Michael Arrevalo

I’m trying to bring a pre-fractured and clustered asset from Houdini into Unreal as a Geometry Collection (via FBX), and I’m having trouble getting the clustering hierarchy to match what I see in Houdini.

In Houdini, I’m creating my own fracture setup (RBD workflow), generating piece IDs and a custom subcluster attribute for grouping. In the Geometry Spreadsheet everything looks correct — the pieces and cluster values are clearly defined and grouped the way I expect.

Before export, I’m converting the attributes to the Chaos-required ones:

unreal_gc_piece

unreal_gc_cluster

unreal_gc_name

I’ve also tested exporting with and without unreal_gc_cluster, depending on whether I want:

Root → Pieces
or

Root → Clusters → Pieces

However, when I import the FBX into Unreal and create a Geometry Collection, the hierarchy in Fracture Mode doesn’t always match what I expect. Sometimes it flattens everything under one root, or adds an extra hierarchy level I wasn’t intending.

So my question is:

When importing a fractured FBX into Unreal, does Chaos strictly read the unreal_gc_* primitive attributes from Houdini to build the hierarchy? Or are there additional requirements (packing, bone generation rules, cluster parent attributes, etc.) that affect how the hierarchy is constructed?