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

@@ -95,7 +95,7 @@ float ambient_occlusion_eval(vec3 normal,
 vec3 safe_normalize(vec3 N);
 float fast_sqrt(float a);
 vec3 cameraVec(vec3 P);
-vec2 btdf_lut(float a, float b, float c);
+vec2 btdf_lut(float a, float b, float c, float d);
 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);

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

@@ -74,7 +74,7 @@ vec3 lut_coords_btdf(float cos_theta, float roughness, float ior)
 }
 
 /* Returns GGX BTDF in first component and fresnel in second. */
-vec2 btdf_lut(float cos_theta, float roughness, float ior)
+vec2 btdf_lut(float cos_theta, float roughness, float ior, float do_multiscatter)
 {
   if (ior <= 1e-5) {
     return vec2(0.0);
@@ -83,16 +83,16 @@ vec2 btdf_lut(float cos_theta, float roughness, float ior)
   if (ior >= 1.0) {
     vec2 split_sum = brdf_lut(cos_theta, roughness);
     float f0 = f0_from_ior(ior);
-    /* Baked IOR for GGX BRDF. */
-    const float specular = 1.0;
-    const float eta_brdf = (2.0 / (1.0 - sqrt(0.08 * specular))) - 1.0;
-    /* Avoid harsh transition coming from ior == 1. */
-    float f90 = fast_sqrt(saturate(f0 / (f0_from_ior(eta_brdf) * 0.25)));
-    float fresnel = F_brdf_single_scatter(vec3(f0), vec3(f90), split_sum).r;
-    /* Setting the BTDF to one is not really important since it is only used for multi-scatter
-     * and it's already quite close to ground truth. */
-    float btdf = 1.0;
-    return vec2(btdf, fresnel);
+    /* 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));
+
+    float brdf = F_brdf_multi_scatter(vec3(f0), vec3(f90), split_sum).r;
+    /* Energy conservation. */
+    float btdf = 1.0 - brdf;
+    /* Assuming the energy loss caused by single-scattering is distributed proportionally in the
+     * reflection and refraction lobes. */
+    return vec2(btdf, brdf) * ((do_multiscatter == 0.0) ? sum(split_sum) : 1.0);
   }
 
   vec3 coords = lut_coords_btdf(cos_theta, roughness, ior);
@@ -101,15 +101,21 @@ vec2 btdf_lut(float cos_theta, float roughness, float ior)
   float layer_floored = floor(layer);
 
   coords.z = lut_btdf_layer_first + layer_floored;
-  vec2 btdf_low = textureLod(utilTex, coords, 0.0).rg;
+  vec2 btdf_brdf_low = textureLod(utilTex, coords, 0.0).rg;
 
   coords.z += 1.0;
-  vec2 btdf_high = textureLod(utilTex, coords, 0.0).rg;
+  vec2 btdf_brdf_high = textureLod(utilTex, coords, 0.0).rg;
 
   /* Manual trilinear interpolation. */
-  vec2 btdf = mix(btdf_low, btdf_high, layer - layer_floored);
+  vec2 btdf_brdf = mix(btdf_brdf_low, btdf_brdf_high, layer - layer_floored);
 
-  return btdf;
+  if (do_multiscatter != 0.0) {
+    /* For energy-conserving BSDF the reflection and refraction lobes should sum to one. Assuming
+     * the energy loss of single-scattering is distributed proportionally in the two lobes. */
+    btdf_brdf /= (btdf_brdf.x + btdf_brdf.y);
+  }
+
+  return btdf_brdf;
 }
 
 /** \} */

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

@@ -38,7 +38,7 @@ void main()
 }
 
 /* Stubs */
-vec2 btdf_lut(float a, float b, float c)
+vec2 btdf_lut(float a, float b, float c, float d)
 {
   return vec2(0.0);
 }

