WebGLRenderer: Add multi-scattering energy compensation for direct lighting (#32072)

* WebGLRenderer: Add multi-scattering energy compensation for direct lighting.

* Updated builds.

* Simplified BRDF_GGX_Multiscatter function.

Author: mrdoob

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

@@ -426,6 +426,48 @@ void computeMultiscattering( const in vec3 normal, const in vec3 viewDir, const
 
 }
 
+// GGX BRDF with multi-scattering energy compensation for direct lighting
+// Based on "Practical Multiple Scattering Compensation for Microfacet Models"
+// https://blog.selfshadow.com/publications/turquin/ms_comp_final.pdf
+vec3 BRDF_GGX_Multiscatter( const in vec3 lightDir, const in vec3 viewDir, const in vec3 normal, const in PhysicalMaterial material ) {
+
+	// Single-scattering BRDF (standard GGX)
+	vec3 singleScatter = BRDF_GGX( lightDir, viewDir, normal, material );
+
+	// Multi-scattering compensation
+	float dotNL = saturate( dot( normal, lightDir ) );
+	float dotNV = saturate( dot( normal, viewDir ) );
+
+	// Precomputed DFG values for view and light directions
+	vec2 dfgV = DFGApprox( vec3(0.0, 0.0, 1.0), vec3(sqrt(1.0 - dotNV * dotNV), 0.0, dotNV), material.roughness );
+	vec2 dfgL = DFGApprox( vec3(0.0, 0.0, 1.0), vec3(sqrt(1.0 - dotNL * dotNL), 0.0, dotNL), material.roughness );
+
+	// Single-scattering energy for view and light
+	vec3 FssEss_V = material.specularColor * dfgV.x + material.specularF90 * dfgV.y;
+	vec3 FssEss_L = material.specularColor * dfgL.x + material.specularF90 * dfgL.y;
+
+	float Ess_V = dfgV.x + dfgV.y;
+	float Ess_L = dfgL.x + dfgL.y;
+
+	// Energy lost to multiple scattering
+	float Ems_V = 1.0 - Ess_V;
+	float Ems_L = 1.0 - Ess_L;
+
+	// Average Fresnel reflectance
+	vec3 Favg = material.specularColor + ( 1.0 - material.specularColor ) * 0.047619; // 1/21
+
+	// Multiple scattering contribution
+	vec3 Fms = FssEss_V * FssEss_L * Favg / ( 1.0 - Ems_V * Ems_L * Favg * Favg + EPSILON );
+
+	// Energy compensation factor
+	float compensationFactor = Ems_V * Ems_L;
+
+	vec3 multiScatter = Fms * compensationFactor;
+
+	return singleScatter + multiScatter;
+
+}
+
 #if NUM_RECT_AREA_LIGHTS > 0
 
 	void RE_Direct_RectArea_Physical( const in RectAreaLight rectAreaLight, const in vec3 geometryPosition, const in vec3 geometryNormal, const in vec3 geometryViewDir, const in vec3 geometryClearcoatNormal, const in PhysicalMaterial material, inout ReflectedLight reflectedLight ) {
@@ -490,7 +532,7 @@ void RE_Direct_Physical( const in IncidentLight directLight, const in vec3 geome
 
 	#endif
 
-	reflectedLight.directSpecular += irradiance * BRDF_GGX( directLight.direction, geometryViewDir, geometryNormal, material );
+	reflectedLight.directSpecular += irradiance * BRDF_GGX_Multiscatter( directLight.direction, geometryViewDir, geometryNormal, material );
 
 	reflectedLight.directDiffuse += irradiance * BRDF_Lambert( material.diffuseColor );
 }