Cycles: improve SSS Fresnel and retro-reflection in Principled BSDF

For details see the "Extending the Disney BRDF to a BSDF with Integrated
Subsurface Scattering" paper.

We split the diffuse BSDF into a lambertian and retro-reflection component.
The retro-reflection component is always handled as a BSDF, while the
lambertian component can be replaced by a BSSRDF.

For the BSSRDF case, we compute Fresnel separately at the entry and exit
points, which may have different normals. As the scattering radius decreases
this converges to the BSDF case.

A downside is that this increases noise for subsurface scattering in the
Principled BSDF, due to some samples going to the retro-reflection component.
However the previous logic (also in 2.93) was simple wrong, using a
non-sensical view direction vector at the exit point. We use an importance
sampling weight estimate for the retro-reflection to try to better balance
samples between the BSDF and BSSRDF.

Differential Revision: https://developer.blender.org/D12801

10 files changed, 125 insertions, 77 deletions

diff --git a/intern/cycles/kernel/closure/bsdf.h b/intern/cycles/kernel/closure/bsdf.h
index 4eb8bcae997..87aa6339f80 100644
--- a/intern/cycles/kernel/closure/bsdf.h
+++ b/intern/cycles/kernel/closure/bsdf.h
@@ -128,7 +128,6 @@ ccl_device_inline int bsdf_sample(const KernelGlobals *kg,
 
   switch (sc->type) {
     case CLOSURE_BSDF_DIFFUSE_ID:
-    case CLOSURE_BSDF_BSSRDF_ID:
       label = bsdf_diffuse_sample(sc,
                                   Ng,
                                   sd->I,
@@ -401,7 +400,6 @@ ccl_device_inline int bsdf_sample(const KernelGlobals *kg,
       break;
 #  ifdef __PRINCIPLED__
     case CLOSURE_BSDF_PRINCIPLED_DIFFUSE_ID:
-    case CLOSURE_BSDF_BSSRDF_PRINCIPLED_ID:
       label = bsdf_principled_diffuse_sample(sc,
                                              Ng,
                                              sd->I,
@@ -481,7 +479,6 @@ ccl_device_inline
   if (!is_transmission) {
     switch (sc->type) {
       case CLOSURE_BSDF_DIFFUSE_ID:
-      case CLOSURE_BSDF_BSSRDF_ID:
         eval = bsdf_diffuse_eval_reflect(sc, sd->I, omega_in, pdf);
         break;
 #ifdef __SVM__
@@ -550,7 +547,6 @@ ccl_device_inline
         break;
 #  ifdef __PRINCIPLED__
       case CLOSURE_BSDF_PRINCIPLED_DIFFUSE_ID:
-      case CLOSURE_BSDF_BSSRDF_PRINCIPLED_ID:
         eval = bsdf_principled_diffuse_eval_reflect(sc, sd->I, omega_in, pdf);
         break;
       case CLOSURE_BSDF_PRINCIPLED_SHEEN_ID:
@@ -576,7 +572,6 @@ ccl_device_inline
   else {
     switch (sc->type) {
       case CLOSURE_BSDF_DIFFUSE_ID:
-      case CLOSURE_BSDF_BSSRDF_ID:
         eval = bsdf_diffuse_eval_transmit(sc, sd->I, omega_in, pdf);
         break;
 #ifdef __SVM__
@@ -637,7 +632,6 @@ ccl_device_inline
         break;
 #  ifdef __PRINCIPLED__
       case CLOSURE_BSDF_PRINCIPLED_DIFFUSE_ID:
-      case CLOSURE_BSDF_BSSRDF_PRINCIPLED_ID:
         eval = bsdf_principled_diffuse_eval_transmit(sc, sd->I, omega_in, pdf);
         break;
       case CLOSURE_BSDF_PRINCIPLED_SHEEN_ID:
diff --git a/intern/cycles/kernel/closure/bsdf_principled_diffuse.h b/intern/cycles/kernel/closure/bsdf_principled_diffuse.h
index 0d611f40096..04963ca1dc5 100644
--- a/intern/cycles/kernel/closure/bsdf_principled_diffuse.h
+++ b/intern/cycles/kernel/closure/bsdf_principled_diffuse.h
@@ -19,50 +19,98 @@
 /* DISNEY PRINCIPLED DIFFUSE BRDF
  *
  * Shading model by Brent Burley (Disney): "Physically Based Shading at Disney" (2012)
+ *
+ * "Extending the Disney BRDF to a BSDF with Integrated Subsurface Scattering" (2015)
+ * For the separation of retro-reflection, "2.3 Dielectric BRDF with integrated
+ * subsurface scattering"
  */
 
