Fix interpretation of subsurface weight in MDL (AcademySoftwareFoundation#2536)

The subsurface weight should not be applied to the expected color but instead enable/disable the effect in general.
This means the semantic of the weight is meant for composition rather than turning down the volume absorption and scattering itself.

With the performance optimizations for Standard Surface and OpenPBR this issue surfaced since weights are not 1.0 anymore.

Author: krohmerNV

source/MaterialXGenMdl/mdl/materialx/pbrlib_1_6.mdl

@@ -344,15 +344,16 @@ export material mx_subsurface_bsdf(
     color  mxp_color     = color(0.18),
     color  mxp_radius    = color(1.0),
     float  mxp_anisotropy = 0.0,
-    float3 mxp_normal    = state::normal() [[ anno::unused() ]]
+    float3 mxp_normal    = state::normal()
 ) [[ 
     anno::usage( "materialx:bsdf") 
 ]]
 = let {
     // https://blog.selfshadow.com/publications/s2017-shading-course/imageworks/s2017_pbs_imageworks_slides_v2.pdf
     // Phase function eccentricity 'g' has been omitted here since we pass that directly 
     // to anisotropic_vdf(directional_bias: g).
-    color C = math::saturate(mxp_weight) * mxp_color;
+    float w = math::saturate(mxp_weight);
+    color C = mxp_color;
     color albedo = color(4.09712) +
            (4.20863f * C) -
            math::sqrt(
@@ -364,7 +365,7 @@ export material mx_subsurface_bsdf(
     color white = color(1.0, 1.0, 1.0);
     color alpha = (white - albedo_sq) / (white - mxp_anisotropy * albedo_sq);
 
-    color radius_inv = white / math::max(color(0.001), mxp_radius);
+    color radius_inv = white / math::max(0.00000001, mxp_radius);
 
     color sigma_s = alpha * radius_inv;
     color sigma_a = radius_inv - sigma_s;
@@ -374,12 +375,16 @@ export material mx_subsurface_bsdf(
         scattering: df::anisotropic_vdf (
             directional_bias: mxp_anisotropy
         ),
-        scattering_coefficient: sigma_s,
-        absorption_coefficient: sigma_a
+        scattering_coefficient: w * sigma_s,
+        absorption_coefficient: w * sigma_a
     ), 
 
     surface: material_surface(
-        scattering: df::diffuse_transmission_bsdf(tint: white)
+        scattering: df::weighted_layer(
+            weight: w,
+            layer: df::diffuse_transmission_bsdf(tint: white),
+            normal: mxp_normal
+        )
     )
 );
 

source/MaterialXTest/MaterialXGenMdl/GenMdl.cpp

@@ -315,7 +315,7 @@ void MdlShaderGeneratorTester::compileSource(const std::vector<mx::FilePath>& so
         if (renderArgs.empty())
         {
             // Assume MDL example DXR is being used and set reasonable arguments automatically
-            renderCommand += " --nogui --res 512 512 --iterations 1024 --max_path_length 3 --noaux --no_firefly_clamp";
+            renderCommand += " --nogui --res 512 512 --iterations 1024 --max_path_length 4 --noaux --no_firefly_clamp";
             renderCommand += " --background 0.073239 0.073239 0.083535";
         }
         else