, or with the @OpInput1-4 string parameters when using VEXpressions.

They can also be accessed numerically as 0-3 with VEX functions that take an input number.


No geometry is wired to this input.


The SOP geometry will be cooked prior to running the VEX and the result wired to this input.

DOP Data

Data in the current simulation to reference. Specified as an object/data, for example, pyro1/vel to refer to the velocity field of the pyro1 object (which will show up as three volume primitives).


Referring to the geometry currently being processed is special as you have to ensure a copy is made so the VEX functions can refer to its original form. This handles that for you.

Nth Context Geometry

These refer to the SOPs wired into the parent DOP Network itself.

Myself (No Reads from Outputs)

Refers the currently processed geometry. Does not make a copy so it is up to the user to ensure no reads are made from any attributes that are bound.

SOP Path

Path to a SOP to wire in.

DOP Data

Object/data path a piece of DOP data in THIS simulation to wire in.



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.


The following examples include this node.

Street Crowd Example Example for Crowd Solver dynamics node

Crowd example showing a street setup with two agent groups

The setup creates two groups of agents. The yellow agents are zombies which follow a path of the street. The blue agents are living pedestrians that wander around until they come into proximity of the zombies and then they swtich into a running state.

Triggers to change agent states are setup in the crowd_sim dopnet. The zombies group uses proximity to the stoplights and the color of the light to transition into a standing state when lights are red. The living group transition into a running state when they get close to the zombie agents.


The animation clips need to be baked out before playing the scene. This should happen automatically if example is created from Crowds shelf. Otherwise save scene file to a location of your choice and click Render on '/obj/bake_cycles' ropnet to write out the files. The default path for the files is ${HIP}/agents.

KeyframedGrains Example for POP Grains dynamics node

This example demonstrates keyframing the internal grains of a solid pighead to create an animated puppet.

See also

Dynamics nodes