Cycles: always perform backface culling for curve, remove option

The hair BSDFs are already designed to assume this, and disabling backface
culling would break them in some cases.

Ref T73778

Depends on D8009

Maniphest Tasks: T73778

Differential Revision: https://developer.blender.org/D8010

1 files changed, 20 insertions, 30 deletions

diff --git a/intern/cycles/kernel/svm/svm_closure.h b/intern/cycles/kernel/svm/svm_closure.h
index 2c57a142692..1ae94f1d766 100644
--- a/intern/cycles/kernel/svm/svm_closure.h
+++ b/intern/cycles/kernel/svm/svm_closure.h
@@ -847,39 +847,29 @@ ccl_device void svm_node_closure_bsdf(KernelGlobals *kg,
     case CLOSURE_BSDF_HAIR_TRANSMISSION_ID: {
       float3 weight = sd->svm_closure_weight * mix_weight;
 
-      if (sd->flag & SD_BACKFACING && sd->type & PRIMITIVE_ALL_CURVE) {
-        /* todo: giving a fixed weight here will cause issues when
-         * mixing multiple BSDFS. energy will not be conserved and
-         * the throughput can blow up after multiple bounces. we
-         * better figure out a way to skip backfaces from rays
-         * spawned by transmission from the front */
-        bsdf_transparent_setup(sd, make_float3(1.0f, 1.0f, 1.0f), path_flag);
-      }
-      else {
-        HairBsdf *bsdf = (HairBsdf *)bsdf_alloc(sd, sizeof(HairBsdf), weight);
+      HairBsdf *bsdf = (HairBsdf *)bsdf_alloc(sd, sizeof(HairBsdf), weight);
 
-        if (bsdf) {
-          bsdf->N = N;
-          bsdf->roughness1 = param1;
-          bsdf->roughness2 = param2;
-          bsdf->offset = -stack_load_float(stack, data_node.z);
+      if (bsdf) {
+        bsdf->N = N;
+        bsdf->roughness1 = param1;
+        bsdf->roughness2 = param2;
+        bsdf->offset = -stack_load_float(stack, data_node.z);
 
-          if (stack_valid(data_node.y)) {
-            bsdf->T = normalize(stack_load_float3(stack, data_node.y));
-          }
-          else if (!(sd->type & PRIMITIVE_ALL_CURVE)) {
-            bsdf->T = normalize(sd->dPdv);
-            bsdf->offset = 0.0f;
-          }
-          else
-            bsdf->T = normalize(sd->dPdu);
+        if (stack_valid(data_node.y)) {
+          bsdf->T = normalize(stack_load_float3(stack, data_node.y));
+        }
+        else if (!(sd->type & PRIMITIVE_ALL_CURVE)) {
+          bsdf->T = normalize(sd->dPdv);
+          bsdf->offset = 0.0f;
+        }
+        else
+          bsdf->T = normalize(sd->dPdu);
 
-          if (type == CLOSURE_BSDF_HAIR_REFLECTION_ID) {
-            sd->flag |= bsdf_hair_reflection_setup(bsdf);
-          }
-          else {
-            sd->flag |= bsdf_hair_transmission_setup(bsdf);
-          }
+        if (type == CLOSURE_BSDF_HAIR_REFLECTION_ID) {
+          sd->flag |= bsdf_hair_reflection_setup(bsdf);
+        }
+        else {
+          sd->flag |= bsdf_hair_transmission_setup(bsdf);
         }
       }