source/blender/draw/engines/eevee_next/shaders/eevee_nodetree_lib.glsl

@@ -316,7 +316,7 @@ vec2 brdf_lut(float cos_theta, float roughness)
 #endif
 }
 
-vec2 btdf_lut(float cos_theta, float roughness, float ior)
+vec2 btdf_lut(float cos_theta, float roughness, float ior, float do_multiscatter)
 {
   if (ior <= 1e-5) {
     return vec2(0.0);
@@ -325,16 +325,16 @@ vec2 btdf_lut(float cos_theta, float roughness, float ior)
   if (ior >= 1.0) {
     vec2 split_sum = brdf_lut(cos_theta, roughness);
     float f0 = F0_from_ior(ior);
-    /* Baked IOR for GGX BRDF. */
-    const float specular = 1.0;
-    const float eta_brdf = (2.0 / (1.0 - sqrt(0.08 * specular))) - 1.0;
-    /* Avoid harsh transition coming from ior == 1. */
-    float f90 = fast_sqrt(saturate(f0 / (F0_from_ior(eta_brdf) * 0.25)));
-    float fresnel = F_brdf_single_scatter(vec3(f0), vec3(f90), split_sum).r;
-    /* Setting the BTDF to one is not really important since it is only used for multi-scatter
-     * and it's already quite close to ground truth. */
-    float btdf = 1.0;
-    return vec2(btdf, fresnel);
+    /* 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));
+
+    float brdf = F_brdf_multi_scatter(vec3(f0), vec3(f90), split_sum).r;
+    /* Energy conservation. */
+    float btdf = 1.0 - brdf;
+    /* Assuming the energy loss caused by single-scattering is distributed proportionally in the
+     * reflection and refraction lobes. */
+    return vec2(btdf, brdf) * ((do_multiscatter == 0.0) ? sum(split_sum) : 1.0);
   }
 
   /* IOR is sin of critical angle. */
@@ -355,11 +355,18 @@ vec2 btdf_lut(float cos_theta, float roughness, float ior)
 
 #ifdef EEVEE_UTILITY_TX
   coords.z = UTIL_BTDF_LAYER + layer_floored;
-  vec2 btdf_low = utility_tx_sample_lut(utility_tx, coords.xy, coords.z).rg;
-  vec2 btdf_high = utility_tx_sample_lut(utility_tx, coords.xy, coords.z + 1.0).rg;
+  vec2 btdf_brdf_low = utility_tx_sample_lut(utility_tx, coords.xy, coords.z).rg;
+  vec2 btdf_brdf_high = utility_tx_sample_lut(utility_tx, coords.xy, coords.z + 1.0).rg;
   /* Manual trilinear interpolation. */
-  vec2 btdf = mix(btdf_low, btdf_high, layer - layer_floored);
-  return btdf;
+  vec2 btdf_brdf = mix(btdf_brdf_low, btdf_brdf_high, layer - layer_floored);
+
+  if (do_multiscatter != 0.0) {
+    /* For energy-conserving BSDF the reflection and refraction lobes should sum to one. Assuming
+     * the energy loss of single-scattering is distributed proportionally in the two lobes. */
+    btdf_brdf /= (btdf_brdf.x + btdf_brdf.y);
+  }
+
+  return btdf_brdf;
 #else
   return vec2(0.0);
 #endif

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

@@ -14,20 +14,17 @@ void node_bsdf_glass(vec4 color,
   vec3 V = cameraVec(g_data.P);
   float NV = dot(N, V);
 
-  vec2 split_sum = btdf_lut(NV, roughness, ior);
-
-  float fresnel = (do_multiscatter != 0.0) ? split_sum.y : F_eta(ior, NV);
-  float btdf = (do_multiscatter != 0.0) ? 1.0 : split_sum.x;
+  vec2 bsdf = btdf_lut(NV, roughness, ior, do_multiscatter);
 
   ClosureReflection reflection_data;
-  reflection_data.weight = fresnel * weight;
+  reflection_data.weight = bsdf.y * weight;
   reflection_data.color = color.rgb;
   reflection_data.N = N;
   reflection_data.roughness = roughness;
 
   ClosureRefraction refraction_data;
-  refraction_data.weight = (1.0 - fresnel) * weight;
-  refraction_data.color = color.rgb * btdf;
+  refraction_data.weight = bsdf.x * weight;
+  refraction_data.color = color.rgb;
   refraction_data.N = N;
   refraction_data.roughness = roughness;
   refraction_data.ior = ior;

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

@@ -64,9 +64,9 @@ void node_bsdf_principled(vec4 base_color,
   vec3 V = cameraVec(g_data.P);
   float NV = dot(N, V);
 
-  float fresnel = (do_multiscatter != 0.0) ? btdf_lut(NV, roughness, ior).y : F_eta(ior, NV);
-  float glass_reflection_weight = fresnel * transmission;
-  float glass_transmission_weight = (1.0 - fresnel) * transmission;
+  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);
 
@@ -144,9 +144,7 @@ void node_bsdf_principled(vec4 base_color,
   /* Refraction. */
   ClosureRefraction refraction_data;
   refraction_data.weight = glass_transmission_weight * weight;
-  float btdf = (do_multiscatter != 0.0) ? 1.0 : btdf_lut(NV, roughness, ior).x;
-
-  refraction_data.color = base_color.rgb * btdf;
+  refraction_data.color = base_color.rgb;
   refraction_data.N = N;
   refraction_data.roughness = roughness;
   refraction_data.ior = ior;