Updates for coat darkening

Author: Mike Bond

packages/dev/core/src/Materials/PBR/openPbrMaterial.ts

@@ -622,7 +622,7 @@ export class OpenPBRMaterial extends OpenPBRMaterialBase {
     public coatDarkening: number;
     @addAccessorsForMaterialProperty("_markAllSubMeshesAsTexturesDirty", "coatDarkening")
     // eslint-disable-next-line @typescript-eslint/no-unused-vars
-    private _coatDarkening: Property<number> = new Property<number>("coat_darkening", 0.0, "vCoatDarkening", 1, 0);
+    private _coatDarkening: Property<number> = new Property<number>("coat_darkening", 1.0, "vCoatDarkening", 1, 0);
 
     /**
      * Defines the amount that interreflections within the coat allow the underlying surface
@@ -1344,6 +1344,8 @@ export class OpenPBRMaterial extends OpenPBRMaterialBase {
         this._coatRoughness;
         this._coatRoughnessTexture;
         this._coatIor;
+        this._coatDarkening;
+        this._coatDarkeningTexture;
         this._geometryNormalTexture;
         this._geometryCoatNormalTexture;
         this._geometryOpacity;

packages/dev/core/src/Shaders/ShadersInclude/openpbrEnvironmentLighting.fx

@@ -124,7 +124,7 @@
         // This uses the geometric series for infinite reflections:
         // T = (1-R) / (1 + R + R² + R³ + ...) = (1-R) / (1/(1-R)) = (1-R)²
         float effectiveReflectance = averageReflectance * coat_weight;
-        float transmission = (1.0 - effectiveReflectance) / (1.0 + effectiveReflectance);
+        float transmission = (1.0 - effectiveReflectance) * (1.0 - effectiveReflectance);
         coatAbsorption = coat_color * mix(1.0, transmission, coat_darkening);
     }
 

packages/dev/core/src/Shaders/ShadersInclude/openpbrIblFunctions.fx

@@ -205,14 +205,10 @@
     vec3 conductorIblFresnel(in ReflectanceParams reflectance, in float NdotV, in float roughness, in vec3 environmentBrdf)
     {
         #if (CONDUCTOR_SPECULAR_MODEL == CONDUCTOR_SPECULAR_MODEL_OPENPBR)
-            // For OpenPBR, we use a different specular lobe for metallic materials and then blend based on metalness. However,
-            // to do this correctly, we really need reflectivityOut to contain separate F0 and F90 values for purely dielectric
-            // and purely metal. Instead, the values are already a mix of dielectric and metallic values.
-            // So, for intermediate metallic values, the result isn't 100% correct but it seems to work well enough in practice.
-            // Because specular weight in OpenPBR removes the specular lobe entirely for metals, we do need the actual dielectric
-            // F0 value to pickup the weight from the dielectric lobe.
-            return getF82Specular(NdotV, reflectance.coloredF0, reflectance.coloredF90, roughness);
-
+            // This is an empirical hack to modify the F0 albedo based on roughness. It's not based on any paper
+            // or anything. Just trying to match results of rough metals in a pathtracer.
+            vec3 albedoF0 = mix(reflectance.coloredF0, pow(reflectance.coloredF0, vec3(1.4)), roughness);
+            return getF82Specular(NdotV, albedoF0, reflectance.coloredF90, roughness);
         #else
             return getReflectanceFromBRDFLookup(reflectance.coloredF0, reflectance.coloredF90, environmentBrdf);
         #endif

packages/dev/core/src/ShadersWGSL/ShadersInclude/openpbrEnvironmentLighting.fx

@@ -126,7 +126,7 @@
         // This uses the geometric series for infinite reflections:
         // T = (1-R) / (1 + R + R² + R³ + ...) = (1-R) / (1/(1-R)) = (1-R)²
         let effectiveReflectance = averageReflectance * coat_weight;
-        let transmission = (1.0f - effectiveReflectance) / (1.0f + effectiveReflectance);
+        let transmission = (1.0f - effectiveReflectance) * (1.0f - effectiveReflectance);
         coatAbsorption = coat_color * mix(1.0f, transmission, coat_darkening);
     }
 

packages/dev/core/src/ShadersWGSL/ShadersInclude/openpbrIblFunctions.fx

@@ -196,13 +196,10 @@
     fn conductorIblFresnel(reflectance: ReflectanceParams, NdotV: f32, roughness: f32, environmentBrdf: vec3f) -> vec3f
     {
         #if (CONDUCTOR_SPECULAR_MODEL == CONDUCTOR_SPECULAR_MODEL_OPENPBR)
-            // For OpenPBR, we use a different specular lobe for metallic materials and then blend based on metalness. However,
-            // to do this correctly, we really need reflectanceOut to contain separate F0 and F90 values for purely dielectric
-            // and purely metal. Instead, the values are already a mix of dielectric and metallic values.
-            // So, for intermediate metallic values, the result isn't 100% correct but it seems to work well enough in practice.
-            // Because specular weight in OpenPBR removes the specular lobe entirely for metals, we do need the actual dielectric
-            // F0 value to pickup the weight from the dielectric lobe.
-            return getF82Specular(NdotV, reflectance.coloredF0, reflectance.coloredF90, roughness);
+            // This is an empirical hack to modify the F0 albedo based on roughness. It's not based on any paper
+            // or anything. Just trying to match results of rough metals in a pathtracer.
+            let albedoF0: vec3f = mix(reflectance.coloredF0, pow(reflectance.coloredF0, vec3f(1.4f)), roughness);
+            return getF82Specular(NdotV, albedoF0, reflectance.coloredF90, roughness);
         #else
             return getReflectanceFromBRDFLookup(reflectance.coloredF0, reflectance.coloredF90, environmentBrdf);
         #endif

packages/dev/inspector/src/components/actionTabs/tabs/propertyGrids/materials/openpbrMaterialPropertyGridComponent.tsx

@@ -450,6 +450,14 @@ export class OpenPBRMaterialPropertyGridComponent extends React.Component<IOpenP
                         step={0.01}
                         onPropertyChangedObservable={this.props.onPropertyChangedObservable}
                     />
+                    <TextureLinkLineComponent
+                        label="Darkening Texture"
+                        texture={material.coatDarkeningTexture}
+                        propertyName="coatDarkeningTexture"
+                        material={material}
+                        onSelectionChangedObservable={this.props.onSelectionChangedObservable}
+                        onDebugSelectionChangeObservable={this._onDebugSelectionChangeObservable}
+                    />
                 </LineContainerComponent>
                 <LineContainerComponent title="EMISSION" selection={this.props.globalState}>
                     <Color3LineComponent

packages/tools/tests/test/visualization/ReferenceImages/OpenPBR-Analytic-Lights-Coat-Darkening-vs-Coat-IOR.png

@@ -2827,10 +2827,20 @@
             "playgroundId": "#GRQHVV#46",
             "excludedEngines": ["webgl1"]
         },
+        {
+            "title": "OpenPBR IBL Coat Darkening vs Coat IOR",
+            "playgroundId": "#GRQHVV#48",
+            "excludedEngines": ["webgl1"]
+        },
         {
             "title": "OpenPBR Analytic Lights Coat Darkening",
             "playgroundId": "#GRQHVV#47",
             "excludedEngines": ["webgl1"]
+        },
+        {
+            "title": "OpenPBR Analytic Lights Coat Darkening vs Coat IOR",
+            "playgroundId": "#GRQHVV#49",
+            "excludedEngines": ["webgl1"]
         }
     ]
 }