Houdini 17.5 Nodes VOP nodes

Snippet VOP node

Runs a VEX snippet to modify the incoming values.

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

This node is similar to the Point Wrangle SOP and other "wrangle" nodes, which let VEX snippets manipulate geometry.


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 when the node cooks. The values on the incoming wires are available as VEX variables. The snippet can modify the variables, then the values are passed as the node’s outputs. Each input has a corresponding output, whether the snippet modifies its value or not.

For example, if you have an incoming wire named foo, you can write the snippet:

foo *= 2

…and the foo output will have double the value of the foo input.

  • 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.

Creating inputs

  • The node has a "multi-input" that can accept any datatype wire. Wiring into this input creates a new input on the node. You can then use the value of the input by referencing its internal name as a variable. You can hover over a VOP node input/output to see its internal name.

  • You can rename an input by number to a fixed name using the Variable name # parameters. For example, you can say the first input will map to the variable foo. This allows you to rewire the input without having to change your code.

  • It’s possible to read and write global variables directly (for example, P and Cd in the SOP context). However, we strongly recommend you don’t use global variables directly. Houdini does not guarantee the order in which VOPs generate their code, so assigning values to globals can give unpredictable results. Instead, you should explicitly wire from the globals node into this node.

  • Unlike the Inline VOP, you do not prefix variables with $. (The snippet will be encapsulated in a function definition.)

Binding geometry attributes to VEX variables

In the SOP context and CVEX shaders applied to geometry, you will usually want to read and write geometry attributes in the snippet. The best way to access geometry attributes is by wiring a Bind VOP into this node.

However, in some cases, you may want to promote snippet parameter while the VOP network is locked, preventing the user from binding variables. In this case, the user can access attributes using the @ syntax explained below.


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.


Code Snippet

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

Include Files

List any files that need to be included at the start of your shader. You can use this facility to write complex functions, and then call them with a single line of VEX code. This is simply a list of files, such as voplib.h shading.h.

Outer Code

This parameter specifies the source code that should appear outside (before) the shader function definition.

Expand Expressions in Code

If you wish to use $ for temporary variables, you have to prevent the expansion of variable names or your temporaries will be expanded. However, using $ is not recommended.

Variable Name #

For each input connected to this operator you can specify the name for the input. If an empty string is specified in the table, the name will be copied from the output connected to each input. This will be the name used inside the code snippet to both read from the input and write to the output.

Bindings to Export

When an ad-hoc binding is created with the @ syntax, it is brought in as a read-only parameter to the generated VEX function. If it matches this string, however, it will be flagged as an export. In VOP SOP contexts, for example, this will cause it to create a new attribute.

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-existant attribute.

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

This applies to all @ bindings, not just attribute bindings, so if you wish to use @Frame you will need to prototype it as float @Frame;

Force Code Generation

Because a snippet may have side effects, such as creating geometry or printing output, the code is always added to the generated VEX code even if the outputs of the snippet are not needed. If you know you have no side effects, you can turn off Force Code Generation and avoid generating the snippet code if the outputs are unused.

Standard Variable Names Only

In some contexts, the attribute names can contain namespace separator characters such as a colon. Such characters are not valid in standard VEX variable names.

When this option is on, the @ binding variable name ends when a non-standard character is encountered.

With this option off, such non-standard characters are accepted as part of the VEX variable name, which is then encoded to avoid compiler errors. This allows binding VEX variables to such attributes.


Next Input

Where the next input value should be connected. Each input will create a corresponding output


The following examples include this node.

Stadium Crowd Example Example for Crowd Solver dynamics node

Crowd example showing a stadium setup

The setup creates a stadium crowd. The rotating cheer_bbox object is used as a bounding box for the agents. When they are inside it it will trigger a transition from a sitting to a cheering state. After a few seconds the cheering crowd sits back down by transitioning into a sitting state.


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.


To only see a section of the crowd for quicker preview there’s a switch node in /obj/crowdsource/switch_all_subsection. When 0 it will show all agents, when set to 1 will only show a small section.

Fuzzy Logic Obstacle Avoidance Example Example for Fuzzy Defuzz VOP node

This example shows agent obstacle avoidance and path following implemented using a fuzzy logic controller.

See also

VOP nodes