EEVEE: change Principled BSDF to match Cycles

Ref: #99447

Co-authored-by: Clément Foucault <foucault.clem@gmail.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/111754

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

@@ -24,7 +24,7 @@ float ior_from_f0(float f0)
 }
 
 /* Simplified form of F_eta(eta, 1.0). */
-float f0_from_ior(float eta)
+float F0_from_ior(float eta)
 {
   float A = (eta - 1.0) / (eta + 1.0);
   return A * A;
@@ -69,7 +69,7 @@ float F_eta(float eta, float cos_theta)
 /* Fresnel color blend base on fresnel factor */
 vec3 F_color_blend(float eta, float fresnel, vec3 f0_color)
 {
-  float f0 = f0_from_ior(eta);
+  float f0 = F0_from_ior(eta);
   float fac = saturate((fresnel - f0) / (1.0 - f0));
   return mix(f0_color, vec3(1.0), fac);
 }

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

@@ -100,6 +100,7 @@ vec2 brdf_lut(float a, float b);
 vec3 F_brdf_multi_scatter(vec3 a, vec3 b, vec2 c);
 vec3 F_brdf_single_scatter(vec3 a, vec3 b, vec2 c);
 float F_eta(float a, float b);
+float F0_from_ior(float a);
 #endif
 
 #ifdef VOLUMETRICS

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

@@ -82,7 +82,7 @@ vec2 btdf_lut(float cos_theta, float roughness, float ior, float do_multiscatter
 
   if (ior >= 1.0) {
     vec2 split_sum = brdf_lut(cos_theta, roughness);
-    float f0 = f0_from_ior(ior);
+    float f0 = F0_from_ior(ior);
     /* Gradually increase `f90` from 0 to 1 when IOR is in the range of [1.0, 1.33], to avoid harsh
      * transition at `IOR == 1`. */
     float f90 = fast_sqrt(saturate(f0 / 0.02));

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

@@ -63,7 +63,7 @@ vec4 screen_space_refraction(vec3 vP, vec3 N, vec3 V, float ior, float roughness
 
     /* Empirical fit for refraction. */
     /* TODO: find a better fit or precompute inside the LUT. */
-    cone_tan *= 0.5 * fast_sqrt(f0_from_ior((ior < 1.0) ? 1.0 / ior : ior));
+    cone_tan *= 0.5 * fast_sqrt(F0_from_ior((ior < 1.0) ? 1.0 / ior : ior));
 
     float cone_footprint = hit_dist * cone_tan;
 

source/blender/gpu/shaders/material/gpu_shader_material_principled.glsl

@@ -53,101 +53,103 @@ void node_bsdf_principled(vec4 base_color,
 {
   /* Match cycles. */
   metallic = clamp(metallic, 0.0, 1.0);
-  transmission = clamp(transmission, 0.0, 1.0) * (1.0 - metallic);
-  float diffuse_weight = (1.0 - transmission) * (1.0 - metallic);
-  float specular_weight = (1.0 - transmission);
-  float clearcoat_weight = max(clearcoat, 0.0) * 0.25;
-  specular = max(0.0, specular);
+  transmission = clamp(transmission, 0.0, 1.0);
+  clearcoat = max(clearcoat, 0.0) * 0.25;
 
   N = safe_normalize(N);
   CN = safe_normalize(CN);
   vec3 V = cameraVec(g_data.P);
   float NV = dot(N, V);
 
-  vec2 glass_bsdf = btdf_lut(NV, roughness, ior, do_multiscatter);
-  float glass_reflection_weight = glass_bsdf.y * transmission;
-  float glass_transmission_weight = glass_bsdf.x * transmission;
-
-  vec3 base_color_tint = tint_from_color(base_color.rgb);
-
-  vec2 split_sum = brdf_lut(NV, roughness);
-
   ClosureTransparency transparency_data;
   transparency_data.weight = weight;
   transparency_data.transmittance = vec3(1.0 - alpha);
   transparency_data.holdout = 0.0;
-
   weight *= alpha;
 
