Houdini Engine for Unreal
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Houdini Asset

Unreal creates a HoudiniAsset when you import a Houdini digital assset file (.hda, .hdalc) into an Unreal project. For education licenses, you can also import .hdanc file types.

The HoudiniAsset's .uasset file asssociates with the **.hda** file and stores a copy of its binary contents. The HDA copy is only used if the source HDA is not found.

If you drag a HoudiniAsset from the Content Browser into the viewport, it spawns a HoudiniAssetActor. This actor uses a HoudiniAssetComponent as its main root component.

The HoudiniAssetComponent is the main component that handles cooking, parameters, inputs and outputs of the digital asset in the level.

To check if your assets run inside Unreal via Houdini Engine, look at the HAPI_CLIENTNAME environment variable and verify it's set to unreal.

Export Houdini digital asset

This covers a basic way to export an asset from Houdini into a Houdini Digital Asset. Visit the Digital Assset documentation to learn how digital assets work and common uses.

  1. In Houdini, select the nodes for your asset. Make sure to set the display flag on the node you want to render.
  2. In the main menu bar, click New Digital Asset From Selection...
  3. Name your digital asset and click Accept.
  4. In the Edit Operation Type Properties pop-up window, make any changes to your settings.
  5. Click Accept.

This creates a new subnet node.


Import asset into Unreal

  1. In Unreal, open the content browser.
  2. Left-click drag your HDA file into the content browser. Your HDA file should appear as a HoudiniAsset.
  3. Left-click drag your HoudiniAsset into the viewport.

Your HoudiniAsset now appears in your viewport.

You can also import Expanded HDA files (HDA folders) into an Unreal project. Expanded HDA files doesn't store a local copy of the asset's content and requires the original file.

Note:** Imported assets uses relative paths if it's on the same drive as your project. Assets on different drives uses absolute paths. You should save the HDA file on the same drive as your project if you use a form of source control.

Updating Assets

After you import your digital asset, you can make revisions in Houdini without having to re-export and re-import your asset.

  1. In Houdini, make your changes to the nodes and/or node network.
  2. Right-click the subnet node and click Save Node Type.
  3. In Unreal's content browser, right-click your HoudiniAsset and click Rebuild All Instances. This triggers a rebuild of all instances of that asset.

You should see your HoudiniAsset updates in your viewport.


Houdini Asset Details


In the Details panel for a HoudiniAssetActor, the Houdini Engine section lists a number of options and actions that can be performed on the Houdini Asset.


The first thing that is displayed under the Houdini Engine logo is the Session Status. This indicates the current status of the Houdini Engine session, and lets you know whether or not the plugin is correctly talking to Houdini.

The statuses can be:

  • Not Started: The Houdini Engine session has not been initialized yet. (the plugin only starts a session after instantiating a new HDA or modifying an existing HDA).
  • Ready: The Houdini Engine Session was successfully created and the plugin is actively communicating with Houdini Engine.
  • Stopped: The Houdini Engine session was manually interupted by the user.
  • Failed: The plugin was unable to start a Houdini Engine session. The logs will likely indicate the reason of the failure.
  • Lost: The plugin had lots contact with the Houdini Engine Session. This likely indicates that either Houdini / HARS was manually interrupted, or that it crashed.
  • Failed - No License: This indicates that the plugin was unable to start a Houdini Engine session due to the lack of Houdini Engine / Houdini License.
  • Disabled: This indicates that the session type is set to None in the plugin settings. The plugin will not try to create a Houdini Engine session, and will not be able to instantiate/cook HDAs.


In the context of a HoudiniAssetActor, Cooking refers to the same concept as in Houdini. The outputs of a Digital Asset inside Houdini are recalculated when a cook is triggered. This generally happens either manually by a forced recook or (optionally) automatically when a parameter or input of the Houdini Asset changes.

Assets can also be cooked or rebuilt via Console Commands or Keyboard Shortcuts .

  • Rebuild Reimports the asset's .hda file and forces the asset to recook even if no change has been detected. This is useful when the underlying .hda has been updated or if something has gotten out of sync. Rebuilding an HDA will update its interface, and any new parameter or input will be visible in the details panel. Value of existing parameter or input will be kept after a rebuild.
  • Recook Triggers a forced cook of the Houdini asset in Houdini, even if its inputs or parameters haven't been modified. This will also force an update to the HDA's outputs. This is mostly useful when the On Parameter/Input change cook trigger is disabled.
  • Reset Parameters Resets all of the asset's parameters to their default state saved in the Digital Asset.
  • Temporary Cook Folder Indicates the directory that will be used to save the temporary output files generated by the Houdini Asset when cooking. This can also be set via the "unreal_temp_folder" attribute.