 #include "kernel/closure/bsdf_util.h"
 
 CCL_NAMESPACE_BEGIN
 
+enum PrincipledDiffuseBsdfComponents {
+  PRINCIPLED_DIFFUSE_FULL = 1,
+  PRINCIPLED_DIFFUSE_LAMBERT = 2,
+  PRINCIPLED_DIFFUSE_LAMBERT_EXIT = 4,
+  PRINCIPLED_DIFFUSE_RETRO_REFLECTION = 8,
+};
+
 typedef ccl_addr_space struct PrincipledDiffuseBsdf {
   SHADER_CLOSURE_BASE;
 
   float roughness;
+  int components;
 } PrincipledDiffuseBsdf;
 
 static_assert(sizeof(ShaderClosure) >= sizeof(PrincipledDiffuseBsdf),
               "PrincipledDiffuseBsdf is too large!");
 
-ccl_device float3 calculate_principled_diffuse_brdf(
+ccl_device int bsdf_principled_diffuse_setup(PrincipledDiffuseBsdf *bsdf)
+{
+  bsdf->type = CLOSURE_BSDF_PRINCIPLED_DIFFUSE_ID;
+  return SD_BSDF | SD_BSDF_HAS_EVAL;
+}
+
+ccl_device float3 bsdf_principled_diffuse_compute_brdf(
     const PrincipledDiffuseBsdf *bsdf, float3 N, float3 V, float3 L, float *pdf)
 {
-  float NdotL = dot(N, L);
+  const float NdotL = dot(N, L);
 
   if (NdotL <= 0) {
     return make_float3(0.0f, 0.0f, 0.0f);
   }
 
-  float NdotV = dot(N, V);
+  const float NdotV = dot(N, V);
+
+  const float FV = schlick_fresnel(NdotV);
+  const float FL = schlick_fresnel(NdotL);
+
+  float f = 0.0f;
 
-  /* H = normalize(L + V);  // Bisector of an angle between L and V.
-   * LH2 = 2 * dot(L, H)^2 = 2cos(x)^2 = cos(2x) + 1 = dot(L, V) + 1,
-   * half-angle x between L and V is at most 90 deg
-   */
-  float LH2 = dot(L, V) + 1;
+  /* Lambertian component. */
+  if (bsdf->components & (PRINCIPLED_DIFFUSE_FULL | PRINCIPLED_DIFFUSE_LAMBERT)) {
+    f += (1.0f - 0.5f * FV) * (1.0f - 0.5f * FL);
+  }
+  else if (bsdf->components & PRINCIPLED_DIFFUSE_LAMBERT_EXIT) {
+    f += (1.0f - 0.5f * FL);
+  }
 
-  float FL = schlick_fresnel(NdotL), FV = schlick_fresnel(NdotV);
-  const float Fd90 = 0.5f + LH2 * bsdf->roughness;
-  float Fd = (1.0f - FL + Fd90 * FL) * (1.0f - FV + Fd90 * FV);
+  /* Retro-reflection component. */
+  if (bsdf->components & (PRINCIPLED_DIFFUSE_FULL | PRINCIPLED_DIFFUSE_RETRO_REFLECTION)) {
+    /* H = normalize(L + V);  // Bisector of an angle between L and V
+     * LH2 = 2 * dot(L, H)^2 = 2cos(x)^2 = cos(2x) + 1 = dot(L, V) + 1,
+     * half-angle x between L and V is at most 90 deg. */
+    const float LH2 = dot(L, V) + 1;
+    const float RR = bsdf->roughness * LH2;
+    f += RR * (FL + FV + FL * FV * (RR - 1.0f));
+  }
 
