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A VEX structured type (usually called a “struct”) is a compound data type made up of named pieces of sub-data (such as integers, floats, vectors, matrices, and/or other structs) called members.

The definition of a struct specifies the names, datatypes, and UI labels of the members, like a template. This defines a new data type, similar to vectors and matrices. You can then create instances of the template and fill in the member values.

You can use structs in VOP networks through the Struct, Struct Pack, and Struct Unpack nodes.


In Houdini 13, structs are not used by Houdini. They are mostly useful for bundling wires together as a form of organization. Future versions of Houdini will integrate structs more deeply. For example, entities such as ramps and noise settings that are currently represented as multiple wires could be alternately represented as a struct.

Defined and ad-hoc structs

Houdini can work with structs in VOPs in two ways.

  • “Defined” structs are saved in a file in the Houdini path and are available everywhere in Houdini. This is useful for types you want to re-use and/or use in more than one network or share between networks. You create instances of defined structs using the Struct node.

  • “Ad-hoc” structs are created “on-the-fly” and are only available in the network containing the node. This is useful when you want to bundle wires together to simplify the network. You create ad-hoc struct instances using the Struct Pack node.

The nodes

  • The Struct node can bundle separate wires into a struct wire. The node requires that you specify (or create) a defined struct type, and creates inputs/outputs based on the defined members.

    You can also use the Struct node to un-bundle a struct wire into separate member wires.

  • The Struct Pack node bundles separate wires into a struct wire. Unlike the Struct node, it doesn’t require you to specify a defined struct type. Instead, it uses the types of any incoming wires (and names you specify in the parameters) to create an ad-hoc struct type.

  • The Struct Unpack node individual members out of an input struct wire by name. Usually you should use the Struct node to unbundle a struct wire into member wires. However, this node lets you write assets/subnetworks that accept any struct type, since it extracts members by name and type, whereas of wiring the outputs of Struct is by position.

    If the input struct has a member of the given name but the wrong type, it will cause an error in the network. Houdini forces you to explicitly set the expected type (instead of Houdini automatically changing the output type based on whatever struct type is wired in) to allow you to wire the outputs consistently.

How to

To...Do this

Create a new defined struct type

  1. Create a Struct node.

  2. In the parameter editor, click Edit.

  3. Use the Create new menu to create a new

  4. Use the VOP structure editor to define the members.

  5. Click Accept.

Create an instance of a defined struct

  1. Create a Struct node.

  2. In the parameter editor, choose the struct type from the Struct name menu.

  3. Wire member data into the inputs. The first output contains the struct instance.

Create an ad-hoc struct

  1. Create a Struct Pack node.

  2. Wire member data into the inputs.

  3. In the parameter editor, you can name the members. This will make wiring and debugging easier. If the name is blank, Houdini uses the name of the wire. In some cases this will be what you want anyway (for example, if the input is from a Parameter node, the default name is the parameter name).

  4. The output contains the struct instance.

Extract the members of a struct as separate wires

  1. Create a Struct node.

  2. Wire the struct into the first input.

  3. If you want to require that the input be of a certain defined type, in the parameter editor, set the Struct name to the type. The node will error if the input is not of that type.

  4. The outputs after the first contain the member data.

Extract one or more members of a struct by name

  1. Create a Struct Unpack node.

  2. Wire the struct into the input.

  3. In the parameter editor, set the Number of members you want to extract.

  4. For each item, set the Member n name and the type you expect the member to be.

    (The node will error if the input struct does not have all the specified members or if they are not of the specified type.)

  5. The outputs contain the extracted values.

Set the value of a struct member

  1. Create a Struct node

  2. Wire the struct into the first input.

  3. Wire new values into the inputs corresponding to the members you want to set.

Modify the value of a struct member

  1. Unbundle the struct into separate wires using Struct or Struct Unpack (see above).

  2. Wire the output(s) you want to change into nodes that modify the value.

  3. Create a second Struct node to re-bundle the struct. Wire the struct (first) output of the unbundling node to the struct (first) input of this node.

  4. Insert nodes between the member outputs of the “unbundling” node and the member inputs of the “rebundling” node to change the values.

Create a new instance of an ad-hoc struct given a wire of that type

  1. Create a Struct node.

  2. Wire the struct into the first input.

  3. Wire member data into the inputs. The first output contains the struct instance.


  • There is a default wire color for structs, but when you edit a struct you can specify a custom color for wires with that struct type.

  • To edit the struct definition files on disk “by hand”, see how structs are stored on disk for information about the file locations and format.

  • When you create a VOP digital asset, you can specify defined structs as input and output types.

  • Since Houdini looks for the definition files on the path (see below), if you have a shared network store on the path, you can define site-wide structs for use by shader writers.

Storage of defined types

The “defined” struct types available across Houdini are loaded from JSON files. Houdini loads any *.json files in a path defined by the HOUDINI_VOP_DEFINITIONS_PATH environment variable. The default path is HOUDINI_PATH/vop. The default file for struct types created using the editor in Houdini is $HOME/HoudiniX.Y/vop/vopdefinitions.json.

Houdini will also load struct definitions from an asset if the asset has a section named VopTypeDefinitions. This section should contain a JSON string in the same format as the files.

The following example shows the basic structure of a struct type definition file. The connectorColor key and label on members are optional.

    "typeDefinitions": [
            "type": "struct",
            "typeName": "StructTest",
            "connectorColor": {
                "type": "RGB",
                "data": [0.25, 0.2, 0.9]
            "variables": [
                    "typeName": "int",
                    "name": "a"
                    "typeName": "vector",
                    "name": "b",
                    "label": "Label B"
                    "typeName": "matrix3",
                    "name": "c"

The full format is defined in a “JSON schema” file in $HFS/houdini/vop/TypeDefinitions.json.schema.

Custom languages

The JSON configuration files can also define custom languages for VOPs.

A language defines certain aspects and behaviours for VOPs that belong to that language. For example, it determines whether VOP node input/output data types support uniform data types (in addition to varying).

The JSON configuration file allows you to define own custom languages and specify their aspects.

Once you define a language and give it a name, you can set the VOP operator render mask to that name, to assign the VOP to that language. VOPs like Parameter or Constant that don’t have any render mask inherit the language from the parent, when they are created. To explicitly set the language on the parent subnet, you can use HOM’s @ref hou.VopNode.setShaderLanguageName() function in the shelf tool that creates that subnet. You can also use that function directly on the Parameter VOP node (etc.) too.



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