Cycles: better path termination for transparency.

We now continue transparent paths after diffuse/glossy/transmission/volume
bounces are exceeded. This avoids unexpected boundaries in volumes with
transparent boundaries. It is also required for MIS to work correctly with
transparent surfaces, as we also continue through these in shadow rays.

The main visible changes is that volumes will now be lit by the background
even at volume bounces 0, same as surfaces.

Fixes T53914 and T54103.

1 files changed, 26 insertions, 5 deletions

diff --git a/intern/cycles/kernel/kernel_shader.h b/intern/cycles/kernel/kernel_shader.h
index 5f2f00c5ceb..8cfd33b808e 100644
--- a/intern/cycles/kernel/kernel_shader.h
+++ b/intern/cycles/kernel/kernel_shader.h
@@ -966,10 +966,21 @@ ccl_device float3 shader_holdout_eval(KernelGlobals *kg, ShaderData *sd)
 /* Surface Evaluation */
 
 ccl_device void shader_eval_surface(KernelGlobals *kg, ShaderData *sd,
-	ccl_addr_space PathState *state, int path_flag, int max_closure)
+	ccl_addr_space PathState *state, int path_flag)
 {
+	/* If path is being terminated, we are tracing a shadow ray or evaluating
+	 * emission, then we don't need to store closures. The emission and shadow
+	 * shader data also do not have a closure array to save GPU memory. */
+	int max_closures;
+	if(path_flag & (PATH_RAY_TERMINATE|PATH_RAY_SHADOW|PATH_RAY_EMISSION)) {
+		max_closures = 0;
+	}
+	else {
+		max_closures = kernel_data.integrator.max_closures;
+	}
+
 	sd->num_closure = 0;
-	sd->num_closure_left = max_closure;
+	sd->num_closure_left = max_closures;
 
 #ifdef __OSL__
 	if(kg->osl)
@@ -1140,13 +1151,23 @@ ccl_device_inline void shader_eval_volume(KernelGlobals *kg,
                                           ShaderData *sd,
                                           ccl_addr_space PathState *state,
                                           ccl_addr_space VolumeStack *stack,
-                                          int path_flag,
-                                          int max_closure)
+                                          int path_flag)
 {
+	/* If path is being terminated, we are tracing a shadow ray or evaluating
+	 * emission, then we don't need to store closures. The emission and shadow
+	 * shader data also do not have a closure array to save GPU memory. */
+	int max_closures;
+	if(path_flag & (PATH_RAY_TERMINATE|PATH_RAY_SHADOW|PATH_RAY_EMISSION)) {
+		max_closures = 0;
+	}
+	else {
+		max_closures = kernel_data.integrator.max_closures;
+	}
+
 	/* reset closures once at the start, we will be accumulating the closures
 	 * for all volumes in the stack into a single array of closures */
 	sd->num_closure = 0;
-	sd->num_closure_left = max_closure;
+	sd->num_closure_left = max_closures;
 	sd->flag = 0;
 	sd->object_flag = 0;