-  float value = M_1_PI_F * NdotL * Fd;
+  float value = M_1_PI_F * NdotL * f;
 
   return make_float3(value, value, value);
 }
 
-ccl_device int bsdf_principled_diffuse_setup(PrincipledDiffuseBsdf *bsdf)
+/* Compute Fresnel at entry point, to be compbined with PRINCIPLED_DIFFUSE_LAMBERT_EXIT
+ * at the exit point to get the complete BSDF. */
+ccl_device_inline float bsdf_principled_diffuse_compute_entry_fresnel(const float NdotV)
+{
+  const float FV = schlick_fresnel(NdotV);
+  return (1.0f - 0.5f * FV);
+}
+
+/* Ad-hoc weight adjusment to avoid retro-reflection taking away half the
+ * samples from BSSRDF. */
+ccl_device_inline float bsdf_principled_diffuse_retro_reflection_sample_weight(
+    PrincipledDiffuseBsdf *bsdf, const float3 I)
+{
+  return bsdf->roughness * schlick_fresnel(dot(bsdf->N, I));
+}
+
+ccl_device int bsdf_principled_diffuse_setup(PrincipledDiffuseBsdf *bsdf, int components)
 {
   bsdf->type = CLOSURE_BSDF_PRINCIPLED_DIFFUSE_ID;
+  bsdf->components = components;
   return SD_BSDF | SD_BSDF_HAS_EVAL;
 }
 
@@ -79,7 +127,7 @@ ccl_device float3 bsdf_principled_diffuse_eval_reflect(const ShaderClosure *sc,
 
   if (dot(N, omega_in) > 0.0f) {
     *pdf = fmaxf(dot(N, omega_in), 0.0f) * M_1_PI_F;
-    return calculate_principled_diffuse_brdf(bsdf, N, V, L, pdf);
+    return bsdf_principled_diffuse_compute_brdf(bsdf, N, V, L, pdf);
   }
   else {
     *pdf = 0.0f;
@@ -115,7 +163,7 @@ ccl_device int bsdf_principled_diffuse_sample(const ShaderClosure *sc,
   sample_cos_hemisphere(N, randu, randv, omega_in, pdf);
 
   if (dot(Ng, *omega_in) > 0) {
-    *eval = calculate_principled_diffuse_brdf(bsdf, N, I, *omega_in, pdf);
+    *eval = bsdf_principled_diffuse_compute_brdf(bsdf, N, I, *omega_in, pdf);
 
 #ifdef __RAY_DIFFERENTIALS__
     // TODO: find a better approximation for the diffuse bounce
diff --git a/intern/cycles/kernel/closure/bssrdf.h b/intern/cycles/kernel/closure/bssrdf.h
index db183887018..d2f8af7910c 100644
--- a/intern/cycles/kernel/closure/bssrdf.h
+++ b/intern/cycles/kernel/closure/bssrdf.h
@@ -277,10 +277,27 @@ ccl_device_inline Bssrdf *bssrdf_alloc(ShaderData *sd, float3 weight)
 ccl_device int bssrdf_setup(ShaderData *sd, Bssrdf *bssrdf, ClosureType type, const float ior)
 {
   int flag = 0;
+
+  /* Add retro-reflection component as separate diffuse BSDF. */
+  if (bssrdf->roughness != FLT_MAX) {
+    PrincipledDiffuseBsdf *bsdf = (PrincipledDiffuseBsdf *)bsdf_alloc(
+        sd, sizeof(PrincipledDiffuseBsdf), bssrdf->weight);
+
+    if (bsdf) {
+      bsdf->N = bssrdf->N;
+      bsdf->roughness = bssrdf->roughness;
+      flag |= bsdf_principled_diffuse_setup(bsdf, PRINCIPLED_DIFFUSE_RETRO_REFLECTION);
+
+      /* Ad-hoc weight adjusment to avoid retro-reflection taking away half the
+       * samples from BSSRDF. */
+      bsdf->sample_weight *= bsdf_principled_diffuse_retro_reflection_sample_weight(bsdf, sd->I);
+    }
+  }
+
+  /* Verify if the radii are large enough to sample without precision issues. */
   int bssrdf_channels = 3;
   float3 diffuse_weight = make_float3(0.0f, 0.0f, 0.0f);
 
