[Bf-blender-cvs] [1ea0a14fd15] cycles-x: Fix wrong MIS weight with transparent surfaces and volumes, leading to noise

[Brecht Van Lommel]

Commit: 1ea0a14fd1565140efbf9a9c5bb483b6a975625e
Author: Brecht Van Lommel
Date:   Mon Jul 19 17:09:29 2021 +0200
Branches: cycles-x
https://developer.blender.org/rB1ea0a14fd1565140efbf9a9c5bb483b6a975625e

Fix wrong MIS weight with transparent surfaces and volumes, leading to noise

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

M	intern/cycles/kernel/integrator/integrator_shade_surface.h
M	intern/cycles/kernel/integrator/integrator_state_template.h

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

diff --git a/intern/cycles/kernel/integrator/integrator_shade_surface.h b/intern/cycles/kernel/integrator/integrator_shade_surface.h
index aafb6f3a072..f0607138db7 100644
--- a/intern/cycles/kernel/integrator/integrator_shade_surface.h
+++ b/intern/cycles/kernel/integrator/integrator_shade_surface.h
@@ -259,7 +259,10 @@ ccl_device_forceinline int integrate_surface_bsdf_bssrdf_bounce(INTEGRATOR_STATE
   }
 
   /* Update path state */
-  if (!(label & LABEL_TRANSPARENT)) {
+  if (label & LABEL_TRANSPARENT) {
+    INTEGRATOR_STATE_WRITE(path, mis_ray_t) += sd->ray_length;
+  }
+  else {
     INTEGRATOR_STATE_WRITE(path, mis_ray_pdf) = bsdf_pdf;
     INTEGRATOR_STATE_WRITE(path, mis_ray_t) = 0.0f;
     INTEGRATOR_STATE_WRITE(path, min_ray_pdf) = fminf(bsdf_pdf,
@@ -288,6 +291,8 @@ ccl_device_forceinline bool integrate_surface_volume_only_bounce(INTEGRATOR_STAT
   INTEGRATOR_STATE_WRITE(ray, dP) = differential_make_compact(sd->dP);
 #  endif
 
+  INTEGRATOR_STATE_WRITE(path, mis_ray_t) += sd->ray_length;
+
   return LABEL_TRANSMIT | LABEL_TRANSPARENT;
 }
 #endif
diff --git a/intern/cycles/kernel/integrator/integrator_state_template.h b/intern/cycles/kernel/integrator/integrator_state_template.h
index 13f565b29de..e3b0d893290 100644
--- a/intern/cycles/kernel/integrator/integrator_state_template.h
+++ b/intern/cycles/kernel/integrator/integrator_state_template.h
@@ -49,7 +49,10 @@ KERNEL_STRUCT_MEMBER(path, uint32_t, rng_hash, KERNEL_FEATURE_PATH_TRACING)
 KERNEL_STRUCT_MEMBER(path, uint32_t, rng_offset, KERNEL_FEATURE_PATH_TRACING)
 /* enum PathRayFlag */
 KERNEL_STRUCT_MEMBER(path, uint32_t, flag, KERNEL_FEATURE_PATH_TRACING)
-/* Multiple importance sampling. */
+/* Multiple importance sampling
+ * The PDF of BSDF sampling at the last scatter point, and distance to the
+ * last scatter point minus the last ray segment. This distance lets us
+ * compute the complete distance through transparent surfaces and volumes. */
 KERNEL_STRUCT_MEMBER(path, float, mis_ray_pdf, KERNEL_FEATURE_PATH_TRACING)
 KERNEL_STRUCT_MEMBER(path, float, mis_ray_t, KERNEL_FEATURE_PATH_TRACING)
 /* Filter glossy. */