WebGLRenderer/WebGPURenderer: Simplify multi-scattering compensation for direct lighting

src/nodes/functions/BSDF/BRDF_GGX_Multiscatter.js

@@ -2,48 +2,29 @@ import BRDF_GGX from './BRDF_GGX.js';
 import DFGApprox from './DFGApprox.js';
 import { normalView } from '../../accessors/Normal.js';
 import { positionViewDirection } from '../../accessors/Position.js';
-import { EPSILON } from '../../math/MathNode.js';
 import { Fn, float } from '../../tsl/TSLBase.js';
 
 // GGX BRDF with multi-scattering energy compensation for direct lighting
-// This provides more accurate energy conservation, especially for rough materials
-// Based on "Practical Multiple Scattering Compensation for Microfacet Models"
-// https://blog.selfshadow.com/publications/turquin/ms_comp_final.pdf
+// Based on "Multiple-Scattering Microfacet Model for Real-Time Image Based Lighting"
+// by Fdez-Agüera: http://www.jcgt.org/published/0008/01/03/
 const BRDF_GGX_Multiscatter = /*@__PURE__*/ Fn( ( { lightDirection, f0, f90, roughness: _roughness, f, USE_IRIDESCENCE, USE_ANISOTROPY } ) => {
 
 	// Single-scattering BRDF (standard GGX)
 	const singleScatter = BRDF_GGX( { lightDirection, f0, f90, roughness: _roughness, f, USE_IRIDESCENCE, USE_ANISOTROPY } );
 
-	// Multi-scattering compensation
-	const dotNL = normalView.dot( lightDirection ).clamp();
+	// Multi-scattering compensation using Fdez-Agüera's approximation
 	const dotNV = normalView.dot( positionViewDirection ).clamp();
+	const fab = DFGApprox( { roughness: _roughness, dotNV } );
 
-	// Precomputed DFG values for view and light directions
-	const dfgV = DFGApprox( { roughness: _roughness, dotNV } );
-	const dfgL = DFGApprox( { roughness: _roughness, dotNV: dotNL } );
+	const FssEss = f0.mul( fab.x ).add( f90.mul( fab.y ) );
+	const Ess = fab.x.add( fab.y );
+	const Ems = float( 1.0 ).sub( Ess );
 
-	// Single-scattering energy for view and light
-	const FssEss_V = f0.mul( dfgV.x ).add( f90.mul( dfgV.y ) );
-	const FssEss_L = f0.mul( dfgL.x ).add( f90.mul( dfgL.y ) );
-
-	const Ess_V = dfgV.x.add( dfgV.y );
-	const Ess_L = dfgL.x.add( dfgL.y );
-
-	// Energy lost to multiple scattering
-	const Ems_V = float( 1.0 ).sub( Ess_V );
-	const Ems_L = float( 1.0 ).sub( Ess_L );
-
-	// Average Fresnel reflectance
 	const Favg = f0.add( f0.oneMinus().mul( 0.047619 ) ); // 1/21
+	const Fms = FssEss.mul( Favg ).div( float( 1.0 ).sub( Ems.mul( Favg ) ) );
 
-	// Multiple scattering contribution
-	// Uses geometric mean of view and light contributions for better energy distribution
-	const Fms = FssEss_V.mul( FssEss_L ).mul( Favg ).div( float( 1.0 ).sub( Ems_V.mul( Ems_L ).mul( Favg ).mul( Favg ) ).add( EPSILON ) );
-
-	// Energy compensation factor
-	const compensationFactor = Ems_V.mul( Ems_L );
-
-	const multiScatter = Fms.mul( compensationFactor );
+	// Add multi-scattering contribution
+	const multiScatter = Fms.mul( Ems );
 
 	return singleScatter.add( multiScatter );
 

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

@@ -434,35 +434,19 @@ vec3 BRDF_GGX_Multiscatter( const in vec3 lightDir, const in vec3 viewDir, const
 	// Single-scattering BRDF (standard GGX)
 	vec3 singleScatter = BRDF_GGX( lightDir, viewDir, normal, material );
 
-	// Multi-scattering compensation
-	float dotNL = saturate( dot( normal, lightDir ) );
+	// Multi-scattering compensation using Fdez-Agüera's approximation
 	float dotNV = saturate( dot( normal, viewDir ) );
+	vec2 fab = DFGApprox( normal, viewDir, material.roughness );
 
-	// 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;
+	vec3 FssEss = material.specularColor * fab.x + material.specularF90 * fab.y;
+	float Ess = fab.x + fab.y;
+	float Ems = 1.0 - Ess;
 
-	// Average Fresnel reflectance
 	vec3 Favg = material.specularColor + ( 1.0 - material.specularColor ) * 0.047619; // 1/21
+	vec3 Fms = FssEss * Favg / ( 1.0 - Ems * Favg );
 
-	// 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;
+	// Add multi-scattering contribution
+	vec3 multiScatter = Fms * Ems;
 
 	return singleScatter + multiScatter;