Cycles: Add optional probabilistic termination of light samples based on their expected contribution

In scenes with many lights, some of them might have a very small contribution to some pixels, but the shadow rays are traced anyways.
To avoid that, this patch adds probabilistic termination to light samples - if the contribution before checking for shadowing is below a user-defined threshold, the sample will be discarded with probability (1 - (contribution / threshold)) and otherwise kept, but weighted more to remain unbiased.
This is the same approach that's also used in path termination based on length.

Note that the rendering remains unbiased with this option, it just adds a bit of noise - but if the setting is used moderately, the speedup gained easily outweighs the additional noise.

Reviewers: #cycles

Subscribers: sergey, brecht

Differential Revision: https://developer.blender.org/D2217

1 files changed, 8 insertions, 4 deletions

diff --git a/intern/cycles/kernel/kernel_path_surface.h b/intern/cycles/kernel/kernel_path_surface.h
index 45f0f1cbfaa..fea503d06e5 100644
--- a/intern/cycles/kernel/kernel_path_surface.h
+++ b/intern/cycles/kernel/kernel_path_surface.h
@@ -49,6 +49,7 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(KernelGlobal
 			for(int j = 0; j < num_samples; j++) {
 				float light_u, light_v;
 				path_branched_rng_2D(kg, &lamp_rng, state, j, num_samples, PRNG_LIGHT_U, &light_u, &light_v);
+				float terminate = path_branched_rng_light_termination(kg, &lamp_rng, state, j, num_samples);
 
 				LightSample ls;
 				if(lamp_light_sample(kg, i, light_u, light_v, ccl_fetch(sd, P), &ls)) {
@@ -57,7 +58,7 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(KernelGlobal
 					if(kernel_data.integrator.pdf_triangles != 0.0f)
 						ls.pdf *= 2.0f;
 
-					if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp)) {
+					if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {
 						/* trace shadow ray */
 						float3 shadow;
 
@@ -79,6 +80,7 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(KernelGlobal
 				float light_t = path_branched_rng_1D(kg, rng, state, j, num_samples, PRNG_LIGHT);
 				float light_u, light_v;
 				path_branched_rng_2D(kg, rng, state, j, num_samples, PRNG_LIGHT_U, &light_u, &light_v);
+				float terminate = path_branched_rng_light_termination(kg, rng, state, j, num_samples);
 
 				/* only sample triangle lights */
 				if(kernel_data.integrator.num_all_lights)
@@ -90,7 +92,7 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(KernelGlobal
 					if(kernel_data.integrator.num_all_lights)
 						ls.pdf *= 2.0f;
 
-					if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp)) {
+					if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {
 						/* trace shadow ray */
 						float3 shadow;
 
@@ -108,11 +110,12 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(KernelGlobal
 		float light_t = path_state_rng_1D(kg, rng, state, PRNG_LIGHT);
 		float light_u, light_v;
 		path_state_rng_2D(kg, rng, state, PRNG_LIGHT_U, &light_u, &light_v);
+		float terminate = path_state_rng_light_termination(kg, rng, state);
 
 		LightSample ls;
 		if(light_sample(kg, light_t, light_u, light_v, ccl_fetch(sd, time), ccl_fetch(sd, P), state->bounce, &ls)) {
 			/* sample random light */
-			if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp)) {
+			if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {
 				/* trace shadow ray */
 				float3 shadow;
 
@@ -210,7 +213,8 @@ ccl_device_inline void kernel_path_surface_connect_light(KernelGlobals *kg, ccl_
 
 	LightSample ls;
 	if(light_sample(kg, light_t, light_u, light_v, ccl_fetch(sd, time), ccl_fetch(sd, P), state->bounce, &ls)) {
-		if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp)) {
+		float terminate = path_state_rng_light_termination(kg, rng, state);
+		if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {
 			/* trace shadow ray */
 			float3 shadow;