Jonathan Mack

Tim Crowson
Attached image:
- green is the diffuse color of the mtlx shader
- blue is the Usd Preview Surface color
- Houdini GL ignores the latter and renders the former (but this seemed to work as expect in 20.0

Basically... I can't seem to author a custom UsdPreviewSurface in 20.5 and actually see it in GL.

no, you cannot. The viewer will use the 'best' shader it knows how to render. Since the HoudiniGL viewer knows how to render a mtlx shader, it will use that.

freshbaked
I can't see your rule for ".exr" but it's probably set to Linear Rec709 or something. Either change it, or in your output filename add "ACEScg" token so it matches the appropriate rule.

You can also explicitly set the output colorspace on the Karma render settings node.

GustavBP
Is there any way around this, other than going through the hassle of creating an unlit surface and connecting to the surface output to preview WIP areas of the shading network?

make sure the network is not isolated from the material. I can be connected to anything, it doesn't have to be contributing meaningfully.