Ray Tracing PSO Issue with Imported Mesh

Development Rendering
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1

I have a rock Mesh (~700k polygons) imported from Houdini. The object rendered just fine in Lumen. However, when I switched to Path Tracing, I got a message “Ray Tracing pso is not ready yet using fallback.”

So far, I have tried reimporting the mesh and set Fallback Triangle percent to 100. I also cleared out any shading cache in my files but nothing seems to work. I will be attaching pictures below.

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The mesh in Lumen

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Path tracing

So far my PC has not experience any lags or other error messages. I would love help since this is for my school project. Thank you!

2

Here is a few quick tips:

1: wait for all shaders to compile before starting pathtracing

2: close and reopen UE

3: Set default RHI to DX12

3

BTW what are you making?

5

TY I’ll try your suggestions and see if those work.

I’m just making a hanging stone table for my Heliot Emil brand exhibition case study project for this semester. Since I’m learning Houdini I want to create some rocks myself instead of relying on Quixel assets ^^

6

Cool. i am just a hobbyist so they solutions i provided may not work. Also do you know anything about depth fade fog Depth Fade fog bug - Development / Rendering - Epic Developer Community Forums. if you can help that would be great.

7

Unfortunately I don’t have enough expertise to help you as I rarely use UE5. For fog I really just either use the built-in Volumetric Fog or Ultra Dynamic Sky, which I think you might want to try the latter.

8

That message is usually a shader compile fallback, not a missing mesh. Path tracing builds a ray-tracing PSO for the material, and if the PSO is not ready yet the engine draws the fallback triangles. Setting Fallback Triangle Percent to 100 makes that fallback the whole mesh, which is why the rock looks different under path tracing even though Lumen is fine.

Things worth checking before reimporting again:

- Material: a 700k poly Houdini mesh often uses a complex shader (virtual textures, custom nodes, or a heavy blend mode). Path tracing has a stricter shader limit. If the material uses a feature path tracing does not support, you get this exact fallback. Try a simpler test material (base color, normal, roughness, metallic only) and see if the PSO message goes away. If it does, the problem is the shader, not the mesh.

- r.RayTracing.ShaderPBRMode and related console variables: make sure you are not in a mode that forces fallback. Also check Project Settings → Rendering → Ray Tracing. Path tracing needs ray tracing enabled, and the project should be restarted after changing that, not just the level.

- Nanite: if the mesh is Nanite, path tracing uses a different representation. The fallback message can show up while Nanite is still compiling or if Nanite and path tracing are fighting over the mesh. Try disabling Nanite on this mesh and see if the PSO becomes ready.

- Shading cache: you cleared it, which is good. After a clean, the first path-trace view can still show the fallback until shaders recompile. Watch the console for “Ray Tracing PSO” or shader compile errors. If there is a compile error, fix that instead of the percent slider.

- GPU / driver: this message also appears when the PSO compile is slow and then times out. If the machine is not lagging, it is more likely a shader issue than a stall. I would still try once with a fresh editor restart and the mesh on a simple material.

I would not keep the fallback at 100 for the final look. Once the PSO is ready, set the fallback percent back down so only unsupported pieces fall back. If you can, post the material’s blend mode and whether the mesh is Nanite. That usually narrows it to one of the ca

ses above.

9

Thank you for the help.

Prior to your comment, I switched back to UE 5.7, disabled Nanite, and exported the mesh with correct smoothing groups from Houdini and everything worked fine. I will try your suggestions again in 5.8 and see how it goes.