Propose an initial graph refactoring framework for shader generation (#2832)

This changelist proposes an initial graph refactoring framework for MaterialX shader generation, replacing hardcoded graph optimizations with a composable set of named refactoring passes.  The following specific changes are included:

- Add a `ShaderGraphRefactor` base class, with three concrete subclasses for node elision, premultiplied BSDF add (valuable in hardware languages), and layer-over-mix distribution (valuable in MDL).
- Add a virtual `setDefaultOptions` method to `ShaderGenerator`, enabling each subclass to set target-appropriate defaults at `GenContext` construction time.
- Add a `MIX` classification bit to ShaderNode for efficient mix node identification.
- Remove hand-authored GLSL and MDL graph overrides for `open_pbr_surface` and `standard_surface`, as these refactoring steps are now generated automatically.

Author: jstone-lucasfilm

libraries/bxdf/genglsl/open_pbr_surface.mtlx

@@ -36,6 +36,9 @@ EMSCRIPTEN_BINDINGS(GenOptions)
         .property("targetDistanceUnit", &mx::GenOptions::targetDistanceUnit)
         .property("addUpstreamDependencies", &mx::GenOptions::addUpstreamDependencies)
         .property("emitColorTransforms", &mx::GenOptions::emitColorTransforms)
+        .property("elideConstantNodes", &mx::GenOptions::elideConstantNodes)
+        .property("premultipliedBsdfAdd", &mx::GenOptions::premultipliedBsdfAdd)
+        .property("distributeLayerOverBsdfMix", &mx::GenOptions::distributeLayerOverBsdfMix)
         .property("hwTransparency", &mx::GenOptions::hwTransparency)
         .property("hwSpecularEnvironmentMethod", &mx::GenOptions::hwSpecularEnvironmentMethod)
         .property("hwDirectionalAlbedoMethod", &mx::GenOptions::hwDirectionalAlbedoMethod)

libraries/bxdf/genglsl/standard_surface.mtlx

@@ -89,6 +89,12 @@ HwShaderGenerator::HwShaderGenerator(TypeSystemPtr typeSystem, SyntaxPtr syntax)
     _tokenSubstitutions[HW::T_CLOSURE_DATA_CONSTRUCTOR] = HW::CLOSURE_DATA_CONSTRUCTOR;
 }
 
+void HwShaderGenerator::applyDefaultOptions(GenOptions& options) const
+{
+    ShaderGenerator::applyDefaultOptions(options);
+    options.premultipliedBsdfAdd = true;
+}
+
 ShaderPtr HwShaderGenerator::createShader(const string& name, ElementPtr element, GenContext& context) const
 {
     // Create the root shader graph

libraries/bxdf/genmdl/open_pbr_surface.mtlx

@@ -43,6 +43,9 @@ class MX_GENHW_API HwShaderGenerator : public ShaderGenerator
     void emitClosureDataArg(const ShaderNode& node, GenContext& context, ShaderStage& stage) const override;
     void emitClosureDataParameter(const ShaderNode& node, GenContext& context, ShaderStage& stage) const override;
 
+    /// Apply the default GenOptions for hardware shader generation.
+    void applyDefaultOptions(GenOptions& options) const override;
+
     /// Logic to indicate whether code to support direct lighting should be emitted.
     /// By default if the graph is classified as a shader, or BSDF node then lighting is assumed to be required.
     /// Derived classes can override this logic.

source/JsMaterialX/JsMaterialXGenShader/JsGenOptions.cpp

@@ -125,6 +125,12 @@ MdlShaderGenerator::MdlShaderGenerator(TypeSystemPtr typeSystem) :
     registerImplementation("IM_image_vector4_" + MdlShaderGenerator::TARGET, ImageNodeMdl::create);
 }
 
+void MdlShaderGenerator::applyDefaultOptions(GenOptions& options) const
+{
+    ShaderGenerator::applyDefaultOptions(options);
+    options.distributeLayerOverBsdfMix = true;
+}
+
 ShaderPtr MdlShaderGenerator::generate(const string& name, ElementPtr element, GenContext& context) const
 {
     // For MDL we cannot cache node implementations between generation calls,

source/MaterialXGenHw/HwShaderGenerator.cpp

@@ -70,6 +70,9 @@ class MX_GENMDL_API MdlShaderGenerator : public ShaderGenerator
     /// Return a unique identifier for the target this generator is for
     const string& getTarget() const override { return TARGET; }
 
+    /// Apply the default GenOptions for MDL shader generation.
+    void applyDefaultOptions(GenOptions& options) const override;
+
     /// Generate a shader starting from the given element, translating
     /// the element and all dependencies upstream into shader code.
     ShaderPtr generate(const string& name, ElementPtr element, GenContext& context) const override;

source/MaterialXGenHw/HwShaderGenerator.h

@@ -20,6 +20,9 @@ GenContext::GenContext(ShaderGeneratorPtr sg) :
         throw ExceptionShaderGenError("GenContext must have a valid shader generator");
     }
 
+    // Apply the generator's default options for its target.
+    _sg->applyDefaultOptions(_options);
+
     // Collect and cache reserved words from the shader generator
     StringSet reservedWords;
 

source/MaterialXGenMdl/MdlShaderGenerator.cpp

@@ -82,6 +82,8 @@ class MX_GENSHADER_API GenOptions
         libraryPrefix("libraries"),
         emitColorTransforms(true),
         elideConstantNodes(true),
+        premultipliedBsdfAdd(false),
+        distributeLayerOverBsdfMix(false),
         hwTransparency(false),
         hwSpecularEnvironmentMethod(SPECULAR_ENVIRONMENT_FIS),
         hwDirectionalAlbedoMethod(DIRECTIONAL_ALBEDO_ANALYTIC),
@@ -141,6 +143,17 @@ class MX_GENSHADER_API GenOptions
     /// Enable eliding constant nodes. Defaults to true.
     bool elideConstantNodes;
 
+    /// Enable replacing BSDF mix nodes with premultiplied add nodes.
+    /// This folds the mix weight into each BSDF's weight input, enabling
+    /// hardware shading languages to skip BSDF evaluation via dynamic
+    /// branching when the weight is zero. Defaults to false.
+    bool premultipliedBsdfAdd;
+
+    /// Enable distributing layer operations over mix nodes.
+    /// Transforms layer(mix(A, B, w), C) into mix(layer(A, C), layer(B, C), w)
+    /// for backends with limited layering capabilities. Defaults to false.
+    bool distributeLayerOverBsdfMix;
+
     /// Sets if transparency is needed or not for HW shaders.
     /// If a surface shader has potential of being transparent
     /// this must be set to true, otherwise no transparency