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

1 files changed, 11 insertions, 9 deletions

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,