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Parameter VOP node

Represents a user-controllable parameter.

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The Parameter node represents the value of one of the parameters the user can set in the shader’s parameter editor interface. For example, in a Balloon shader, you could define a parameter for the Balloon Color, allowing the user to control the shader’s color.

Parameters

Name

The name of the new parameter, both in the VEX function declaration, the definition of any OP type that uses the VOPNET, and the VOP tile’s output. If a parameter by this name already exists, this operator will reference it and will disable most of of its own fields, such as the Parameter Type and Parameter Label.

The name must not be a reserved keyword.

Prefix

The optional prefix to prepend to the parameter name. The prefix changes the final name of the new parameter, both in the VEX function declaration, the definition of any OP type that uses the VOPNET, and the VOP tile’s output.

Postfix

The optional postfix to append to the parameter name. The postfix changes the final name of the new parameter, both in the VEX function declaration, the definition of any OP type that uses the VOPNET, and the VOP tile’s output.

Label

A human-readable label for this parameter. Appears in the owner node’s parameters and the tooltip when the user hovers over the output on the VOP tile.

Show Label

Toggles the display of the parameter label in the owner node’s parameters.

Type

Specifies the VEX data type of the new parameter. It can also specify how the parameter should be represented in an OP dialog. For example, a VEX vector can be viewed as 3 float values or as a color value.

Default Values

Depending on the parameter type selected, it represents the default value of the parameter in the VEX function and in the OP type that uses this VOP network.

Comment

Brief description of the parameter. The comment is included after the parameter declaration in the generated code.

Use This Node to Set Parameter Attributes

When several Parameter VOPs have the same Parameter Name, only one of the nodes is used to control the attributes of the parameter. Houdini decides which node to use for this by searching all the nodes according to the user defined order, then traversing inside subnets. If none of them have this parameter turned on, the first one found is used to set the parameter attributes. If one or more do have this option turned on, the first one with this option is used, even if another node (with this option turned off) is found first.

Use Own Export Context

When several Parameter or Bind VOPs have the same Parameter Name, some other node will determine export context. However, it may be desirable to have each Parameter VOP exporting in a different context. In such cases, setting this parameter will enable this node to specify own export context, independent of the primary definer node.

Use Input Value If Parameter Not Bound

If true, the VEX function parameter is checked to see if it is bound to an attribute in the current VEX context (e.g. point color). If not bound, the input value is is assigned to the VEX parameter. In a Surface VOP context, a VEX parameter is bound if the geometry being shaded has an attribute with the VEX parameter name.

Has Corresponding Connector

If true, the shader node parameter defined by this Parameter VOP will also have a corresponding input connector, providing such shader nodes can have input and output connectors.

Export

Specifies whether the new parameter can be exported to other contexts (written to as well as read from). If set to Always or When Input is Connected, this operator gets an input. The value wired into this input is then assigned to the exported parameter. In a Surface network, exported parameters can be used to create deep rasters. In SOP and POP networks, the exported parameters create new geometry attributes.

Never

The parameter will not be exported.

Always

The parameter will be exported.

When Input is Connected

The parameter will only be exported if the node’s input is ultimately connected to another VOP.

Export in Context

The context or contexts that the parameter is exported to. Use whitespace characters as separators when specifying multiple contexts.

Provide Menu

If the parameter type is String or Integer, then you can provide a menu of choices instead of a string entry field or an integer slider.

No Menu

No menu is provided and the entry field or slider is displayed instead.

Menu from Items

A menu is generated from the list of items specified by the Menu Items parameter.

Menu from Script

A menu is generated from the script specified by the Menu Script parameter.

Menu Items

A list of pairs of strings. The first string in each pair is the parameter value if that menu item is chosen. The second string is the label that appears in the dialog for that menu item.

The strings are separated by spaces. If you want a value or label to contain a space, surround the item with double quotes.

Menu Script

A script that populates the generated menu. You can write the script in either Hscript or Python. We highly recommend using Python.

A Python script should generate a list (or tuple) of strings where the odd items are values and the even items are labels, for example:

["10", "Light", "50", "Medium", "100", "Heavy"]

Houdini handles the Python script in an odd way: if the script field contains a single line, Houdini treats it as an expression and expects it to evaluate to a list. If it has multiple lines, Houdini treats it as a function body and it must end with a return statement:

result = []
for input_node in hou.pwd().inputs():
    result += [input_node.path(), input_node.name()]
return result

For Hscript, the script should print a space-separated list of the form:

echo "token1" "Label 1" "token2" "Label 2" ...

