Fix T52021: Shadow catcher renders wrong when catcher object is behind transparent object

Tweaked the path radiance summing and alpha to accommodate for possible contribution of
light by transparent surface bounces happening prior to shadow catcher intersection.

This commit will change the way how shadow catcher results looks when was behind semi
transparent object, but the old result seemed to be fully wrong: there were big artifacts
when alpha-overing the result on some actual footage.

1 files changed, 7 insertions, 4 deletions

diff --git a/intern/cycles/kernel/kernel_accumulate.h b/intern/cycles/kernel/kernel_accumulate.h
index 175bd6b9737..9ed16aceb55 100644
--- a/intern/cycles/kernel/kernel_accumulate.h
+++ b/intern/cycles/kernel/kernel_accumulate.h
@@ -220,7 +220,9 @@ ccl_device_inline void path_radiance_init(PathRadiance *L, int use_light_pass)
 #ifdef __SHADOW_TRICKS__
 	L->path_total = make_float3(0.0f, 0.0f, 0.0f);
 	L->path_total_shaded = make_float3(0.0f, 0.0f, 0.0f);
-	L->shadow_color = make_float3(0.0f, 0.0f, 0.0f);
+	L->shadow_background_color = make_float3(0.0f, 0.0f, 0.0f);
+	L->shadow_radiance_sum = make_float3(0.0f, 0.0f, 0.0f);
+	L->shadow_throughput = 0.0f;
 #endif
 
 #ifdef __DENOISING_FEATURES__
@@ -680,11 +682,12 @@ ccl_device_inline float3 path_radiance_sum_shadowcatcher(KernelGlobals *kg,
 	const float shadow = path_radiance_sum_shadow(L);
 	float3 L_sum;
 	if(kernel_data.background.transparent) {
-		*alpha = 1.0f-shadow;
-		L_sum = make_float3(0.0f, 0.0f, 0.0f);
+		*alpha = 1.0f - L->shadow_throughput * shadow;
+		L_sum = L->shadow_radiance_sum;
 	}
 	else {
-		L_sum = L->shadow_color * shadow;
+		L_sum = L->shadow_background_color * L->shadow_throughput * shadow +
+		        L->shadow_radiance_sum;
 	}
 	return L_sum;
 }