Cycles: improve sample stratification on area lights for path tracing.

Previously we used a 1D sequence to select a light, and another 2D sequence
to sample a point on the light. For multiple lights this meant each light
would get a random subset of a 2D stratified sequence, which is not
guaranteed to be stratified anymore.

Now we use only a 2D sequence, split into segments along the X axis, one for
each light. The samples that fall within a segment then each are a stratified
sequence, at least in the limit. So for example for two lights, we split up
the unit square into two segments [0,0.5[ x [0,1[ and [0.5,1[ x [0,1[.

This doesn't make much difference in most scenes, mainly helps if you have a
few large area lights or some types of HDR backgrounds.

1 files changed, 4 insertions, 7 deletions

diff --git a/intern/cycles/kernel/kernel_path_surface.h b/intern/cycles/kernel/kernel_path_surface.h
index 6c3a444e48a..e798fcc6a2c 100644
--- a/intern/cycles/kernel/kernel_path_surface.h
+++ b/intern/cycles/kernel/kernel_path_surface.h
@@ -85,17 +85,16 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(
 			float num_samples_inv = num_samples_adjust/num_samples;
 
 			for(int j = 0; j < num_samples; j++) {
-				float light_t = path_branched_rng_1D(kg, state->rng_hash, state, j, num_samples, PRNG_LIGHT);
 				float light_u, light_v;
 				path_branched_rng_2D(kg, state->rng_hash, state, j, num_samples, PRNG_LIGHT_U, &light_u, &light_v);
 				float terminate = path_branched_rng_light_termination(kg, state->rng_hash, state, j, num_samples);
 
 				/* only sample triangle lights */
 				if(kernel_data.integrator.num_all_lights)
-					light_t = 0.5f*light_t;
+					light_u = 0.5f*light_u;
 
 				LightSample ls;
-				if(light_sample(kg, light_t, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
+				if(light_sample(kg, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
 					/* Same as above, probability needs to be corrected since the sampling was forced to select a mesh light. */
 					if(kernel_data.integrator.num_all_lights)
 						ls.pdf *= 2.0f;
@@ -118,13 +117,12 @@ ccl_device_noinline void kernel_branched_path_surface_connect_light(
 	}
 	else {
 		/* sample one light at random */
-		float light_t = path_state_rng_1D(kg, state, PRNG_LIGHT);
 		float light_u, light_v;
 		path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v);
 		float terminate = path_state_rng_light_termination(kg, state);
 
 		LightSample ls;
-		if(light_sample(kg, light_t, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
+		if(light_sample(kg, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
 			/* sample random light */
 			if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {
 				/* trace shadow ray */
@@ -238,7 +236,6 @@ ccl_device_inline void kernel_path_surface_connect_light(KernelGlobals *kg,
 #endif
 
 	/* sample illumination from lights to find path contribution */
-	float light_t = path_state_rng_1D(kg, state, PRNG_LIGHT);
 	float light_u, light_v;
 	path_state_rng_2D(kg, state, PRNG_LIGHT_U, &light_u, &light_v);
 
@@ -251,7 +248,7 @@ ccl_device_inline void kernel_path_surface_connect_light(KernelGlobals *kg,
 #endif
 
 	LightSample ls;
-	if(light_sample(kg, light_t, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
+	if(light_sample(kg, light_u, light_v, sd->time, sd->P, state->bounce, &ls)) {
 		float terminate = path_state_rng_light_termination(kg, state);
 		if(direct_emission(kg, sd, emission_sd, &ls, state, &light_ray, &L_light, &is_lamp, terminate)) {
 			/* trace shadow ray */