-  /* Verify if the radii are large enough to sample without precision issues. */
   if (bssrdf->radius.x < BSSRDF_MIN_RADIUS) {
     diffuse_weight.x = bssrdf->weight.x;
     bssrdf->weight.x = 0.0f;
@@ -304,17 +321,13 @@ ccl_device int bssrdf_setup(ShaderData *sd, Bssrdf *bssrdf, ClosureType type, co
     /* Add diffuse BSDF if any radius too small. */
 #ifdef __PRINCIPLED__
     if (bssrdf->roughness != FLT_MAX) {
-      float roughness = bssrdf->roughness;
-      float3 N = bssrdf->N;
-
       PrincipledDiffuseBsdf *bsdf = (PrincipledDiffuseBsdf *)bsdf_alloc(
           sd, sizeof(PrincipledDiffuseBsdf), diffuse_weight);
 
       if (bsdf) {
-        bsdf->type = CLOSURE_BSDF_BSSRDF_PRINCIPLED_ID;
-        bsdf->N = N;
-        bsdf->roughness = roughness;
-        flag |= bsdf_principled_diffuse_setup(bsdf);
+        bsdf->N = bssrdf->N;
+        bsdf->roughness = bssrdf->roughness;
+        flag |= bsdf_principled_diffuse_setup(bsdf, PRINCIPLED_DIFFUSE_LAMBERT);
       }
     }
     else
@@ -323,7 +336,6 @@ ccl_device int bssrdf_setup(ShaderData *sd, Bssrdf *bssrdf, ClosureType type, co
       DiffuseBsdf *bsdf = (DiffuseBsdf *)bsdf_alloc(sd, sizeof(DiffuseBsdf), diffuse_weight);
 
       if (bsdf) {
-        bsdf->type = CLOSURE_BSDF_BSSRDF_ID;
         bsdf->N = bssrdf->N;
         flag |= bsdf_diffuse_setup(bsdf);
       }
diff --git a/intern/cycles/kernel/integrator/integrator_shade_surface.h b/intern/cycles/kernel/integrator/integrator_shade_surface.h
index a24473addcc..27338f824c0 100644
--- a/intern/cycles/kernel/integrator/integrator_shade_surface.h
+++ b/intern/cycles/kernel/integrator/integrator_shade_surface.h
@@ -378,11 +378,11 @@ ccl_device bool integrate_surface(INTEGRATOR_STATE_ARGS,
     }
 
 #ifdef __SUBSURFACE__
-    if (INTEGRATOR_STATE(path, flag) & PATH_RAY_SUBSURFACE) {
+    if (path_flag & PATH_RAY_SUBSURFACE) {
       /* When coming from inside subsurface scattering, setup a diffuse
        * closure to perform lighting at the exit point. */
+      subsurface_shader_data_setup(INTEGRATOR_STATE_PASS, &sd, path_flag);
       INTEGRATOR_STATE_WRITE(path, flag) &= ~PATH_RAY_SUBSURFACE;
-      subsurface_shader_data_setup(INTEGRATOR_STATE_PASS, &sd);
     }
 #endif
 
