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, 23 insertions, 6 deletions

diff --git a/intern/cycles/kernel/closure/bsdf_transparent.h b/intern/cycles/kernel/closure/bsdf_transparent.h
index 22ca7f3847e..79ee9dc4537 100644
--- a/intern/cycles/kernel/closure/bsdf_transparent.h
+++ b/intern/cycles/kernel/closure/bsdf_transparent.h
@@ -35,21 +35,38 @@
 
 CCL_NAMESPACE_BEGIN
 
-ccl_device void bsdf_transparent_setup(ShaderData *sd, const float3 weight)
+ccl_device void bsdf_transparent_setup(ShaderData *sd, const float3 weight, int path_flag)
 {
 	if(sd->flag & SD_TRANSPARENT) {
 		sd->closure_transparent_extinction += weight;
+
+		for(int i = 0; i < sd->num_closure; i++) {
+			ShaderClosure *sc = &sd->closure[i];
+
+			if(sc->type == CLOSURE_BSDF_TRANSPARENT_ID) {
+				sc->weight += weight;
+				sc->sample_weight += fabsf(average(weight));
+				break;
+			}
+		}
 	}
 	else {
 		sd->flag |= SD_BSDF|SD_TRANSPARENT;
 		sd->closure_transparent_extinction = weight;
-	}
 
-	ShaderClosure *bsdf = bsdf_alloc(sd, sizeof(ShaderClosure), weight);
+		if(path_flag & PATH_RAY_TERMINATE) {
+			/* In this case the number of closures is set to zero to disable
+			 * all others, but we still want to get transparency so increase
+			 * the number just for this. */
+			sd->num_closure_left = 1;
+		}
+
+		ShaderClosure *bsdf = bsdf_alloc(sd, sizeof(ShaderClosure), weight);
 
-	if(bsdf) {
-		bsdf->N = sd->N;
-		bsdf->type = CLOSURE_BSDF_TRANSPARENT_ID;
+		if(bsdf) {
+			bsdf->N = sd->N;
+			bsdf->type = CLOSURE_BSDF_TRANSPARENT_ID;
+		}
 	}
 }