Cycles: remove hair minimum width support.

This never really worked as it was supposed to. The main goal of this is to
turn noise from sampling tiny hairs into multiple layers of transparency that
do not need to be sampled stochastically. However the implementation of this
worked by randomly discarding hair intersections in BVH traversal, which
defeats the purpose.

If it ever comes back, it's best implemented outside the kernel as a preprocess
that changes hair radius before BVH building. This would also make it work with
Embree, where it's not supported now. But it's not so clear anymore that with
many AA samples and GPU rendering this feature is as helpful as it once was for
CPU raytracers with few AA samples.

The benefit of removing this feature is improved hair ray tracing performance,
tested on NVIDIA Titan Xp:

bmw27: +0.37%
classroom: +0.26%
fishy_cat: -7.36%
koro: -12.98%
pabellon: -0.12%

Differential Revision: https://developer.blender.org/D4532

1 files changed, 4 insertions, 8 deletions

diff --git a/intern/cycles/kernel/kernel_shadow.h b/intern/cycles/kernel/kernel_shadow.h
index 6af1369feab..07201819030 100644
--- a/intern/cycles/kernel/kernel_shadow.h
+++ b/intern/cycles/kernel/kernel_shadow.h
@@ -103,8 +103,7 @@ ccl_device bool shadow_blocked_opaque(KernelGlobals *kg,
                                       Intersection *isect,
                                       float3 *shadow)
 {
-  const bool blocked = scene_intersect(
-      kg, *ray, visibility & PATH_RAY_SHADOW_OPAQUE, isect, NULL, 0.0f, 0.0f);
+  const bool blocked = scene_intersect(kg, *ray, visibility & PATH_RAY_SHADOW_OPAQUE, isect);
 #ifdef __VOLUME__
   if (!blocked && state->volume_stack[0].shader != SHADER_NONE) {
     /* Apply attenuation from current volume shader. */
@@ -319,8 +318,7 @@ ccl_device bool shadow_blocked_transparent_stepped_loop(KernelGlobals *kg,
       if (bounce >= kernel_data.integrator.transparent_max_bounce) {
         return true;
       }
-      if (!scene_intersect(
-              kg, *ray, visibility & PATH_RAY_SHADOW_TRANSPARENT, isect, NULL, 0.0f, 0.0f)) {
+      if (!scene_intersect(kg, *ray, visibility & PATH_RAY_SHADOW_TRANSPARENT, isect)) {
         break;
       }
       if (!shader_transparent_shadow(kg, isect)) {
@@ -376,8 +374,7 @@ ccl_device bool shadow_blocked_transparent_stepped(KernelGlobals *kg,
                                                    Intersection *isect,
                                                    float3 *shadow)
 {
-  bool blocked = scene_intersect(
-      kg, *ray, visibility & PATH_RAY_SHADOW_OPAQUE, isect, NULL, 0.0f, 0.0f);
+  bool blocked = scene_intersect(kg, *ray, visibility & PATH_RAY_SHADOW_OPAQUE, isect);
   bool is_transparent_isect = blocked ? shader_transparent_shadow(kg, isect) : false;
   return shadow_blocked_transparent_stepped_loop(
       kg, sd, shadow_sd, state, visibility, ray, isect, blocked, is_transparent_isect, shadow);
@@ -436,8 +433,7 @@ ccl_device_inline bool shadow_blocked(KernelGlobals *kg,
    * TODO(sergey): Check why using record-all behavior causes slowdown in such
    * cases. Could that be caused by a higher spill pressure?
    */
-  const bool blocked = scene_intersect(
-      kg, *ray, visibility & PATH_RAY_SHADOW_OPAQUE, &isect, NULL, 0.0f, 0.0f);
+  const bool blocked = scene_intersect(kg, *ray, visibility & PATH_RAY_SHADOW_OPAQUE, &isect);
   const bool is_transparent_isect = blocked ? shader_transparent_shadow(kg, &isect) : false;
   if (!blocked || !is_transparent_isect || max_hits + 1 >= SHADOW_STACK_MAX_HITS) {
     return shadow_blocked_transparent_stepped_loop(