WebGLRenderer: Fix specular occlusion for smooth metals

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

@@ -4,7 +4,7 @@ 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;
+	reflectedLight.indirectDiffuse *= computeMultiBounceAO( ambientOcclusion, material.diffuseColor );
 
 	#if defined( USE_CLEARCOAT ) 
 		clearcoatSpecularIndirect *= ambientOcclusion;

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

@@ -588,10 +588,27 @@ 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 ambient occlusion approximation for diffuse lighting
+// ref: https://www.activision.com/cdn/research/Practical_Real_Time_Strategies_for_Accurate_Indirect_Occlusion_NEW%20VERSION_COLOR.pdf
+vec3 computeMultiBounceAO( const in float ambientOcclusion, 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;
+
+	float x = ambientOcclusion;
+
+	return max( vec3( x ), ( ( x * a + b ) * x + c ) * x );
+
+}
+
 // 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 specularOcclusion = saturate( pow( dotNV + ambientOcclusion, exp2( - 16.0 * roughness - 1.0 ) ) - 1.0 + ambientOcclusion );
+
+	// Smoothly disable specular occlusion for very smooth surfaces to prevent over-darkening
+	return mix( 1.0, specularOcclusion, smoothstep( 0.01, 0.1, roughness ) );
 
 }
 `;