Holdouts do not work in Path Tracer in UE 5.6

Development Rendering
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1

Holdouts in Path Tracer in 5.6 do not work correctly. The mesh with holdout enabled is supposed to be pure black (transparent in render) but instead it still has the diffuse color (pass). In UE 5.5 it works fine in both Lumen and Path Tracer but in UE 5.6 it works only in Lumen.

How it looks in Path Tracer (incorrect):

[Image Removed]

How it looks in Lumen (correct):

[Image Removed]

2

Steps to Reproduce

  1. Create any new project in UE 5.6 (E. g. Automotive Photo Studio template project)
  2. Enable `Support Hardware Ray Tracing`, `Path Tracing`, `Alpha Output` and `Support Primitive Alpha Holdout (Deferred)` in the Project Settings
  3. Enable holdout on a mesh
  4. Switch between `Lit` and `Path Tracing`
3

Hi Josef, yes this got broken. We are looking into it and will fix asap. Thanks for reporting!

4

Fix is here:

https://github.com/EpicGames/UnrealEngine/commit/57202bdc14beac2d717941b52783739872bf3baf

It will be in the next hotfix as well.

5

Thanks for the response! Could you please update us on this topic once it’s fixed? We’re building Unreal from the source, so if you could provide us with the commit id containing the fix, that would be perfect.