Houdini Engine for Unreal
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The asset parameters that are exposed on a Houdini Asset will be displayed in the Details Panel in Unreal. They can be easily manipulated and modified in Unreal, and will automatically trigger a recook of the asset when modified if the OnParameterChange cook trigger is enabled in the Asset Options.

Some dynamic parameter options, such as Disable When and Hide When are supported, and will be reflected in the Unreal UI. If a numerical parameter's unit is set in Houdini, it will be displayed next to its value in the details panel as well.

Please note that parameters that are marked as invisible, or whose parent are marked as invisible in Houdini will not be displayed or accessible in Unreal.

In Version 2, it is now possible to edit parameters when selecting multiple HDAs of the same type. When multiple HDAs are selected, the values displayed will be that of the first selected HDA. Changing a parameter will alos apply the change for that parameter to all the selected HDAs.

By default, a parameter's tooltip in Unreal will be the following: "Label (Name) Type Tuple_Size".

If you want to customize that tooltip, you can simply edit the parameter's help in the type properties dialog, and that help string will be used as the parameter's tooltip in the plug-in instead of the default string.

Version 2 stores the default values per parameter. The "Revert" arrow and "Reset Parameters" buttons are now only enabled/visible when there are non-default parameters.

Houdini assets support a wide range of parameter types and configurations. However, not all parameter types and configurations are supported by the Unreal plug-in.

Following is a list of all the supported parameter types:


Integer parameters are supported, as well as Integer tuples.

Values can be edited either by typing in the fields or by using the slider. The min and max range of the parameter can be used to constrain the values of the parameter and the slider. If not set, the default range for the slider is [0 10].


Special Types of Integer parameters are also supported:

  • Int Vector 2, Int Vector 3 and Int Vector 4 are displayed in the same way as their Int Tuple equivalent.
  • Logarithmic Integer parameters are properly supported in Version 2, so their UI slider be logarithmic unlike in version 1.
  • Min Max Integer will be displayed as an Int Tuple 2 parameter.

It is also possible to create Integer Menus.



Float parameters are supported, as well as Float tuples.

Float parameters with a tuple size of 3 will automatically be recognized and displayed as a XYZ Vector3 parameter. Their values will also be automatically converted from Houdini to Unreal's Coordinate Systems. If you want to ignore the automatic swapping for a Vector3 / Float3 parameter, you can add the hengine_noswap Tag to the parameter.

Values can be edited either by typing in the fields or by using the slider. The min and max range of the parameter can be used to constrain the values of the parameter and the slider. If not set, the default range for the slider will be [0 10].


Special Types of Float parameters are also supported:

  • Angles will be displayed as a normal float parameter, and its unit will be displayed in Unreal if it is set to "deg" in Houdini.
  • Direction Vectors and Float Vector 3 will be displayed as a XYZ Vector, and their values properly converted to Houdini's Coordinate Systems.
  • Logarithmic Floats are properly supported in Version 2, so their UI slider be logarithmic unlike in version 1.
  • Min Max Floats will be displayed as a Float Tuple 2 parameter.
  • Float Vector 2, Float Vector 4, UV and UVW will be displayed in the same way as their Float Tuple equivalent.


String parameters are supported, as well as string tuples and string Menus.

Multiline strings are now supported in version 2 of the plugin.


In version 2, string parameters now support Drag & Drop. When an asset is dropped on a string parameter, its value is automatically set to the dropped asset's reference. This is especially useful when using unreal_instance / unreal_material attributes.

Additionally, String parameters can be turned into an asset picker, that sets the string value to the selected asset's reference. To do so, add the "unreal_ref" tag (with its value set to 1) to the string parameter. When using this tag, you can also use the "unreal_ref_class" tag to specify a class used to filter the asset visible in the picker. (for example, setting the tag value to "MaterialInterface" will list all Materials and MaterialInstances).


Button parameters are supported. Clicking on them will change their value to 1 in Houdini and trigger their Callback Script if it's been configured.

Note that a button parameter channel-linked to a node inside the asset must be made editable in that asset's Type Properties in order to work.


In version 2, Button Strip parameters are also supported.


Color parameters are supported.


The color can be choosen using the standard Unreal color widget that is displayed after clicking on the color parameter.


File parameters are supported (as well as File - Geometry and File - Images parameters. The edit box will display the path to the selected file. You can select a file by clicking on the browse button. When setting a file parameter's value, paths relative to the unreal project are converted to absolute paths in Houdini. The parameter's "Browse Mode" is used to decide if a "New File" or "Open File" dialog is opened when browsing.


In version 2, the support for File parameters has been improved: custom extensions, directory, new files are now supported. You also have the ability to preset the default directory that will be used when opening the Browse dialog by using the "default_dir" parameter tag.


Some types of Folder parameters are supported by the plug-in:

  • Tabs folder are now properly represented as tabs in the details panel. Clicking on a tab will display the corresponding child parameters it contains.
  • Radio are also supported, and will display a radio button in the tab.
  • Simple and Collapsible folders are also supported.

When parenting Folders, the parent's visiblity will modify its children's.

In version 2, the UI for tabs, radio, and collapsible folders has been improved.



Labels parameters are supported and can be used to display text in the detail panel of an asset.


In version 2 of the plugin, Column labels are supported, but will display their values as rows.


Multiparm Block type folders are supported, but List, Scroll and Tabs Multiparm blocks will all be represented as an array of parameters.

The first line will allow you to define the size of the MultiParam array, and either to add or remove elements in it, or to clear it.

All the parameters will then be listed, with additional x and + buttons that will allow you to remove the current element or to insert a new one.

Multiparms nested inside other multiparms are supported since Houdini 16.0.640.



Both Float Ramps and Color Ramps are supported by the plug-in.

All the key points of the ramps are displayed in an array under the ramp. This array allows you to set the point values, their interpolation, and delete or add new points.

Right-clicking on a key will allow you to choose its interpolation type. You can also right click on the ramp to add a new key.


Color ramps work in a similar way than Float Ramps. You can set the color of each point by clicking on its color parameter, and choosing it in the Color Picker.


In Version 2, the UI for Ramps parameters has been improved, and they gained the ability to easily turn off auto-update while editing them. When turned off, this let you edit the ramp without causing constant recooks of the houdini asset. Re-enabling auto-update will commit all the changes made to the ramp parameter and trigger a cook.


It is possible to add separator parameters to your asset. They will work as advertised in the details panel of the asset in Unreal.


Toggle parameters are supported and will be displayed as checkboxes in the detail panels. It is possible to use them to control dynamic options such as Disable When and Hide When on other parameters.


Operator Path

Operator Path parameters are supported if the Op Filter option is set to "Any Object" or "Any SOP". Geometries can be connected to these parameters if they refer to an Object Merge node.

They will have the same behaviors as Inputs, as the connected geometry will be sent through Houdini Engine, and will be available as an input node on the Houdini side. The path of the input node is then set to the parameter.


See Object Path Inputs for more details.


Single Selection Menus can also be used with Integer and String parameters. However, Mini Menu, Field, and Multi Selection Menu are not supported. Also, Menu Scripts cannot be used to generate menu items dynamically.