Hello,

I have created a running bison herd using the new Crowd SOP workflow:
https://www.youtube.com/watch?v=z8Z5PsnWFto [www.youtube.com]

I am trying to export the bison herd Agent Crowd SOP network to multiple FBX files; one FBX file per bison, as per this tutorial.

https://www.youtube.com/watch?v=tqc24KukrSY [www.youtube.com]

i.e. for importing into Unreal Engine 5.4.3.

However, when I try to import an exported FBX file from Houdini 20.5.278-Py3.11 into Unreal 5.4.3, I get the error:
Multiple roots are found in the bone hierarchy. We only support single root bone.

The source FBX file I'm using for the Bison run cycle has only one node at the root level. That one imports successfully into Unreal.
However, when I use the Agent Character Unpack SOP, downstream from my Crowd SOPs, it appears to move the geometry nodes to the root level for some reason.

https://www.sidefx.com/docs/houdini/nodes/sop/kinefx--agentcharacterunpack.html [www.sidefx.com]

The only difference I can tell from the FBX export tutorial and what I'm doing is they are using the older DOPs-based crowd workflow. But that shouldn't change the agent character skeleton hierarchy, right?

I have tried appending Skeleton SOPs or Parent Joint SOPS to move the multiple nodes from the root level as children of a single root node, but that didn't work.

What am I missing?
How do I ensure the FBX files that are being exported have only one root node?

Thanks

Here is a zip file of a test Houdini scene file that reads in the source bison run cycle FBX file, feeds it through a Crowd SOP workflow, and then writes it back out to an FBX file.
The source FBX file loads into Unreal because it has a single root node. The output FBX file does not import into Unreal because it has multiple root-level nodes.