Shaders: Improve energy conservation for diffuse and sheen lighting. (#33985)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

src/nodes/functions/PhysicalLightingModel.js

@@ -619,7 +619,12 @@ class PhysicalLightingModel extends LightingModel {
 
 		}
 
-		reflectedLight.directDiffuse.addAssign( irradiance.mul( BRDF_Lambert( { diffuseColor: diffuseContribution } ) ) );
+		// Light reflected by the specular interface is not available to the diffuse layer ( glTF fresnel_mix )
+		const halfDir = lightDirection.add( positionViewDirection ).normalize();
+		const dotVH = positionViewDirection.dot( halfDir ).clamp();
+		const F = F_Schlick( { f0: specularColor, f90: specularF90, dotVH } );
+
+		reflectedLight.directDiffuse.addAssign( irradiance.mul( BRDF_Lambert( { diffuseColor: diffuseContribution } ) ).mul( F.oneMinus() ) );
 
 		let specularBRDF = BRDF_GGX( { lightDirection, f0: specularColorBlended, f90: 1, roughness, f: this.iridescenceFresnel, USE_IRIDESCENCE: this.iridescence, USE_ANISOTROPY: this.anisotropy } );
 
@@ -733,12 +738,20 @@ class PhysicalLightingModel extends LightingModel {
 
 		const { irradiance, reflectedLight } = builder.context;
 
-		const diffuse = irradiance.mul( BRDF_Lambert( { diffuseColor: diffuseContribution } ) ).toVar();
+		// Energy reflected by the specular lobe is not available to the diffuse layer
+		const singleScattering = vec3().toVar();
+		const multiScattering = vec3().toVar();
+
+		this.computeMultiscattering( singleScattering, multiScattering, specularF90, specularColor, this.iridescenceF0Dielectric );
+
+		const diffuse = irradiance.mul( BRDF_Lambert( { diffuseColor: diffuseContribution } ) ).mul( singleScattering.add( multiScattering ).oneMinus() ).toVar();
 
 		if ( this.sheen === true ) {
 
 			const sheenAlbedo = IBLSheenBRDF( { normal: normalView, viewDir: positionViewDirection, roughness: sheenRoughness } );
 
+			this.sheenSpecularIndirect.addAssign( irradiance.mul( sheen, sheenAlbedo, 1 / Math.PI ) );
+
 			const sheenEnergyComp = sheen.r.max( sheen.g ).max( sheen.b ).mul( sheenAlbedo ).oneMinus();
 
 			diffuse.mulAssign( sheenEnergyComp );
@@ -766,7 +779,8 @@ class PhysicalLightingModel extends LightingModel {
 					normal: normalView,
 					viewDir: positionViewDirection,
 					roughness: sheenRoughness
-				} )
+				} ),
+				1 / Math.PI
 			) );
 
 		}
@@ -888,7 +902,7 @@ class PhysicalLightingModel extends LightingModel {
 
 		if ( this.sheen === true ) {
 
-			const sheenLight = outgoingLight.add( this.sheenSpecularDirect, this.sheenSpecularIndirect.mul( 1.0 / Math.PI ) );
+			const sheenLight = outgoingLight.add( this.sheenSpecularDirect, this.sheenSpecularIndirect );
 
 			outgoingLight.assign( sheenLight );
 

src/renderers/shaders/ShaderChunk/lights_physical_pars_fragment.glsl.js

@@ -542,17 +542,38 @@ void RE_Direct_Physical( const in IncidentLight directLight, const in vec3 geome
 
 	reflectedLight.directSpecular += irradiance * specularBRDF * energyCompensation;
 
-	reflectedLight.directDiffuse += irradiance * BRDF_Lambert( material.diffuseContribution );
+	// Light reflected by the specular interface is not available to the diffuse layer ( glTF fresnel_mix )
+	vec3 halfDir = normalize( directLight.direction + geometryViewDir );
+	float dotVH = saturate( dot( geometryViewDir, halfDir ) );
+	vec3 F = F_Schlick( material.specularColor, material.specularF90, dotVH );
+
+	reflectedLight.directDiffuse += irradiance * BRDF_Lambert( material.diffuseContribution ) * ( 1.0 - F );
 }
 
 void RE_IndirectDiffuse_Physical( const in vec3 irradiance, const in vec3 geometryPosition, const in vec3 geometryNormal, const in vec3 geometryViewDir, const in vec3 geometryClearcoatNormal, const in PhysicalMaterial material, inout ReflectedLight reflectedLight ) {
 
-	vec3 diffuse = irradiance * BRDF_Lambert( material.diffuseContribution );
+	// Energy reflected by the specular lobe is not available to the diffuse layer
+	vec3 singleScattering = vec3( 0.0 );
+	vec3 multiScattering = vec3( 0.0 );
+
+	#ifdef USE_IRIDESCENCE
+
+		computeMultiscatteringIridescence( geometryNormal, geometryViewDir, material.specularColor, material.specularF90, material.iridescence, material.iridescenceF0Dielectric, material.roughness, singleScattering, multiScattering );
+
+	#else
+
+		computeMultiscattering( geometryNormal, geometryViewDir, material.specularColor, material.specularF90, material.roughness, singleScattering, multiScattering );
+
+	#endif
+
+	vec3 diffuse = irradiance * BRDF_Lambert( material.diffuseContribution ) * ( 1.0 - singleScattering - multiScattering );
 
 	#ifdef USE_SHEEN
 
 		float sheenAlbedo = IBLSheenBRDF( geometryNormal, geometryViewDir, material.sheenRoughness );
 
+		sheenSpecularIndirect += irradiance * material.sheenColor * sheenAlbedo * RECIPROCAL_PI;
+
 		float sheenEnergyComp = 1.0 - max3( material.sheenColor ) * sheenAlbedo;
 
 		diffuse *= sheenEnergyComp;