Menu Script Language

The language used by the script populating the menu.

Invisible

Do not show the parameter in the shader’s parameter interface. The parameter still exists and will be used to generate shader strings.

Help tab

The contents of the text field appear in the help browser when the user clicks the help button for a node of this operator type in the parameter editor.

This help can be HTML, or you can use a simple but powerful wiki format to create documentation that looks like the native Houdini help. See how to write wiki-format help.

Inputs

Unbound or Export Value

The value assigned to the new parameter if the Export Parameter toggle is checked or if the parameter is not bound when Use Input Value If Parameter Not Bound is checked.

Outputs

Parameter

The value of this VEX function parameter.

Is Parameter Bound

True if the new parameter is bound to an attribute.

Examples

RampParameter Example for Parameter VOP node

This example shows how to control the particle colours using the temperature attributes from a pyro simulation using a Ramp Parameter VOP node.

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parameterVOP Example for Parameter VOP node

This example shows how inherit age as a custom attribute in to a surface shader to control opacity of particles with a ramp. It is accomplished using the Attribute Create SOP and a Parameter VOP.

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The following examples include this node.

CrowdHeightField Example for Crowd Solver dynamics node

Stadium Crowd Example Example for Crowd Solver dynamics node

Street Crowd Example Example for Crowd Solver dynamics node

DensityViscosity Example for FLIP Solver dynamics node

FlipColorMix Example for FLIP Solver dynamics node

FlipColumn Example for FLIP Solver dynamics node

FlipFluidWire Example for FLIP Solver dynamics node

DiffuseSmoke Example for Gas Diffuse dynamics node

grass

FluidGlass Example for Particle Fluid Solver dynamics node

CurveForce Example for POP Curve Force dynamics node

2dfluid Example for Smoke Object dynamics node

BreakWire Example for Wire Solver dynamics node

IndirectLightBox Example for Indirect Light object node

StickyDonut Example for Sticky object node

MotionVector Example for Mantra render node

Volume Rendering - Metaballs as Volume Example for Mantra render node

RampReference

Down Hill Lava Flow Example for Material shader node

FirePit Example for Material shader node

StyleDisplacement Example for Material shader node

Basic RIS Shading Example Example for RIS Shader Network shader node

VolumeNoiseIso Example for Mantra: VEX Volume Procedural shader node

BlendAttr Example for Attribute Composite geometry node

RandomMaterial Example for Attribute String Edit geometry node

FluffyTorus Example for Bake Volume geometry node

PathAnalysis Example for Find Shortest Path geometry node

ColourAdvect Example for Fluid Source geometry node

Clumping Example for Fur geometry node

FurBall Example for Fur geometry node

FurBallWorkflow Example for Fur geometry node

FurPipelineExample Example for Fur geometry node

FurTextureMap Example for Fur geometry node

Shaved Example for Fur geometry node

PaintPoints Example for Paint geometry node

AlphaOmega Example for Points from Volume geometry node

UnpackWithStyle Example for Unpack geometry node

volumeanalysis_grad Example for Volume Analysis geometry node

barycenter Example for Volume Reduce geometry node

volumesurface_explicitgrade Example for Volume Surface geometry node

volumesurface_simple Example for Volume Surface geometry node

ImportVolumes Example for Volume VOP geometry node

pscaleVOPSOP Example for Add Attribute VOP node

GroupPainted Example for Add Point to Group VOP node

VOPpointgroup Example for Add Point to Group VOP node

WornMetal Example for Curvature VOP node

ST_Colour

SimpleInsideSubnet

SimpleOutsideSubnet

CrinkleSphere Example for Inline Code VOP node

IntersectGrid Example for Intersect VOP node

RampParameter Example for Parameter VOP node

parameterVOP Example for Parameter VOP node

PointCloudIterateAverage Example for Point Cloud Iterate VOP node

PointCloudLookup Example for Point Cloud Iterate VOP node

RaytraceVopShader Example for Ray Trace VOP node

ParticleVolumeCollision Example for Volume Sample VOP node

See also

VOP nodes