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You can create new surface and particle node types using VOPs/VEX. These nodes evaluate their internal VOP network or VEX program to manipulate point attributes on incoming geometry/particles.
You can define how the node modifies the incoming geometry using a VOP network or with a textual VEX source code.
Define the node’s behavior with a VOP network ¶
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In the network editor, go to the VEX Builder Networks level (Right-click
objin the path and choose Other networks ▸ vex). -
Create a
SOP Type network. -
In the parameter editor for the SOP Type node, set the SOP type name to the name you want the node to appear as in the Tab menu.
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In the network editor, double-click the SOP Type node to go inside and create the VOP network.
Define the node’s behavior with textual VEX code ¶
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Choose File ▸ New Operator Type.
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Set the name and label of the new node type, set the Operator style to VEX type, and set the Network Type to Geometry Operator.
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Click Accept.
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Use the Code tab of the new digital asset to edit the VEX definition. The function with a return type of
sopis the “main” function Houdini will run to manipulate the geometry.
How to wire/write the VOPs/VEX ¶
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Parameter expressions in VOP nodes are not evaluated per-point. You must use VOPs to manipulate the point data.
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You can use
Parameter VOP nodes nodes (or function parameters in VEX) to create a user interface for the node (controls for the parameters appear in the VOP SOP node’s parameter editor interface). -
If an attribute exists on the input geometry with the same name as a parameter, the parameter value will be overridden by the attribute. (Note that this refers to the “internal” parameter name, not the human-readable parameter label.)
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To access point attributes:
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In VOPs, create a
Global variables VOP node to access common point attributes (from the VOP SOP node’s first input) and feed it through other VOP nodes. For other attributes, you can use a parameter with the same name and type (so it will be overridden by the attribute) or use the 
Import Attribute VOP. -
In VEX, global variables are available for common point attributes (from the first input) such as
P. For other attributes, you can use a parameter with the same name and type (so it will be overridden by the attribute) or use the import() function.
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Only the first input’s geometry can be manipulated by the network/program. You can access information from the other inputs using the
Import Attribute VOP or the import() function in VEX. -
To add an attribute to the geometry…
In VOPs use a Parameter node and set Export to Always or When input is connected.
In VEX use a function parameter marked as
export(e.g.export float a = 0;), or the addattribute() function (especially to add/change an attribute whose name is not known at compile time, i.e. the attribute name is controlled by the user interface). -
In VEX code, the function with a return type of
soporpopis the “main” function that Houdini will call. You can define other “helper” functions alongside this main function as needed.
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