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, 1 insertions, 2 deletions

diff --git a/intern/cycles/kernel/kernel_types.h b/intern/cycles/kernel/kernel_types.h
index 3f62b726b6a..0c6b4b401f0 100644
--- a/intern/cycles/kernel/kernel_types.h
+++ b/intern/cycles/kernel/kernel_types.h
@@ -1372,8 +1372,7 @@ typedef struct KernelCurves {
   int curveflags;
   int subdivisions;
 
-  float minimum_width;
-  float maximum_width;
+  int pad1, pad2;
 } KernelCurves;
 static_assert_align(KernelCurves, 16);