three.js/examples/materialx/roughness_test.mtlx

<?xml version="1.0"?>
<materialx version="1.39">
  <surfacematerial name="mat_roughness_test" type="material" nodedef="ND_surfacematerial">
    <input name="surfaceshader" type="surfaceshader" nodename="surface_shader1" />
  </surfacematerial>
  <standard_surface name="surface_shader1" type="surfaceshader" nodedef="ND_standard_surface_surfaceshader">
    <input name="base_color" type="color3" value="0.8, 0.8, 0.8" />
    <input name="roughness" type="float" output="out" nodegraph="roughness_map" />
  </standard_surface>
  <nodegraph name="roughness_map">
    <image name="roughness_image" type="float">
      <input name="file" type="filename" value="resources/Images/grid.png" />
      <input name="default" type="float" value="0.5" />
    </image>
    <output name="out" type="float" nodename="roughness_image" />
  </nodegraph>
</materialx>