Cycles: Fallback to diffuse BSDF for the indirect SSS rays when BSSRDF is hit

This is actually how it was intended to work, just didn't notice it wasn't
really happening in the main ray loop.

Solves some memory issues reported in T46880.

1 files changed, 14 insertions, 6 deletions

diff --git a/intern/cycles/kernel/kernel_path.h b/intern/cycles/kernel/kernel_path.h
index 251d7809941..850bfb22b3c 100644
--- a/intern/cycles/kernel/kernel_path.h
+++ b/intern/cycles/kernel/kernel_path.h
@@ -448,14 +448,22 @@ ccl_device bool kernel_path_subsurface_scatter(
 
 	/* do bssrdf scatter step if we picked a bssrdf closure */
 	if(sc) {
-		/* We should never have two consecutive BSSRDF bounces,
-		 * the second one should be converted to a diffuse BSDF to
-		 * avoid this.
-		 */
-		kernel_assert(ss_indirect->num_rays == 0);
-
 		uint lcg_state = lcg_state_init(rng, state, 0x68bc21eb);
 
+		/* If indirect ray hits BSSRDF we replace it with diffuse BSDF. */
+		if(ss_indirect->num_rays) {
+			float bssrdf_u, bssrdf_v;
+			path_state_rng_2D(kg, rng, state, PRNG_BSDF_U, &bssrdf_u, &bssrdf_v);
+			subsurface_scatter_step(kg,
+			                        sd,
+			                        state->flag,
+			                        sc,
+			                        &lcg_state,
+			                        bssrdf_u, bssrdf_v,
+			                        false);
+			return false;
+		}
+
 		SubsurfaceIntersection ss_isect;
 		float bssrdf_u, bssrdf_v;
 		path_state_rng_2D(kg, rng, state, PRNG_BSDF_U, &bssrdf_u, &bssrdf_v);