[Bf-blender-cvs] [aed9b6faee3] blender-v2.93-release: Fix some Cycles random walk SSS precision issues with small radius

[Brecht Van Lommel]

Commit: aed9b6faee39f8049ca9a072f37d2c0c2ea3a760
Author: Brecht Van Lommel
Date:   Wed Apr 21 17:32:32 2021 +0200
Branches: blender-v2.93-release
https://developer.blender.org/rBaed9b6faee39f8049ca9a072f37d2c0c2ea3a760

Fix some Cycles random walk SSS precision issues with small radius

===================================================================

M	intern/cycles/kernel/kernel_subsurface.h

===================================================================

diff --git a/intern/cycles/kernel/kernel_subsurface.h b/intern/cycles/kernel/kernel_subsurface.h
index c75958e79c5..0a56f867158 100644
--- a/intern/cycles/kernel/kernel_subsurface.h
+++ b/intern/cycles/kernel/kernel_subsurface.h
@@ -605,6 +605,13 @@ ccl_device_noinline
     if (hit) {
       t = ray->t;
     }
+    else if (bounce == 0) {
+      /* Restore original position if nothing was hit after the first bounce.
+       * Otherwise if the ray_offset() to avoid self-intersection is relatively
+       * large compared to the scattering radius, we go never backup high enough
+       * to exit the surface. */
+      ray->P = sd->P;
+    }
 
     /* Advance to new scatter location. */
     ray->P += t * ray->D;