'Surface' node does not work in MaterialX network

[pablode]

Tested with latest master (1.0.77) and Blender 3.0.1.

To reproduce:

1. open Blender, select 'USD Hydra' render engine
2. create a cube, assign a new material, convert it to MaterialX
3. change MaterialX graph to look like this:
   
4. observe that RPR Preview render is black
5. export USD scene to file
6. see that diffuse node is missing from exported material graph

def "Materials"
{
    def Material "surfacematerial_2"
    {
        token inputs:bsdf (
            renderType = "BSDF"
        )
        token inputs:edf (
            renderType = "EDF"
        )
        float inputs:opacity = 1
        token outputs:mtlx:surface.connect = </Cube/Material_001/Materials/surfacematerial_2/ND_surface.outputs:surface>

        def Shader "ND_surface"
        {
            uniform token info:id = "ND_surface"
            token inputs:bsdf (
                renderType = "BSDF"
            )
            token inputs:bsdf.connect = </Cube/Material_001/Materials/surfacematerial_2.inputs:bsdf>
            token inputs:edf (
                renderType = "EDF"
            )
            token inputs:edf.connect = </Cube/Material_001/Materials/surfacematerial_2.inputs:edf>
            float inputs:opacity.connect = </Cube/Material_001/Materials/surfacematerial_2.inputs:opacity>
            token outputs:surface
        }
    }
}

[DagerD]

Hi.

Seems like USD (our is 21.11) doesn't fully support MaterialX nodes. The MatX file we are generating is correct.

[pablode]

I think this is the issue we're talking about:
PixarAnimationStudios/OpenUSD#1502

Wrapping the nodes in a nodegraph seems to work.

[DagerD]

Yes, you are right. But this method doesn't make all nodes work.

[pablode]

I would expect this to be fixed in the latest USD version as multiple MaterialX issues have been addressed since USD 21.11:
PixarAnimationStudios/OpenUSD#1785
PixarAnimationStudios/OpenUSD#1784
PixarAnimationStudios/OpenUSD#1629
PixarAnimationStudios/OpenUSD#1581

I've implemented construction of complex graphs in my converter and it works:
https://github.com/pablode/guc/blob/97d156f0d1974aea6ba363e8ce3f0157cda02334/src/libguc/src/materialx.cpp#L388

[DagerD]

We have plans to update USD to latest and we will retest this issue.

[pablode]

Great to hear! I've recently did some work to update RprUSD to v22.05 - I've set up a draft PR here:
GPUOpen-LibrariesAndSDKs/RadeonProRenderUSD#557

[bsavery]

@DagerD please add a task to our sprint to upgrade.

[DagerD]

Created ticket BLEN-169.