Handle backsurface in MDL generator (#2684)

Author: jreichel-nvidia

resources/Materials/TestSuite/stdlib/shader/backsurface.mtlx

@@ -0,0 +1,13 @@
+<?xml version="1.0"?>                                                                          
+<materialx version="1.39" colorspace="lin_rec709">    
+  <standard_surface name="SR_surface" type="surfaceshader">      
+    <input name="base_color" type="color3" value="1, 0, 0" />                   
+  </standard_surface>                                                                          
+  <standard_surface name="SR_backsurface" type="surfaceshader">       
+    <input name="base_color" type="color3" value="0, 1, 0" />                   
+  </standard_surface>                                                                          
+  <surfacematerial name="backsurface" type="material">                   
+    <input name="surfaceshader" type="surfaceshader" nodename="SR_surface" />                     
+    <input name="backsurfaceshader" type="surfaceshader" nodename="SR_backsurface" />                
+  </surfacematerial>                                                                           
+</materialx>                                                                                   

source/MaterialXGenMdl/Nodes/MaterialNodeMdl.cpp

@@ -39,6 +39,14 @@ void MaterialNodeMdl::emitFunctionCall(const ShaderNode& _node, GenContext& cont
         const ShaderNode* surfaceshaderNode = surfaceshaderInput->getConnection()->getNode();
         shadergen.emitFunctionCall(*surfaceshaderNode, context, stage);
 
+        // Emit the function call for upstream backsurface shader.
+        ShaderInput* backsurfaceshaderInput = node.getInput(ShaderNode::BACKSURFACESHADER);
+        if (backsurfaceshaderInput->getConnectedSibling())
+        {
+            const ShaderNode* backsurfaceshaderNode = backsurfaceshaderInput->getConnection()->getNode();
+            shadergen.emitFunctionCall(*backsurfaceshaderNode, context, stage);
+        }
+
         shadergen.emitLineBegin(stage);
 
         // Emit the output and function name.

source/MaterialXGenShader/ShaderNode.cpp

@@ -148,6 +148,7 @@ const string ShaderNode::CONSTANT = "constant";
 const string ShaderNode::DOT = "dot";
 const string ShaderNode::IMAGE = "image";
 const string ShaderNode::SURFACESHADER = "surfaceshader";
+const string ShaderNode::BACKSURFACESHADER = "backsurfaceshader";
 const string ShaderNode::BSDF_R = "R";
 const string ShaderNode::BSDF_T = "T";
 const string ShaderNode::TEXTURE2D_GROUPNAME = "texture2d";

source/MaterialXGenShader/ShaderNode.h

@@ -367,6 +367,7 @@ class MX_GENSHADER_API ShaderNode
     static const string DOT;
     static const string IMAGE;
     static const string SURFACESHADER;
+    static const string BACKSURFACESHADER;
     static const string BSDF_R;
     static const string BSDF_T;
     static const string TRANSFORM_POINT;