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, 33 insertions, 5 deletions

diff --git a/intern/cycles/kernel/kernel_accumulate.h b/intern/cycles/kernel/kernel_accumulate.h
index 623c1dcaaa1..6c3ee6b8098 100644
--- a/intern/cycles/kernel/kernel_accumulate.h
+++ b/intern/cycles/kernel/kernel_accumulate.h
@@ -96,7 +96,7 @@ ccl_device_inline bool bsdf_eval_is_zero(BsdfEval *eval)
 	}
 }
 
-ccl_device_inline void bsdf_eval_mul(BsdfEval *eval, float3 value)
+ccl_device_inline void bsdf_eval_mul(BsdfEval *eval, float value)
 {
 #ifdef __PASSES__
 	if(eval->use_light_pass) {
@@ -115,6 +115,36 @@ ccl_device_inline void bsdf_eval_mul(BsdfEval *eval, float3 value)
 	}
 }
 
+ccl_device_inline void bsdf_eval_mul3(BsdfEval *eval, float3 value)
+{
+#ifdef __PASSES__
+	if(eval->use_light_pass) {
+		eval->diffuse *= value;
+		eval->glossy *= value;
+		eval->transmission *= value;
+		eval->subsurface *= value;
+		eval->scatter *= value;
+
+		/* skipping transparent, this function is used by for eval(), will be zero then */
+	}
+	else
+		eval->diffuse *= value;
+#else
+	eval->diffuse *= value;
+#endif
+}
+
+ccl_device_inline float3 bsdf_eval_sum(BsdfEval *eval)
+{
+#ifdef __PASSES__
+	if(eval->use_light_pass) {
+		return eval->diffuse + eval->glossy + eval->transmission + eval->subsurface + eval->scatter;
+	}
+	else
+#endif
+	return eval->diffuse;
+}
+
 /* Path Radiance
  *
  * We accumulate different render passes separately. After summing at the end
@@ -193,8 +223,7 @@ ccl_device_inline void path_radiance_bsdf_bounce(PathRadiance *L, ccl_addr_space
 		}
 		else {
 			/* transparent bounce before first hit, or indirectly visible through BSDF */
-			float3 sum = (bsdf_eval->diffuse + bsdf_eval->glossy + bsdf_eval->transmission + bsdf_eval->transparent +
-						  bsdf_eval->subsurface + bsdf_eval->scatter) * inverse_pdf;
+			float3 sum = (bsdf_eval_sum(bsdf_eval) + bsdf_eval->transparent) * inverse_pdf;
 			*throughput *= sum;
 		}
 	}
@@ -264,8 +293,7 @@ ccl_device_inline void path_radiance_accum_light(PathRadiance *L, float3 through
 		}
 		else {
 			/* indirectly visible lighting after BSDF bounce */
-			float3 sum = bsdf_eval->diffuse + bsdf_eval->glossy + bsdf_eval->transmission + bsdf_eval->subsurface + bsdf_eval->scatter;
-			L->indirect += throughput*sum*shadow;
+			L->indirect += throughput*bsdf_eval_sum(bsdf_eval)*shadow;
 		}
 	}
 	else