string[] primvars:arnold:shader = [""] results in magenta shader even if material:binding is defined

[hollov]

To Reproduce
Context: I export separate USD layers for shader (from LookDev) and for geo cache (from Anim). And this two are composited in lighting (pretty standard imho)

I've attached 3 usd files but here's how to reproduce the behavior:
LKDV

1. Create a cube in Maya and assign e.g. an aiStandardSurface material
2. Export it to USD with assigned materials. I used "USD Export" with jobContext=[Arnold]
3. Result: LKDV.usd

ANM

1. [Optional] animate the cube
2. Assign some exotic shader that'll result in primvars:arnold:shader = [""]. It was dx11Shader for me.
3. Export one or more frames to USD without material; just geo. (Again, I used "USD Export" with jobContext=[Arnold] but with exportMaterials=0)
4. Result: ANM.usd

LIGHT:
Composit the two layers (ANM over LKDV).

Expected behavior
I'd expect that the LKDV material is visible in the composited USD scene because of the material:binding prim var (despite the fact that dx11Shader resulted in primvars:arnold:shader = [""] in ANM.usd). But the cube has magenta color instead

Used Software Versions

• Maya 2024.2
• mtoa-5.5.4
• mayausd-0.32.0_202505060957-eeb3aaa
• OS: Windows 11 Pro 25H2

arnold_usd_magenta_repro.zip

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-17735

[cpichard]

Hi @hollov,
arnold-usd will look at the composed stage and will use primvars:arnold:shader if it is set, it overrides material:binding. If you want to use material:binding you should remove primvars:arnold:shader. You assign shaders in maya ? are you using lookdevx ?

[hollov]

Hi @cpichard,
No, I don't use lookdevx.
Yes, I assign shaders in Maya. I assign a dx11Shader, export with Export Selected... with Arnold job context (or it is called "Plug-in Configurations" on the GUI) and primvars:arnold:shader = [""] is written in the USD file.

So I see two potential ways to fix this:

• dx11Shader (and other similar "exotic" shaders) shouldn't result in primvars:arnold:shader = [""]
• OR don't override material:binding if primvars:arnold:shader is effectively empty

If you want to use material:binding you should remove primvars:arnold:shader.

BTW my current workaround is exactly that: I post process the USD file that is exported by Maya and remove the empty primvars:arnold:shader primvars ;)