diff --git a/intern/cycles/kernel/integrator/integrator_state_template.h b/intern/cycles/kernel/integrator/integrator_state_template.h
index 15998ee6edf..0fe47cf13bc 100644
--- a/intern/cycles/kernel/integrator/integrator_state_template.h
+++ b/intern/cycles/kernel/integrator/integrator_state_template.h
@@ -99,7 +99,6 @@ KERNEL_STRUCT_BEGIN(subsurface)
 KERNEL_STRUCT_MEMBER(subsurface, float3, albedo, KERNEL_FEATURE_SUBSURFACE)
 KERNEL_STRUCT_MEMBER(subsurface, float3, radius, KERNEL_FEATURE_SUBSURFACE)
 KERNEL_STRUCT_MEMBER(subsurface, float, anisotropy, KERNEL_FEATURE_SUBSURFACE)
-KERNEL_STRUCT_MEMBER(subsurface, float, roughness, KERNEL_FEATURE_SUBSURFACE)
 KERNEL_STRUCT_END(subsurface)
 
 /********************************** Volume Stack ******************************/
diff --git a/intern/cycles/kernel/integrator/integrator_subsurface.h b/intern/cycles/kernel/integrator/integrator_subsurface.h
index 7ca676351db..2d15c82322a 100644
--- a/intern/cycles/kernel/integrator/integrator_subsurface.h
+++ b/intern/cycles/kernel/integrator/integrator_subsurface.h
@@ -47,7 +47,7 @@ ccl_device int subsurface_bounce(INTEGRATOR_STATE_ARGS, ShaderData *sd, const Sh
 
   /* Setup ray into surface. */
   INTEGRATOR_STATE_WRITE(ray, P) = sd->P;
-  INTEGRATOR_STATE_WRITE(ray, D) = sd->N;
+  INTEGRATOR_STATE_WRITE(ray, D) = bssrdf->N;
   INTEGRATOR_STATE_WRITE(ray, t) = FLT_MAX;
   INTEGRATOR_STATE_WRITE(ray, dP) = differential_make_compact(sd->dP);
   INTEGRATOR_STATE_WRITE(ray, dD) = differential_zero_compact();
@@ -56,13 +56,20 @@ ccl_device int subsurface_bounce(INTEGRATOR_STATE_ARGS, ShaderData *sd, const Sh
   INTEGRATOR_STATE_WRITE(isect, Ng) = sd->Ng;
   INTEGRATOR_STATE_WRITE(isect, object) = sd->object;
 
-  /* Pass BSSRDF parameters. */
-  const uint32_t path_flag = INTEGRATOR_STATE_WRITE(path, flag);
-  INTEGRATOR_STATE_WRITE(path, flag) = (path_flag & ~PATH_RAY_CAMERA) |
-                                       ((sc->type == CLOSURE_BSSRDF_BURLEY_ID) ?
-                                            PATH_RAY_SUBSURFACE_DISK :
-                                            PATH_RAY_SUBSURFACE_RANDOM_WALK);
-  INTEGRATOR_STATE_WRITE(path, throughput) *= shader_bssrdf_sample_weight(sd, sc);
+  uint32_t path_flag = (INTEGRATOR_STATE(path, flag) & ~PATH_RAY_CAMERA) |
+                       ((sc->type == CLOSURE_BSSRDF_BURLEY_ID) ? PATH_RAY_SUBSURFACE_DISK :
+                                                                 PATH_RAY_SUBSURFACE_RANDOM_WALK);
+
+  /* Compute weight, optionally including Fresnel from entry point. */
+  float3 weight = shader_bssrdf_sample_weight(sd, sc);
+#  ifdef __PRINCIPLED__
+  if (bssrdf->roughness != FLT_MAX) {
+    path_flag |= PATH_RAY_SUBSURFACE_USE_FRESNEL;
+  }
+#  endif
+
+  INTEGRATOR_STATE_WRITE(path, throughput) *= weight;
+  INTEGRATOR_STATE_WRITE(path, flag) = path_flag;
 
   /* Advance random number offset for bounce. */
   INTEGRATOR_STATE_WRITE(path, rng_offset) += PRNG_BOUNCE_NUM;
@@ -73,15 +80,17 @@ ccl_device int subsurface_bounce(INTEGRATOR_STATE_ARGS, ShaderData *sd, const Sh
     }
   }
 
