three.js/examples/materialx/texture_opacity_test.mtlx

<?xml version="1.0"?>
<materialx version="1.39">
  <surfacematerial name="mat_texture_opacity_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="0.2, 0.6, 0.8" />
    <input name="opacity" type="float" output="out" nodegraph="opacity_texture_graph" />
    <input name="specular_roughness" type="float" value="0.2" />
    <input name="metalness" type="float" value="0.0" />
  </standard_surface>
  <nodegraph name="opacity_texture_graph">
    <image name="opacity_texture" type="float">
      <input name="file" type="filename" value="resources/Images/grid.png" />
      <input name="default" type="float" value="1.0" />
    </image>
    <output name="out" type="float" nodename="opacity_texture" />
  </nodegraph>
</materialx>