MaterialX: Unlit materials (single-output nodegraphs) don't work

[hybridherbst]

Description

Seems that nodegraphs that don't use a surface model (aka: are unlit) are not supported right now, e.g. this one doesn't work:

<?xml version="1.0"?>
<materialx version="1.38" colorspace="lin_rec709">
  <nodegraph name="just_unlit">
    <mix name="mix_color3" type="color3">
      <input name="fg" type="color3" value="0, 0.0986187, 0.186275" />
      <input name="bg" type="color3" value="0.735294, 0.735294, 0.735294" />
      <input name="mix" type="float" value="0.6" />
    </mix>
    <output name="out" type="color3" nodename="mix_color3" />
  </nodegraph>
</materialx>

The above works fine in https://academysoftwarefoundation.github.io/MaterialX/.

I think the relevant spec section is this, saying that if there's only a single output (and no surfacematerial) then that should be used, and it doesn't matter what its name is.

@sunag I think this should become a MeshBasicNodeMaterial, right?
Basically checking here if any surfacematerial was found, and if not, using the first output instead for an Unlit material.

Reproduction steps

1. Load the above MaterialX with MaterialXLoader
2. Note it returns zero materials

(Side note: after discussion on the MaterialX slack there are additional cases – not really relevant for this issue, just interesting for the future. This file is also valid and contains two materials (one implicit unlit one and another explicit surfacematerial): two-materials.mtlx.zip)

[sunag]

@sunagAcho que isso deveria se tornar um MeshBasicNodeMaterial, certo?

Yes, that would be the most appropriate.

Wouldn't it be more useful to expose the nodegraphs as we do with materials?

[hybridherbst]

The main point why I opened this issue is that loading the nodegraph-only .mtlx I posted above should return a single unlit material, without me having to do anything extra.

A way to access individual nodes might also be useful in the future – some viewers allow „picking“ which node should be displayed as result – but I think that might be possible on the NodeMaterial already?

[hybridherbst]

@sunag would it be possible for you to take a look at this issue? I'm not quite sure what the right approach would be. Would be great to see unlit MaterialX support.