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, 30 insertions, 20 deletions

diff --git a/intern/cycles/kernel/kernel_path_state.h b/intern/cycles/kernel/kernel_path_state.h
index a16c20cbee6..15d81fcddf4 100644
--- a/intern/cycles/kernel/kernel_path_state.h
+++ b/intern/cycles/kernel/kernel_path_state.h
@@ -75,6 +75,9 @@ ccl_device_inline void path_state_next(KernelGlobals *kg, ccl_addr_space PathSta
 	if(label & LABEL_TRANSPARENT) {
 		state->flag |= PATH_RAY_TRANSPARENT;
 		state->transparent_bounce++;
+		if(state->transparent_bounce >= kernel_data.integrator.transparent_max_bounce) {
+			state->flag |= PATH_RAY_TERMINATE_IMMEDIATE;
+		}
 
 		if(!kernel_data.integrator.transparent_shadows)
 			state->flag |= PATH_RAY_MIS_SKIP;
@@ -86,6 +89,10 @@ ccl_device_inline void path_state_next(KernelGlobals *kg, ccl_addr_space PathSta
 	}
 
 	state->bounce++;
+	if(state->bounce >= kernel_data.integrator.max_bounce) {
+		state->flag |= PATH_RAY_TERMINATE_AFTER_TRANSPARENT;
+	}
+
 	state->flag &= ~(PATH_RAY_ALL_VISIBILITY|PATH_RAY_MIS_SKIP);
 
 #ifdef __VOLUME__
@@ -95,6 +102,9 @@ ccl_device_inline void path_state_next(KernelGlobals *kg, ccl_addr_space PathSta
 		state->flag &= ~PATH_RAY_TRANSPARENT_BACKGROUND;
 
 		state->volume_bounce++;
+		if(state->volume_bounce >= kernel_data.integrator.max_volume_bounce) {
+			state->flag |= PATH_RAY_TERMINATE_AFTER_TRANSPARENT;
+		}
 	}
 	else
 #endif
@@ -104,10 +114,18 @@ ccl_device_inline void path_state_next(KernelGlobals *kg, ccl_addr_space PathSta
 			state->flag |= PATH_RAY_REFLECT;
 			state->flag &= ~PATH_RAY_TRANSPARENT_BACKGROUND;
 
-			if(label & LABEL_DIFFUSE)
+			if(label & LABEL_DIFFUSE) {
 				state->diffuse_bounce++;
-			else
+				if(state->diffuse_bounce >= kernel_data.integrator.max_diffuse_bounce) {
+					state->flag |= PATH_RAY_TERMINATE_AFTER_TRANSPARENT;
+				}
+			}
+			else {
 				state->glossy_bounce++;
+				if(state->glossy_bounce >= kernel_data.integrator.max_glossy_bounce) {
+					state->flag |= PATH_RAY_TERMINATE_AFTER_TRANSPARENT;
+				}
+			}
 		}
 		else {
 			kernel_assert(label & LABEL_TRANSMIT);
@@ -119,6 +137,9 @@ ccl_device_inline void path_state_next(KernelGlobals *kg, ccl_addr_space PathSta
 			}
 
 			state->transmission_bounce++;
+			if(state->transmission_bounce >= kernel_data.integrator.max_transmission_bounce) {
+				state->flag |= PATH_RAY_TERMINATE_AFTER_TRANSPARENT;
+			}
 		}
 
 		/* diffuse/glossy/singular */
@@ -162,13 +183,13 @@ ccl_device_inline float path_state_continuation_probability(KernelGlobals *kg,
                                                             ccl_addr_space PathState *state,
                                                             const float3 throughput)
 {
-	if(state->flag & PATH_RAY_TRANSPARENT) {
-		/* Transparent rays are treated separately with own max bounces. */
-		if(state->transparent_bounce >= kernel_data.integrator.transparent_max_bounce) {
-			return 0.0f;
-		}
+	if(state->flag & PATH_RAY_TERMINATE_IMMEDIATE) {
+		/* Ray is to be terminated immediately. */
+		return 0.0f;
+	}
+	else if(state->flag & PATH_RAY_TRANSPARENT) {
 		/* Do at least one bounce without RR. */
-		else if(state->transparent_bounce <= 1) {
+		if(state->transparent_bounce <= 1) {
 			return 1.0f;
 		}
 #ifdef __SHADOW_TRICKS__
@@ -179,19 +200,8 @@ ccl_device_inline float path_state_continuation_probability(KernelGlobals *kg,
 #endif
 	}
 	else {
-		/* Test max bounces for various ray types. */
-		if((state->bounce >= kernel_data.integrator.max_bounce) ||
-		   (state->diffuse_bounce >= kernel_data.integrator.max_diffuse_bounce) ||
-		   (state->glossy_bounce >= kernel_data.integrator.max_glossy_bounce) ||
-#ifdef __VOLUME__
-		   (state->volume_bounce >= kernel_data.integrator.max_volume_bounce) ||
-#endif
-		   (state->transmission_bounce >= kernel_data.integrator.max_transmission_bounce))
-		{
-			return 0.0f;
-		}
 		/* Do at least one bounce without RR. */
-		else if(state->bounce <= 1) {
+		if(state->bounce <= 1) {
 			return 1.0f;
 		}
 #ifdef __SHADOW_TRICKS__