WebGLRenderer: Improve ambient occlusion with multi-bounce and better specular occlusion

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

@@ -4,13 +4,14 @@ export default /* glsl */`
 	// reads channel R, compatible with a combined OcclusionRoughnessMetallic (RGB) texture
 	float ambientOcclusion = ( texture2D( aoMap, vAoMapUv ).r - 1.0 ) * aoMapIntensity + 1.0;
 
-	reflectedLight.indirectDiffuse *= ambientOcclusion;
+	// Multi-bounce AO accounts for inter-reflections based on surface albedo
+	reflectedLight.indirectDiffuse *= gtaoMultiBounce( ambientOcclusion, material.diffuseColor );
 
-	#if defined( USE_CLEARCOAT ) 
+	#if defined( USE_CLEARCOAT )
 		clearcoatSpecularIndirect *= ambientOcclusion;
 	#endif
 
-	#if defined( USE_SHEEN ) 
+	#if defined( USE_SHEEN )
 		sheenSpecularIndirect *= ambientOcclusion;
 	#endif
 

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

@@ -546,10 +546,31 @@ void RE_IndirectSpecular_Physical( const in vec3 radiance, const in vec3 irradia
 #define RE_IndirectDiffuse		RE_IndirectDiffuse_Physical
 #define RE_IndirectSpecular		RE_IndirectSpecular_Physical
 
+// Multi-bounce approximation for ambient occlusion.
+// Accumulates light from multiple surface bounces based on albedo.
+// ref: https://www.activision.com/cdn/research/Practical_Real_Time_Strategies_for_Accurate_Indirect_Occlusion_NEW%20VERSION_COLOR.pdf
+vec3 gtaoMultiBounce( const in float ao, const in vec3 albedo ) {
+
+	vec3 a = 2.0404 * albedo - 0.3324;
+	vec3 b = -4.7951 * albedo + 0.6417;
+	vec3 c = 2.7552 * albedo + 0.6903;
+
+	return max( vec3( ao ), ( ( ao * a + b ) * ao + c ) * ao );
+
+}
+
+// Improved specular occlusion with better roughness handling.
 // ref: https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf
 float computeSpecularOcclusion( const in float dotNV, const in float ambientOcclusion, const in float roughness ) {
 
-	return saturate( pow( dotNV + ambientOcclusion, exp2( - 16.0 * roughness - 1.0 ) ) - 1.0 + ambientOcclusion );
+	float a = roughness * roughness;
+	float specularOcclusion = saturate( pow( dotNV + ambientOcclusion, exp2( - 16.0 * a - 1.0 ) ) - 1.0 + ambientOcclusion );
+
+	// Reduce occlusion for very rough surfaces
+	float roughnessFactor = a * a;
+	specularOcclusion = mix( specularOcclusion, ambientOcclusion, roughnessFactor );
+
+	return specularOcclusion;
 
 }
 `;