Detecting MaterialX translucency in Storm

ashwin.bhat · February 26, 2024, 10:13pm

Hi

We are running into an issue with how storm detects if a material is translucent .
_GetMaterialTag checks for the presence of specific token opacity, transmission etc to determine if the material is Translucent

github.com

PixarAnimationStudios/OpenUSD/blob/dev/pxr/imaging/hdSt/materialXFilter.cpp#L753


      
          auto const& opacityConnIt = terminal.inputConnections.find(_tokens->opacity);
          if (opacityConnIt != terminal.inputConnections.end()) {
              return HdStMaterialTagTokens->translucent.GetString();
          }
          
          auto const& transmissionConnIt = terminal.inputConnections.find(
                                                      _tokens->transmission);
          isTranslucent = (transmissionConnIt != terminal.inputConnections.end());
          
          // Then check the opacity and transmission parameter value
          if (!isTranslucent) {
              for (auto const& currParam : terminal.parameters) {
          
                  // UsdPreviewSurface
                  if (currParam.first == _tokens->opacity && 
                      currParam.second.IsHolding<float>()) {
                      isTranslucent = currParam.second.Get<float>() < 1.0f;
                      break;
                  }
                  // StandardSurface
                  if (currParam.first == _tokens->opacity &&

But later that tag is not used for shadergeneration

github.com

PixarAnimationStudios/OpenUSD/blob/2d6099f3c5e72038edd446d8083a3e716b6e5408/pxr/imaging/hdSt/materialXFilter.cpp#L87


      
          
          
          ////////////////////////////////////////////////////////////////////////////////
          // Shader Gen Functions
          
          // Generate the Glsl Pixel Shader based on the given mxContext and mxElement
          // Based on MaterialXViewer Material::generateShader()
          static mx::ShaderPtr
          _GenMaterialXShader(mx::GenContext & mxContext, mx::ElementPtr const& mxElem)
          {
              bool hasTransparency = mx::isTransparentSurface(mxElem,
                                         mxContext.getShaderGenerator().getTarget());
          
              mx::GenContext materialContext = mxContext;
              materialContext.getOptions().hwTransparency = hasTransparency;
              materialContext.getOptions().hwShadowMap = 
                  materialContext.getOptions().hwShadowMap && !hasTransparency;
          
              // MaterialX v1.38.5 added Transmission Refraction method as the default
              // method, this maintains the previous Transmission Opacity behavior.
          #if MATERIALX_MAJOR_VERSION >= 1 && MATERIALX_MINOR_VERSION >= 38 && \

The main problem with this approach is that there could be a node that wraps standard surface and defines translucent using a different name.
e.g. <input name="opacity" type="color3" value="1, 1, 1" interfacename="surface_cutout" />

github.com

autodesk-forks/MaterialX/blob/adsk_contrib/dev/contrib/adsk/libraries/adsklib/adsklib_ng.mtlx#L432


      
              <input name="in2" type="float" value="360.0" />
            </divide>
            <modulo name="specular_rotation_value" type="float">
              <input name="in1" type="float" nodename="div_rotation0" />
            </modulo>
          
            <!-- map values to standard surface -->
            <output name="out" type="surfaceshader" nodename="standard_surface0" />
            <standard_surface name="standard_surface0" type="surfaceshader" version="1.0.1">
              <input name="normal" type="vector3" value="1, 1, 1" interfacename="surface_normal" />
              <input name="opacity" type="color3" value="1, 1, 1" interfacename="surface_cutout" />
              <input name="metalness" type="float" value="1" />
              <input name="base" type="float" value="1.0" />
              <input name="base_color" type="color3" nodename="base_color_value" />
              <input name="specular_color" type="color3" interfacename="surface_albedo" />
              <input name="specular_roughness" type="float" nodename="specular_roughness_value" />
              <input name="specular_anisotropy" type="float" nodename="specular_anisotropy_value" />
              <input name="specular_rotation" type="float" nodename="specular_rotation_value" />
            </standard_surface>
          </nodegraph>

I’m proposing that we change this so that storm always generates a mtlx doc and depends on mx::isTransparentSurface to detect if material is translucent.
Does anyone foresee any issues with this?

Thanks
Ashwin

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Detecting MaterialX translucency in Storm

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