+  /* First layer: Sheen */
+  /* TODO: Maybe sheen should be specular. */
+  vec3 sheen_color = sheen * sheen_tint.rgb * principled_sheen(NV, sheen_roughness);
+  ClosureDiffuse diffuse_data;
+  diffuse_data.color = weight * sheen_color;
+  diffuse_data.N = N;
+  /* Attenuate lower layers */
+  weight *= (1.0 - max_v3(sheen_color));
+
+  /* Second layer: Clearcoat */
+  ClosureReflection clearcoat_data;
+  clearcoat_data.N = CN;
+  clearcoat_data.roughness = clearcoat_roughness;
+  float coat_ior = 1.5;
+  float coat_NV = dot(clearcoat_data.N, V);
+  float reflectance = btdf_lut(coat_NV, clearcoat_data.roughness, coat_ior, 0.0).y;
+  clearcoat_data.weight = weight * clearcoat * reflectance;
+  clearcoat_data.color = vec3(1.0);
+  /* Attenuate lower layers */
+  weight *= (1.0 - reflectance * clearcoat);
+
+  /* Attenuated by sheen and clearcoat. */
   ClosureEmission emission_data;
   emission_data.weight = weight;
   emission_data.emission = emission.rgb * emission_strength;
 
-  /* Diffuse. */
-  ClosureDiffuse diffuse_data;
-  diffuse_data.weight = diffuse_weight * weight;
-  diffuse_data.color = mix(base_color.rgb, subsurface_color.rgb, subsurface);
-  /* Sheen Coarse approximation: We reuse the diffuse radiance and just scale it. */
-  diffuse_data.color += sheen * sheen_tint.rgb * principled_sheen(NV, sheen_roughness);
-  diffuse_data.N = N;
-  diffuse_data.sss_radius = subsurface_radius * subsurface;
-  diffuse_data.sss_id = uint(do_sss);
-
-  /* NOTE(@fclem): We need to blend the reflection color but also need to avoid applying the
-   * weights so we compute the ratio. */
-  float reflection_weight = specular_weight + glass_reflection_weight;
-  float reflection_weight_inv = safe_rcp(reflection_weight);
-  specular_weight *= reflection_weight_inv;
-  glass_reflection_weight *= reflection_weight_inv;
-
-  /* Reflection. */
+  /* Metallic component */
   ClosureReflection reflection_data;
-  reflection_data.weight = reflection_weight * weight;
   reflection_data.N = N;
   reflection_data.roughness = roughness;
+  vec2 split_sum = brdf_lut(NV, roughness);
   if (true) {
-    vec3 dielectric_f0_color = mix(vec3(1.0), base_color_tint, specular_tint);
-    vec3 metallic_f0_color = base_color.rgb;
-    vec3 f0 = mix((0.08 * specular) * dielectric_f0_color, metallic_f0_color, metallic);
-    /* Cycles does this blending using the microfacet fresnel factor. However, our fresnel
-     * is already baked inside the split sum LUT. We approximate by changing the f90 color
-     * directly in a non linear fashion. */
-    vec3 f90 = mix(f0, vec3(1.0), fast_sqrt(specular));
-
-    vec3 reflection_brdf = (do_multiscatter != 0.0) ? F_brdf_multi_scatter(f0, f90, split_sum) :
-                                                      F_brdf_single_scatter(f0, f90, split_sum);
-    reflection_data.color = reflection_brdf * specular_weight;
-  }
-  if (true) {
-    /* Poor approximation since we baked the LUT using a fixed IOR. */
-    vec3 f0 = mix(vec3(1.0), base_color.rgb, specular_tint);
+    vec3 f0 = base_color.rgb;
     vec3 f90 = vec3(1.0);
-
-    vec3 glass_brdf = (do_multiscatter != 0.0) ? F_brdf_multi_scatter(f0, f90, split_sum) :
-                                                 F_brdf_single_scatter(f0, f90, split_sum);
-
-    /* Avoid 3 glossy evaluation. Use the same closure for glass reflection. */
-    reflection_data.color += glass_brdf * glass_reflection_weight;
+    vec3 metallic_brdf = (do_multiscatter != 0.0) ? F_brdf_multi_scatter(f0, f90, split_sum) :
+                                                    F_brdf_single_scatter(f0, f90, split_sum);
+    reflection_data.color = weight * metallic * metallic_brdf;
+    /* Attenuate lower layers */
+    weight *= (1.0 - metallic);
   }
 