+  /* Pass BSSRDF parameters. */
   INTEGRATOR_STATE_WRITE(subsurface, albedo) = bssrdf->albedo;
   INTEGRATOR_STATE_WRITE(subsurface, radius) = bssrdf->radius;
-  INTEGRATOR_STATE_WRITE(subsurface, roughness) = bssrdf->roughness;
   INTEGRATOR_STATE_WRITE(subsurface, anisotropy) = bssrdf->anisotropy;
 
   return LABEL_SUBSURFACE_SCATTER;
 }
 
-ccl_device void subsurface_shader_data_setup(INTEGRATOR_STATE_ARGS, ShaderData *sd)
+ccl_device void subsurface_shader_data_setup(INTEGRATOR_STATE_ARGS,
+                                             ShaderData *sd,
+                                             const uint32_t path_flag)
 {
   /* Get bump mapped normal from shader evaluation at exit point. */
   float3 N = sd->N;
@@ -95,21 +104,16 @@ ccl_device void subsurface_shader_data_setup(INTEGRATOR_STATE_ARGS, ShaderData *
   sd->num_closure_left = kernel_data.max_closures;
 
   const float3 weight = one_float3();
-  const float roughness = INTEGRATOR_STATE(subsurface, roughness);
 
 #  ifdef __PRINCIPLED__
-  if (roughness != FLT_MAX) {
+  if (path_flag & PATH_RAY_SUBSURFACE_USE_FRESNEL) {
     PrincipledDiffuseBsdf *bsdf = (PrincipledDiffuseBsdf *)bsdf_alloc(
         sd, sizeof(PrincipledDiffuseBsdf), weight);
 
     if (bsdf) {
       bsdf->N = N;
-      bsdf->roughness = roughness;
-      sd->flag |= bsdf_principled_diffuse_setup(bsdf);
-
-      /* replace CLOSURE_BSDF_PRINCIPLED_DIFFUSE_ID with this special ID so render passes
-       * can recognize it as not being a regular Disney principled diffuse closure */
-      bsdf->type = CLOSURE_BSDF_BSSRDF_PRINCIPLED_ID;
+      bsdf->roughness = FLT_MAX;
+      sd->flag |= bsdf_principled_diffuse_setup(bsdf, PRINCIPLED_DIFFUSE_LAMBERT_EXIT);
     }
   }
   else
@@ -120,10 +124,6 @@ ccl_device void subsurface_shader_data_setup(INTEGRATOR_STATE_ARGS, ShaderData *
     if (bsdf) {
       bsdf->N = N;
       sd->flag |= bsdf_diffuse_setup(bsdf);
-
-      /* replace CLOSURE_BSDF_DIFFUSE_ID with this special ID so render passes
-       * can recognize it as not being a regular diffuse closure */
-      bsdf->type = CLOSURE_BSDF_BSSRDF_ID;
     }
   }
 }
diff --git a/intern/cycles/kernel/kernel_shader.h b/intern/cycles/kernel/kernel_shader.h
index a50f3fb214b..d1b53832793 100644
--- a/intern/cycles/kernel/kernel_shader.h
+++ b/intern/cycles/kernel/kernel_shader.h
@@ -159,7 +159,7 @@ ccl_device_forceinline bool _shader_bsdf_exclude(ClosureType type, uint light_sh
     return false;
   }
   if (light_shader_flags & SHADER_EXCLUDE_DIFFUSE) {
-    if (CLOSURE_IS_BSDF_DIFFUSE(type) || CLOSURE_IS_BSDF_BSSRDF(type)) {
+    if (CLOSURE_IS_BSDF_DIFFUSE(type)) {
       return true;
     }
   }
@@ -201,8 +201,7 @@ ccl_device_inline float _shader_bsdf_multi_eval(const KernelGlobals *kg,
         float3 eval = bsdf_eval(kg, sd, sc, omega_in, is_transmission, &bsdf_pdf);
 
