Shader graph reconnection does not update IPR — renderer becomes unresponsive

[podorvan]

When assigning an Arnold Material Builder to a Mesh primitive and switching to the Arnold renderer in Hydra's IPR, the render state becomes unresponsive after specific shader graph modifications. Specifically, if you navigate inside the Arnold Material Builder and change the Material Output connection from Standard Surface to a texture node (e.g., checkerboard), the raw texture renders as expected. However, switching the Material Output back to Standard Surface produces no visible update — the IPR render remains frozen on the previous state. No further shader graph changes have any effect until the renderer is manually restarted.

Steps to reproduce the behavior:

1. Create a Mesh primitive and assign an Arnold Material Builder to it.
2. Switch the Hydra renderer to Arnold and start IPR.
3. Dive inside the Arnold Material Builder.
4. Disconnect Standard Surface from the Material Output and connect the Checkerboard node instead.
5. Observe the raw texture render — this is expected behavior.
6. Reconnect Standard Surface back to the Material Output.
7. Observe that the IPR render does not update; further shader edits have no effect until the renderer is restarted manually.

Reconnecting Standard Surface to the Material Output — or any subsequent shader graph change — should immediately trigger an IPR update, reflecting the correct shaded result without requiring a manual renderer restart.

• Arnold: HTOA 6.4.6.1

• USD: 23.11

• OS: Windows 11 Pro

• Houdini 21.0.596

• The issue does not reproduce in Houdini 20.5 with HTOA.

• A workaround is available: referencing the problematic primitive (with its assigned material) into another prim restores normal IPR behavior and shader responsiveness.

arnold_ipr.zip