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Overview ¶
Houdini Engine for Unreal uses a headless version of Houdini meaning an internal Houdini scene is running in the background hidden from the user.
Houdini Engine creates and manages an internal Houdini scene when:
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a Houdini asset is instantiated in Unreal
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objects from Unreal are connected to one of your HDA’s input, and the Unreal geometry is transfered to an input node in Houdini
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an asset parameter is modified in Unreal and needs to be mirrored in the Houdini scene
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manipulators like curves and handles are modified in Unreal, and need to be replicated and linked to the Houdini scene
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geometries from the Houdini asset outputs are replicated into Unreal.
The plugin doesn’t create a session immediately after opening your Unreal project or loading a level that contains Houdini Asset. A session is only created when one of the asset in the level is modified, or if a new Houdini Asset is instantiated. Only assets that need to be cooked will be created in the Houdini scene, this means that a Houdini Engine session that has been manually started will contain no HDAs.
When debugging HDAs, make sure the assets to debug are created in the Houdini scene by recooking or rebuilding them.
Debugging Workflows ¶
You may want inspect the internal Houdini scene when an asset doesn’t behave as expected.
These three methods launches or saves the current internal Houdini scene and allows you to debug any issues on the Unreal asset. It’s also useful to see the data and metadata available in the Houdini scene.
There are three methods to debug your assets:
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Use Session Sync
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Open the scene in Houdini
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Save out a .hip file
Note
All three methods require a Houdini license to run a Houdini UI version.
Use SessionSync ¶
This method uses a live version of Houdini, synced with Unreal and allows you to send iterations between Houdini and Unreal.
You must be connected to a Session Sync session. See Start a SessionSync documentation.
If you use a regular Houdini Engine session, you can switch to Session Sync:
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In the Houdini Engine menu in Unreal, select Open Houdini Engine Session Sync.
This will terminate your current Houdini Engine session, start a UI version of Houdini with Session Sync, and connect the Unreal plugin to it.
When you open a session a sync, the Houdini scene is empty. Your Houdini assets will appear after you use it in Unreal such as instantiate, modify, or recook assets. You can also rebuild or recook an HDA to force it to appear.
If you want to debug a Houdini Asset that is currently not in the level.
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In Unreal’s Content Drawer, select your HDA asset.
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your asset, and select Instantiate at the origin. -
Your asset now appears at the origin in Houdini.
An advantage of debugging in Session Sync is the live sesssion of Houdini. You can unlock your HDA in Houdini and modify its content to fix the issue immediately. To unlock your asset:
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your node and select Allow Editing of Contents.
Any changes to the HDA will automatically trigger a cook and update of the outputs on the unreal side.
You can save your changes to the HDA by on the node and select Save Node Type. To revert your changes, select Match Current Defininition .
Open scene in Houdini ¶
This method opens the current internal Houdini scene in a new Houdini application.
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In the Houdini Engine menu in Unreal, select Open Scene in Houdini.
A new Houdini application lauches with the internal Houdini scene.
Save out HIP file ¶
This method saves out the current internal Houdini scene to a .hip file.
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In the Houdini Engine menu in Unreal, select Save Houdini scene (HIP).
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Save the file.
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Open the .hip file in your Houdini application.