         if (bsdf_pdf != 0.0f) {
-          const bool is_diffuse = (CLOSURE_IS_BSDF_DIFFUSE(sc->type) ||
-                                   CLOSURE_IS_BSDF_BSSRDF(sc->type));
+          const bool is_diffuse = CLOSURE_IS_BSDF_DIFFUSE(sc->type);
           bsdf_eval_accum(result_eval, is_diffuse, eval * sc->weight, 1.0f);
           sum_pdf += bsdf_pdf * sc->sample_weight;
         }
@@ -320,8 +319,7 @@ ccl_device int shader_bsdf_sample_closure(const KernelGlobals *kg,
   label = bsdf_sample(kg, sd, sc, randu, randv, &eval, omega_in, domega_in, pdf);
 
   if (*pdf != 0.0f) {
-    const bool is_diffuse = (CLOSURE_IS_BSDF_DIFFUSE(sc->type) ||
-                             CLOSURE_IS_BSDF_BSSRDF(sc->type));
+    const bool is_diffuse = CLOSURE_IS_BSDF_DIFFUSE(sc->type);
     bsdf_eval_init(bsdf_eval, is_diffuse, eval * sc->weight);
 
     if (sd->num_closure > 1) {
@@ -401,8 +399,7 @@ ccl_device float3 shader_bsdf_diffuse(const KernelGlobals *kg, const ShaderData
   for (int i = 0; i < sd->num_closure; i++) {
     const ShaderClosure *sc = &sd->closure[i];
 
-    if (CLOSURE_IS_BSDF_DIFFUSE(sc->type) || CLOSURE_IS_BSSRDF(sc->type) ||
-        CLOSURE_IS_BSDF_BSSRDF(sc->type))
+    if (CLOSURE_IS_BSDF_DIFFUSE(sc->type) || CLOSURE_IS_BSSRDF(sc->type))
       eval += sc->weight;
   }
 
diff --git a/intern/cycles/kernel/kernel_types.h b/intern/cycles/kernel/kernel_types.h
index eb681683d6a..00457695e53 100644
--- a/intern/cycles/kernel/kernel_types.h
+++ b/intern/cycles/kernel/kernel_types.h
@@ -263,32 +263,34 @@ enum PathRayFlag {
   /* Perform subsurface scattering. */
   PATH_RAY_SUBSURFACE_RANDOM_WALK = (1 << 20),
   PATH_RAY_SUBSURFACE_DISK = (1 << 21),
-  PATH_RAY_SUBSURFACE = (PATH_RAY_SUBSURFACE_RANDOM_WALK | PATH_RAY_SUBSURFACE_DISK),
+  PATH_RAY_SUBSURFACE_USE_FRESNEL = (1 << 22),
+  PATH_RAY_SUBSURFACE = (PATH_RAY_SUBSURFACE_RANDOM_WALK | PATH_RAY_SUBSURFACE_DISK |
+                         PATH_RAY_SUBSURFACE_USE_FRESNEL),
 
   /* Contribute to denoising features. */
-  PATH_RAY_DENOISING_FEATURES = (1 << 22),
+  PATH_RAY_DENOISING_FEATURES = (1 << 23),
 
   /* Render pass categories. */
-  PATH_RAY_REFLECT_PASS = (1 << 23),
-  PATH_RAY_TRANSMISSION_PASS = (1 << 24),
-  PATH_RAY_VOLUME_PASS = (1 << 25),
+  PATH_RAY_REFLECT_PASS = (1 << 24),
+  PATH_RAY_TRANSMISSION_PASS = (1 << 25),
+  PATH_RAY_VOLUME_PASS = (1 << 26),
   PATH_RAY_ANY_PASS = (PATH_RAY_REFLECT_PASS | PATH_RAY_TRANSMISSION_PASS | PATH_RAY_VOLUME_PASS),
 