Most Houdini assets need to have some of their temporary cooked content written to a uasset file. Static Meshes, Materials, Textures, Landscape Infos etc. will be stored in the Temporary Cook Folder (by default Game/HoudiniEngine/Temp). All unused and unreferenced assets stored in this folder can be cleaned up automatically by using the Clean Houdini Engine Temp Folder entry in the Houdini Engine menu.


Baking refers to the creation of native Unreal assets or actors based on the outputs of the Houdini asset. Contrary to cooked outputs, which are still owned by the Houdini Asset Actor, and will be destroyed with it, baked output are independant, and are not tied to the Houdini Asset that generated them.

  • Bake Pressing this button will bake all the outputs of the Houdini Asset. The type of asset produced when baking can be control by the combo box next to the bake button.
  • Actor Can be used to bake the current asset outputs to Actors in the world. Baking to Actors will bake all outputs and then spawns new individual Unreal-native actors for each outputa. The new actors will be added to a new folder in the World Outliner.
  • Blueprint Can be used to bake the current asset outputs to a BluePrint Actor in the content browser. All of the Houdini Asset's outputs are baked to .uasset files in the bake folder, and a new Blueprint Actor class is created, which has components that uses all the baked outputs.
  • Foliage DEPRECATED. See the unreal_foliage attribute to specifcy foliage.
  • World Outliner: Not implemented in V2 Bakes a single Static Mesh and sets it on the first World Outliner input's actor, then disconnect the input. This option is only available if the asset has at least one world outliner input and generates one static mesh.

Bake Options can also be used to control or automate some of the baking process:

  • Remove HDA output after Bake After baking, all the temporary/cook outputs of the HDA will be automatically destroyed. This is especially usefull when baking to Actors, as this avoids having both baked and cooked outputs overlapping.
  • Auto Bake When enabled, the temporary outputs of the Houdini Assets are automnatically baked after a succesfull cook.
  • Recenter Baked Actors When enabled, this option automatically recenters the position of all the baked Actors, making them use the center of their bounding box instead of the HDA's location.
  • Replace previous Bake When enabled, baking will always overwrite the previous bake's results. When disabled, the baked files will be created incrementally, and will never replace previous bake files.
  • Bake Folder This can be used to set the directory where baked output assets will be saved if the "unreal_bake_folder" attribute is not set on the output. The Bake Folder is determined by checking the "unreal_bake_folder" attribute first (prim, detail), then this property, and lastly the default folder configured in the plugin settings.

All Houdini assets present in the level can be replaced automatically by their baked counterpart by using the Bake And Replace All Houdini Assets entry in the file menu. This process cannot be reverted and should only be used with final content.

Baking all or only selected Houdini Asset Actors can also be achieved via Console Commands or Keyboard Shortcuts .

Asset Options

Cook Triggers

  • On Parameter/Input change When any parameter or input is changed on the HDA, a cook of the asset will be triggered. Disabling this allows parameters/inputs to be changed without recooking the ouputs. Cooking must then be performed manually by pressing on the Recook button. This may be desired for complex assets that have a long cooking time.
  • On Transform change When enabled, translating, rotating or scaling the Houdini Asset Actor in Unreal will automatically trigger a recook of the HDA.
  • On Asset Input Cook When using another HDA in an Asset Input, this option lets you decide if this HDA should be recooked automatically when its input HDA is cooked (ie, after modifying its parameters or inputs).


  • Push transform to Houdini Indicates whether or not the Unreal world transform should be set on the digital asset's input node in Houdini.
  • Do not generate outputs Prevents the Houdini Asset from generating outputs. The asset will be cooked in Houdini, but will not produce anything in Unreal. This can be useful when chaining multiple Houdini Assets together via Asset Inputs.
  • Output templated geos When enabled, outputs will be created for templated nodes found in the HDA. Such templated outputs are editor-only, and will, by default, use a wireframe material. They can be useful to visualize helper geometry (like a cutting plane for example) directly in the Unreal Editor.

Input Curves & Editable Output Curves

  • Enable Curve Settings By default the settings in this section get their values from the plugin's default settings. To override any of these settings on a particular Houdini Asset Component, enable this setting.
  • Disable rot & scale Attributes If this is checked, the plugin will not create rot and scale attributes on the points of input curves and editable output curves that are sent to Houdini by this component. The default value for this setting can be configured in the plugin settings: SettingCurves.

Help and Debug

  • Show Cook log Displays the contents of the Houdini cook log. This is useful for troubleshooting/debugging the Digital Asset.
  • Asset Help Displays the contents of the Digital Asset help. The help is created via the Help tab in the Operator Type Properties window for the Digital Asset in Houdini.