Eevee: Fix fresnel factor to match Unreal Engine specular input.

source/blender/draw/engines/eevee/shaders/bsdf_common_lib.glsl

@@ -89,9 +89,9 @@ vec3 mul(mat3 m, vec3 v) { return m * v; }
 mat3 mul(mat3 m1, mat3 m2) { return m1 * m2; }
 
 float saturate(float a) { return clamp(a, 0.0, 1.0); }
-vec2 saturate(vec2 a) { return vec2(saturate(a.x), saturate(a.y)); }
-vec3 saturate(vec3 a) { return vec3(saturate(a.x), saturate(a.y), saturate(a.z)); }
-vec4 saturate(vec4 a) { return vec4(saturate(a.x), saturate(a.y), saturate(a.z), saturate(a.w)); }
+vec2 saturate(vec2 a) { return clamp(a, 0.0, 1.0); }
+vec3 saturate(vec3 a) { return clamp(a, 0.0, 1.0); }
+vec4 saturate(vec4 a) { return clamp(a, 0.0, 1.0); }
 
 float distance_squared(vec2 a, vec2 b) { a -= b; return dot(a, a); }
 float distance_squared(vec3 a, vec3 b) { a -= b; return dot(a, a); }
@@ -285,7 +285,28 @@ vec3 mrp_area(LightData ld, ShadingData sd, vec3 dir, inout float roughness, out
 vec3 F_schlick(vec3 f0, float cos_theta)
 {
 	float fac = pow(1.0 - cos_theta, 5);
-	return f0 + (1.0 - f0) * fac;
+
+	/* Unreal specular matching : if specular color is below 2% intensity,
+	 * (using green channel for intensity) treat as shadowning */
+	return saturate(50.0 * f0.g) * fac + (1.0 - fac) * f0;
+}
+
+/* Fresnel approximation for LTC area lights (not MRP) */
+vec3 F_area(vec3 f0, vec2 lut)
+{
+	vec2 fac = normalize(lut.xy);
+
+	/* Unreal specular matching : if specular color is below 2% intensity,
+	 * (using green channel for intensity) treat as shadowning */
+	return saturate(50.0 * f0.g) * fac.y + fac.x * f0;
+}
+
+/* Fresnel approximation for LTC area lights (not MRP) */
+vec3 F_ibl(vec3 f0, vec2 lut)
+{
+	/* Unreal specular matching : if specular color is below 2% intensity,
+	 * (using green channel for intensity) treat as shadowning */
+	return saturate(50.0 * f0.g) * lut.y + lut.x * f0;
 }
 
 /* GGX */

source/blender/draw/engines/eevee/shaders/bsdf_direct_lib.glsl

@@ -135,9 +135,7 @@ vec3 direct_ggx_sphere(LightData ld, ShadingData sd, float roughness, vec3 f0)
 	bsdf *= lut.b; /* Bsdf intensity */
 	bsdf *= M_1_2PI * M_1_PI;
 
-	/* Rough fresnel approximation using the LUT */
-	lut.xy = normalize(lut.xy);
-	vec3 spec = bsdf * lut.y + f0 * bsdf * lut.x;
+	vec3 spec = F_area(f0, lut.xy) * bsdf;
 #else
 	float energy_conservation;
 	vec3 L = mrp_sphere(ld, sd, sd.spec_dominant_dir, roughness, energy_conservation);
@@ -165,9 +163,7 @@ vec3 direct_ggx_rectangle(LightData ld, ShadingData sd, float roughness, vec3 f0
 	bsdf *= lut.b; /* Bsdf intensity */
 	bsdf *= M_1_2PI;
 
-	/* Rough fresnel approximation using the LUT */
-	lut.xy = normalize(lut.xy);
-	vec3 spec = bsdf * lut.y + f0 * bsdf * lut.x;
+	vec3 spec = F_area(f0, lut.xy) * bsdf;
 #else
 	float energy_conservation;
 	vec3 L = mrp_area(ld, sd, sd.spec_dominant_dir, roughness, energy_conservation);

source/blender/draw/engines/eevee/shaders/lit_surface_frag.glsl

@@ -273,7 +273,7 @@ vec3 eevee_surface_lit(vec3 world_normal, vec3 albedo, vec3 f0, float roughness,
 	vec2 uv = lut_coords(dot(sd.N, sd.V), sqrt(roughness));
 	vec2 brdf_lut = texture(brdfLut, uv).rg;
 	vec3 Li = textureLod(probeFiltered, sd.spec_dominant_dir, roughness * lodMax).rgb;
-	indirect_radiance += Li * brdf_lut.y + f0 * Li * brdf_lut.x;
+	indirect_radiance += Li * F_ibl(f0, brdf_lut);
 
 	indirect_radiance += spherical_harmonics(sd.N, shCoefs) * albedo;