   /* Shadow ray is for a light or surface. */
-  PATH_RAY_SHADOW_FOR_LIGHT = (1 << 26),
+  PATH_RAY_SHADOW_FOR_LIGHT = (1 << 27),
 
   /* A shadow catcher object was hit and the path was split into two. */
-  PATH_RAY_SHADOW_CATCHER_HIT = (1 << 27),
+  PATH_RAY_SHADOW_CATCHER_HIT = (1 << 28),
 
   /* A shadow catcher object was hit and this path traces only shadow catchers, writing them into
    * their dedicated pass for later division.
    *
    * NOTE: Is not covered with `PATH_RAY_ANY_PASS` because shadow catcher does special handling
    * which is separate from the light passes. */
-  PATH_RAY_SHADOW_CATCHER_PASS = (1 << 28),
+  PATH_RAY_SHADOW_CATCHER_PASS = (1 << 29),
 
   /* Path is evaluating background for an approximate shadow catcher with non-transparent film. */
-  PATH_RAY_SHADOW_CATCHER_BACKGROUND = (1 << 29),
+  PATH_RAY_SHADOW_CATCHER_BACKGROUND = (1 << 30),
 };
 
 /* Configure ray visibility bits for rays and objects respectively,
diff --git a/intern/cycles/kernel/svm/svm_closure.h b/intern/cycles/kernel/svm/svm_closure.h
index b3f7fee8a63..e55f76a4400 100644
--- a/intern/cycles/kernel/svm/svm_closure.h
+++ b/intern/cycles/kernel/svm/svm_closure.h
@@ -221,7 +221,7 @@ ccl_device_noinline int svm_node_closure_bsdf(
             bsdf->roughness = roughness;
 
             /* setup bsdf */
-            sd->flag |= bsdf_principled_diffuse_setup(bsdf);
+            sd->flag |= bsdf_principled_diffuse_setup(bsdf, PRINCIPLED_DIFFUSE_FULL);
           }
         }
         else if (subsurface > CLOSURE_WEIGHT_CUTOFF) {
@@ -255,7 +255,7 @@ ccl_device_noinline int svm_node_closure_bsdf(
           bsdf->roughness = roughness;
 
           /* setup bsdf */
-          sd->flag |= bsdf_principled_diffuse_setup(bsdf);
+          sd->flag |= bsdf_principled_diffuse_setup(bsdf, PRINCIPLED_DIFFUSE_FULL);
         }
       }
 #  endif
diff --git a/intern/cycles/kernel/svm/svm_types.h b/intern/cycles/kernel/svm/svm_types.h
index e846e4af259..6f6c101fb69 100644
--- a/intern/cycles/kernel/svm/svm_types.h
+++ b/intern/cycles/kernel/svm/svm_types.h
@@ -538,8 +538,6 @@ typedef enum ClosureType {
   CLOSURE_BSDF_HAIR_TRANSMISSION_ID,
 
   /* Special cases */
-  CLOSURE_BSDF_BSSRDF_ID,
-  CLOSURE_BSDF_BSSRDF_PRINCIPLED_ID,
   CLOSURE_BSDF_TRANSPARENT_ID,
 
   /* BSSRDF */
@@ -569,8 +567,6 @@ typedef enum ClosureType {
    (type == CLOSURE_BSDF_HAIR_PRINCIPLED_ID))
 #define CLOSURE_IS_BSDF_TRANSMISSION(type) \
   (type >= CLOSURE_BSDF_REFRACTION_ID && type <= CLOSURE_BSDF_HAIR_TRANSMISSION_ID)
-#define CLOSURE_IS_BSDF_BSSRDF(type) \
-  (type == CLOSURE_BSDF_BSSRDF_ID || type == CLOSURE_BSDF_BSSRDF_PRINCIPLED_ID)
 #define CLOSURE_IS_BSDF_SINGULAR(type) \
   (type == CLOSURE_BSDF_REFLECTION_ID || type == CLOSURE_BSDF_REFRACTION_ID || \
    type == CLOSURE_BSDF_TRANSPARENT_ID)