Cycles: shadow function optimization for transparent shadows (CPU only).

Old algorithm:

Raytrace from one transparent surface to the next step by step. To minimize
overhead in cases where we don't need transparent shadows, we first trace a
regular shadow ray. We check if the hit primitive was potentially transparent,
and only in that case start marching. this gives extra ray cast for the cases
were we do want transparency.

New algorithm:

We trace a single ray. If it hits any opaque surface, or more than a given
number of transparent surfaces is hit, then we consider the geometry to be
entirely blocked. If not, all transparent surfaces will be recorded and we
will shade them one by one to determine how much light is blocked. This all
happens in one scene intersection function.

Recording all hits works well in some cases but may be slower in others. If
we have many semi-transparent hairs, one intersection may be faster because
you'd be reinteresecting the same hairs a lot with each step otherwise. If
however there is mostly binary transparency then we may be recording many
unnecessary intersections when one of the first surfaces blocks all light.

We found that this helps quite nicely in some scenes, on koro.blend this can
give a 50% reduction in render time, on the pabellon barcelona scene and a
forest scene with transparent leaves it was 30%. Some other files rendered
maybe 1% or 2% slower, but this seems a reasonable tradeoff.

Differential Revision: https://developer.blender.org/D473

1 files changed, 29 insertions, 4 deletions

diff --git a/intern/cycles/kernel/geom/geom_object.h b/intern/cycles/kernel/geom/geom_object.h
index 71ad4a55088..431cd6321d5 100644
--- a/intern/cycles/kernel/geom/geom_object.h
+++ b/intern/cycles/kernel/geom/geom_object.h
@@ -377,7 +377,7 @@ ccl_device_inline float3 bvh_inverse_direction(float3 dir)
 
 /* Transform ray into object space to enter static object in BVH */
 
-ccl_device_inline void bvh_instance_push(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t, const float tmax)
+ccl_device_inline void bvh_instance_push(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t)
 {
 	Transform tfm = object_fetch_transform(kg, object, OBJECT_INVERSE_TRANSFORM);
 
@@ -393,7 +393,7 @@ ccl_device_inline void bvh_instance_push(KernelGlobals *kg, int object, const Ra
 
 /* Transorm ray to exit static object in BVH */
 
-ccl_device_inline void bvh_instance_pop(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t, const float tmax)
+ccl_device_inline void bvh_instance_pop(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t)
 {
 	if(*t != FLT_MAX) {
 		Transform tfm = object_fetch_transform(kg, object, OBJECT_TRANSFORM);
@@ -405,10 +405,23 @@ ccl_device_inline void bvh_instance_pop(KernelGlobals *kg, int object, const Ray
 	*idir = bvh_inverse_direction(*dir);
 }
 
+/* Same as above, but returns scale factor to apply to multiple intersection distances */
+
+ccl_device_inline void bvh_instance_pop_factor(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t_fac)
+{
+	Transform tfm = object_fetch_transform(kg, object, OBJECT_TRANSFORM);
+	*t_fac = len(transform_direction(&tfm, 1.0f/(*idir)));
+
+	*P = ray->P;
+	*dir = bvh_clamp_direction(ray->D);
+	*idir = bvh_inverse_direction(*dir);
+}
+
+
 #ifdef __OBJECT_MOTION__
 /* Transform ray into object space to enter motion blurred object in BVH */
 
-ccl_device_inline void bvh_instance_motion_push(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t, Transform *tfm, const float tmax)
+ccl_device_inline void bvh_instance_motion_push(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t, Transform *tfm)
 {
 	Transform itfm;
 	*tfm = object_fetch_transform_motion_test(kg, object, ray->time, &itfm);
@@ -425,7 +438,7 @@ ccl_device_inline void bvh_instance_motion_push(KernelGlobals *kg, int object, c
 
 /* Transorm ray to exit motion blurred object in BVH */
 
-ccl_device_inline void bvh_instance_motion_pop(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t, Transform *tfm, const float tmax)
+ccl_device_inline void bvh_instance_motion_pop(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t, Transform *tfm)
 {
 	if(*t != FLT_MAX)
 		*t *= len(transform_direction(tfm, 1.0f/(*idir)));
@@ -434,6 +447,18 @@ ccl_device_inline void bvh_instance_motion_pop(KernelGlobals *kg, int object, co
 	*dir = bvh_clamp_direction(ray->D);
 	*idir = bvh_inverse_direction(*dir);
 }
+
+/* Same as above, but returns scale factor to apply to multiple intersection distances */
+
+ccl_device_inline void bvh_instance_motion_pop_factor(KernelGlobals *kg, int object, const Ray *ray, float3 *P, float3 *dir, float3 *idir, float *t_fac, Transform *tfm)
+{
+	*t_fac = len(transform_direction(tfm, 1.0f/(*idir)));
+
+	*P = ray->P;
+	*dir = bvh_clamp_direction(ray->D);
+	*idir = bvh_inverse_direction(*dir);
+}
+
 #endif
 
 CCL_NAMESPACE_END