Cycles: Improve smooth thin glass shadow

By allocating transparent BSDF for non-camera rays when roughness is
sufficiently low

Pull Request: https://projects.blender.org/blender/blender/pulls/159798

intern/cycles/kernel/closure/bsdf_microfacet.h

@@ -1268,8 +1268,20 @@ ccl_device_inline void bsdf_thin_glass_transmission_setup(KernelGlobals kg,
                                                           const Spectrum weight,
                                                           const float3 N,
                                                           const float roughness,
-                                                          const float ior)
+                                                          const float ior,
+                                                          const PathRayVisibility ray_visibility,
+                                                          const uint32_t path_flag)
 {
+  const float transmission_roughness = bsdf_thin_glass_transmission_roughness(roughness, ior);
+  if (!(ray_visibility & PATH_RAY_VISIBILITY_CAMERA) &&
+      roughness_is_almost_specular(transmission_roughness, transmission_roughness))
+  {
+    /* Smooth thin glass does not bend the ray and is effectively transparent, allocate transparent
+     * closures for non-camera rays to improve sampling and keep render passes intact. */
+    bsdf_transparent_setup(sd, weight, path_flag);
+    return;
+  }
+
   /* Model double refraction events as one reflection event with the incident ray mirrored along
    * the surface. */
   ccl_private MicrofacetBsdf *bsdf = (ccl_private MicrofacetBsdf *)bsdf_alloc(
@@ -1277,7 +1289,7 @@ ccl_device_inline void bsdf_thin_glass_transmission_setup(KernelGlobals kg,
   if (bsdf) {
     bsdf->N = -N;
     bsdf->T = zero_float3();
-    bsdf->alpha_x = bsdf->alpha_y = bsdf_thin_glass_transmission_roughness(roughness, ior);
+    bsdf->alpha_x = bsdf->alpha_y = transmission_roughness;
     bsdf->ior = 1.0f;
     bsdf->fresnel_type = MicrofacetFresnel::NONE;
     bsdf->energy_scale = 1.0f;
@@ -1343,7 +1355,9 @@ ccl_device void bsdf_thin_glass_setup(KernelGlobals kg,
                                       const float ior,
                                       const FresnelThinFilm thinfilm,
                                       ccl_private Spectrum *r_reflectance,
-                                      ccl_private Spectrum *r_transmittance)
+                                      ccl_private Spectrum *r_transmittance,
+                                      const PathRayVisibility ray_visibility,
+                                      const uint32_t path_flag)
 {
   const float cos_theta_i = dot(N, sd->wi);
   bsdf_thin_glass_fresnel(kg,
@@ -1362,7 +1376,8 @@ ccl_device void bsdf_thin_glass_setup(KernelGlobals kg,
         kg, sd, reflection_tint, *r_reflectance * weight, N, roughness);
   }
   if (!is_zero(*r_transmittance)) {
-    bsdf_thin_glass_transmission_setup(kg, sd, *r_transmittance * weight, N, roughness, ior);
+    bsdf_thin_glass_transmission_setup(
+        kg, sd, *r_transmittance * weight, N, roughness, ior, ray_visibility, path_flag);
   }
 }
 

intern/cycles/kernel/osl/closures_setup.h

@@ -517,7 +517,7 @@ ccl_device void osl_closure_generalized_schlick_bsdf_setup(
 ccl_device void osl_closure_thin_glass_setup(KernelGlobals kg,
                                              ccl_private ShaderData *sd,
                                              const PathRayVisibility path_visibility,
-                                             const uint32_t /*path_flag*/,
+                                             const uint32_t path_flag,
                                              const float3 weight,
                                              const ccl_private ThinGlassClosure *closure,
                                              float3 *layer_albedo)
@@ -563,7 +563,9 @@ ccl_device void osl_closure_thin_glass_setup(KernelGlobals kg,
                         closure->ior,
                         thinfilm,
                         &reflectance,
-                        &transmittance);
+                        &transmittance,
+                        path_visibility,
+                        path_flag);
 
   if (layer_albedo != nullptr) {
     *layer_albedo = transmittance * !!has_transmission + reflectance * !!has_reflection;

intern/cycles/kernel/svm/closure.h

@@ -379,7 +379,9 @@ ccl_device
                                   ior,
                                   thinfilm,
                                   &reflectance,
-                                  &transmittance);
+                                  &transmittance,
+                                  ray_visibility,
+                                  path_flag);
           }
           else {
             ccl_private MicrofacetBsdf *bsdf = (ccl_private MicrofacetBsdf *)bsdf_alloc(

intern/cycles/scene/shader_nodes.cpp

@@ -30,6 +30,7 @@
 #include "util/string.h"
 #include "util/transform.h"
 
+#include "kernel/closure/bsdf_microfacet.h"
 #include "kernel/svm/color_util.h"
 #include "kernel/svm/mapping_util.h"
 #include "kernel/svm/math_util.h"
@@ -2739,7 +2740,22 @@ void PrincipledBsdfNode::simplify_settings(Scene * /* scene */)
 
 bool PrincipledBsdfNode::has_surface_transparent()
 {
-  return (input("Alpha")->link != nullptr || alpha < (1.0f - CLOSURE_WEIGHT_CUTOFF));
+  if (input("Alpha")->link != nullptr || alpha < (1.0f - CLOSURE_WEIGHT_CUTOFF)) {
+    return true;
+  }
+
+  /* Smooth thin glass are treated as transparent for non-camera rays. */
+  if ((input("Thin Wall")->link || thin_wall) && has_nonzero_weight("Transmission Weight")) {
+    if (input("Roughness")->link || input("IOR")->link) {
+      return true;
+    }
+
+    const float transmission_roughness = bsdf_thin_glass_transmission_roughness(sqr(roughness),
+                                                                                ior);
+    return roughness_is_almost_specular(transmission_roughness, transmission_roughness);
+  }
+
+  return false;
 }
 
 bool PrincipledBsdfNode::is_thin_wall()

tests/files/render/principled_bsdf/cycles_renders/principled_bsdf_thin_glass.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
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-size 33284
+oid sha256:59f3ab7dd107fb2d0a340e38e35e7e7a19df6c2cfd3103a15f3b90e14a9b5019
+size 33101