-  ClosureReflection clearcoat_data;
-  clearcoat_data.weight = clearcoat_weight * weight;
-  clearcoat_data.N = CN;
-  clearcoat_data.roughness = clearcoat_roughness;
-  if (true) {
-    float NV = dot(clearcoat_data.N, V);
-    vec2 split_sum = brdf_lut(NV, clearcoat_data.roughness);
-    vec3 brdf = F_brdf_single_scatter(vec3(0.04), vec3(1.0), split_sum);
-    clearcoat_data.color = brdf;
-  }
-
-  /* Refraction. */
+  /* Transmission component */
   ClosureRefraction refraction_data;
-  refraction_data.weight = glass_transmission_weight * weight;
-  refraction_data.color = base_color.rgb;
-  refraction_data.N = N;
-  refraction_data.roughness = roughness;
-  refraction_data.ior = ior;
+  /* TODO: change `specular_tint` to rgb. */
+  vec3 reflection_tint = mix(vec3(1.0), base_color.rgb, specular_tint);
+  if (true) {
+    vec2 bsdf = btdf_lut(NV, roughness, ior, do_multiscatter);
+
+    reflection_data.color += weight * transmission * bsdf.y * reflection_tint;
+
+    refraction_data.weight = weight * transmission * bsdf.x;
+    refraction_data.color = base_color.rgb;
+    refraction_data.N = N;
+    refraction_data.roughness = roughness;
+    refraction_data.ior = ior;
+    /* Attenuate lower layers */
+    weight *= (1.0 - transmission);
+  }
+
+  /* Specular component */
+  if (true) {
+    vec3 f0 = F0_from_ior(ior) * 2.0 * specular * reflection_tint;
+    vec3 f90 = vec3(1.0);
+    vec3 specular_brdf = (do_multiscatter != 0.0) ? F_brdf_multi_scatter(f0, f90, split_sum) :
+                                                    F_brdf_single_scatter(f0, f90, split_sum);
+    reflection_data.color += weight * specular_brdf;
+    /* Attenuate lower layers */
+    weight *= (1.0 - max_v3(specular_brdf));
+  }
+
+  /* Diffuse component */
+  if (true) {
+    vec3 diffuse_color = mix(base_color.rgb, subsurface_color.rgb, subsurface);
+    diffuse_data.sss_radius = subsurface_radius * subsurface;
+    diffuse_data.sss_id = uint(do_sss);
+    diffuse_data.color += weight * diffuse_color;
+  }
+
+  /* Adjust the weight of picking the closure. */
+  reflection_data.weight = avg(reflection_data.color);
+  reflection_data.color *= safe_rcp(reflection_data.weight);
+  diffuse_data.weight = avg(diffuse_data.color);
+  diffuse_data.color *= safe_rcp(diffuse_data.weight);
 
   /* Ref. #98190: Defines are optimizations for old compilers.
    * Might become unnecessary with EEVEE-Next. */

source/blender/nodes/shader/nodes/node_shader_bsdf_principled.cc

@@ -170,7 +170,8 @@ static int node_shader_gpu_bsdf_principled(GPUMaterial *mat,
   }
 #endif
 
-  bool use_diffuse = socket_not_one(SOCK_METALLIC_ID) && socket_not_one(SOCK_TRANSMISSION_ID);
+  bool use_diffuse = socket_not_zero(SOCK_SHEEN_ID) ||
+                     (socket_not_one(SOCK_METALLIC_ID) && socket_not_one(SOCK_TRANSMISSION_ID));
   bool use_subsurf = socket_not_zero(SOCK_SUBSURFACE_ID) && use_diffuse;
   bool use_refract = socket_not_one(SOCK_METALLIC_ID) && socket_not_zero(SOCK_TRANSMISSION_ID);
   bool use_transparency = socket_not_one(SOCK_ALPHA_ID);