Refactor the coat calculation

Author: portsmouth

reference/open_pbr_surface.mtlx

@@ -459,7 +459,9 @@
       <input name="mix" type="float" interfacename="base_metalness" />
     </mix>
 
-    <!-- Coat darkening calculation  -->
+    <!-- Coat Layer -->
+
+    <!--  Coat inter-reflection darkening darkening calculation   -->
     <!-- approximate Kcoat, "internal diffuse reflection coefficient" of coat  -->
     <subtract name="one_minus_coat_F0" type="float">
       <input name="in1" type="float" value="1.0" />
@@ -494,74 +496,75 @@
       <input name="bg" type="color3" nodename="Edielectric" />
       <input name="mix" type="float" interfacename="base_metalness" />
     </mix>
-    <!-- final base darkening factor due to coat:  base_darkening = (1 - Kcoat) / (1 - Ebase*Kcoat)  -->
+    <!-- final base inter-reflection darkening darkening factor due to coat,
+         where Tcoat2 = coat_color:
+              base_darkening = (1 - Kcoat) / (1 - Ebase*Kcoat*Tcoat2) -->
     <multiply name="Ebase_Kcoat" type="color3">
       <input name="in1" type="color3" nodename="Ebase" />
       <input name="in2" type="float" nodename="Kcoat" />
     </multiply>
-    <multiply name="Ebase_Kcoat_T2" type="color3">
+    <multiply name="Ebase_Kcoat_Tcoat2" type="color3">
       <input name="in1" type="color3" nodename="Ebase_Kcoat" />
       <input name="in2" type="color3" interfacename="coat_color" />
     </multiply>
     <subtract name="one_minus_Kcoat" type="float">
       <input name="in1" type="float" value="1.0" />
       <input name="in2" type="float" nodename="Kcoat" />
     </subtract>
-    <subtract name="one_minus_Ebase_Kcoat_T2" type="color3">
+    <subtract name="one_minus_Ebase_Kcoat_Tcoat2" type="color3">
       <input name="in1" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="in2" type="color3" nodename="Ebase_Kcoat_T2" />
+      <input name="in2" type="color3" nodename="Ebase_Kcoat_Tcoat2" />
     </subtract>
     <convert name="one_minus_Kcoat_color" type="color3">
       <input name="in" type="float" nodename="one_minus_Kcoat" />
     </convert>
     <divide name="base_darkening" type="color3">
       <input name="in1" type="color3" nodename="one_minus_Kcoat_color" />
-      <input name="in2" type="color3" nodename="one_minus_Ebase_Kcoat" />
+      <input name="in2" type="color3" nodename="one_minus_Ebase_Kcoat_Tcoat2" />
     </divide>
+
+    <!-- inter-reflection darkening, modulated by coat_darkening -->
     <mix name="modulated_base_darkening" type="color3">
       <input name="fg" type="color3" nodename="base_darkening" />
       <input name="bg" type="color3" value="1.0, 1.0, 1.0" />
       <input name="mix" type="float" interfacename="coat_darkening" />
     </mix>
-    <mix name="coat_modulated_base_darkening" type="color3">
-      <input name="fg" type="color3" nodename="modulated_base_darkening" />
-      <input name="bg" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="mix" type="float" interfacename="coat_weight" />
-    </mix>
     <multiply name="darkened_base_substrate" type="BSDF">
       <input name="in1" type="BSDF" nodename="base_substrate" />
-      <input name="in2" type="color3" nodename="coat_modulated_base_darkening" />
+      <input name="in2" type="color3" nodename="modulated_base_darkening" />
     </multiply>
-
-    <!-- Coat Layer -->
-    <mix name="coat_attenuation" type="color3">
-      <input name="fg" type="color3" interfacename="coat_color" />
-      <input name="bg" type="color3" value="1.0, 1.0, 1.0" />
-      <input name="mix" type="float" interfacename="coat_weight" />
-    </mix>
-    <multiply name="coat_substrate_attenuated" type="BSDF">
+    <!-- base_substrate, attenuated due to absorption color, and inter-reflection darkening  -->
+    <multiply name="base_substrate_attenuated" type="BSDF">
       <input name="in1" type="BSDF" nodename="darkened_base_substrate" />
-      <input name="in2" type="color3" nodename="coat_attenuation" />
+      <input name="in2" type="color3" nodename="coat_color" />
     </multiply>
-
+    <!-- coat BSDF  -->
     <open_pbr_anisotropy name="coat_roughness_vector" type="vector2">
       <input name="roughness" type="float" interfacename="coat_roughness" />
       <input name="anisotropy" type="float" interfacename="coat_roughness_anisotropy" />
     </open_pbr_anisotropy>
     <dielectric_bsdf name="coat_bsdf" type="BSDF">
-      <input name="weight" type="float" interfacename="coat_weight" />
+      <input name="weight" type="float" value="1.0" />
       <input name="ior" type="float" interfacename="coat_ior" />
       <input name="roughness" type="vector2" nodename="coat_roughness_vector" />
       <input name="normal" type="vector3" interfacename="geometry_coat_normal" />
       <input name="tangent" type="vector3" interfacename="geometry_coat_tangent" />
       <input name="scatter_mode" type="string" value="R" />
     </dielectric_bsdf>
-    <layer name="coat_layer" type="BSDF">
+    <!-- coated base  -->
+    <layer name="coated_base_substrate" type="BSDF">
       <input name="top" type="BSDF" nodename="coat_bsdf" />
-      <input name="base" type="BSDF" nodename="coat_substrate_attenuated" />
+      <input name="base" type="BSDF" nodename="base_substrate_attenuated" />
     </layer>
+    <!-- statistical mix of coated and uncoated base  -->
+    <mix name="partially_coated_base" type="color3">
+      <input name="fg" type="BSDF" nodename="coated_base_substrate" />
+      <input name="bg" type="BSDF" nodename="base_substrate" />
+      <input name="mix" type="float" interfacename="coat_weight" />
+    </mix>
+
 
-    <!-- Fuzz Layer -->
+    <!--  Fuzz Layer  -->
     <sheen_bsdf name="fuzz_bsdf" type="BSDF">
       <input name="weight" type="float" interfacename="fuzz_weight" />
       <input name="color" type="color3" interfacename="fuzz_color" />
@@ -571,7 +574,7 @@
     </sheen_bsdf>
     <layer name="fuzz_layer" type="BSDF">
       <input name="top" type="BSDF" nodename="fuzz_bsdf" />
-      <input name="base" type="BSDF" nodename="coat_layer" />
+      <input name="base" type="BSDF" nodename="partially_coated_base" />
     </layer>
 
     <!-- Emission Layer -->