Fix: Cycles: Inconsistency in transparent bounces for NEE and forward path

Note: this is a partial fix, that makes NEE and forward path consistent
only when `max_transparent_bounce > 0`. It is much more involved to make
forward path tracing support a max transparent bounce of 0, but since we
don't expect people to set up a very low number of transparent bounces,
it is less important to support that specific case.

Pull Request: https://projects.blender.org/blender/blender/pulls/138098

intern/cycles/kernel/integrator/intersect_shadow.h

@@ -59,7 +59,7 @@ ccl_device_forceinline int integrate_shadow_max_transparent_hits(KernelGlobals k
   const int transparent_max_bounce = kernel_data.integrator.transparent_max_bounce;
   const int transparent_bounce = INTEGRATOR_STATE(state, shadow_path, transparent_bounce);
 
-  return max(transparent_max_bounce - transparent_bounce - 1, 0);
+  return max(transparent_max_bounce - transparent_bounce, 0);
 }
 
 #ifdef __TRANSPARENT_SHADOWS__

intern/cycles/kernel/integrator/path_state.h

@@ -116,6 +116,8 @@ ccl_device_inline void path_state_next(KernelGlobals kg,
 
     flag |= PATH_RAY_TRANSPARENT;
     if (transparent_bounce >= kernel_data.integrator.transparent_max_bounce) {
+      /* FIXME: `transparent_max_bounce` could be 0, but `transparent_bounce` is at least 1 when we
+       * enter this path. */
       flag |= PATH_RAY_TERMINATE_ON_NEXT_SURFACE;
     }
 

intern/cycles/scene/integrator.cpp

@@ -185,6 +185,8 @@ void Integrator::device_update(Device *device, DeviceScene *dscene, Scene *scene
   device_free(device, dscene);
 
   /* integrator parameters */
+
+  /* Plus one so that a bounce of 0 indicates no global illumination, only direct illumination. */
   kintegrator->min_bounce = min_bounce + 1;
   kintegrator->max_bounce = max_bounce + 1;
 
@@ -194,7 +196,10 @@ void Integrator::device_update(Device *device, DeviceScene *dscene, Scene *scene
   kintegrator->max_volume_bounce = max_volume_bounce + 1;
 
   kintegrator->transparent_min_bounce = transparent_min_bounce + 1;
-  kintegrator->transparent_max_bounce = transparent_max_bounce + 1;
+
+  /* Unlike other type of bounces, 0 transparent bounce means there is no transparent bounce in the
+   * scene. */
+  kintegrator->transparent_max_bounce = transparent_max_bounce;
 
   kintegrator->ao_bounces = (ao_factor != 0.0f) ? ao_bounces : 0;
   kintegrator->ao_bounces_distance = ao_distance;