Cycles: Fix wrong hair render results when using BVH motion steps

The issue here was mainly coming from minimal pixel width feature
which is quite commonly enabled in production shots.

This feature will use some probabilistic heuristic in the curve
intersection function to check whether we need to return intersection
or not. This probability is calculated for every intersection check.
Now, when we use multiple BVH nodes for curve primitives we increase
probability of that primitive to be considered a good intersection
for us. This is similar to increasing minimal width of curve.

What is worst here is that change in the intersection probability
fully depends on exact layout of BVH, meaning probability might
change differently depending on a view angle, the way how builder
binned the primitives and such. This makes it impossible to do
simple check like dividing probability by number of BVH steps.

Other solution might have been to split BVH into fully independent
trees, but that will increase memory usage of all the static
objects in the scenes, which is also not something desirable.

For now used most simple but robust approach: store BVH primitives
time and test it in curve intersection functions. This solves the
regression, but has two downsides:

- Uses more memory.

  which isn't surprising, and ANY solution to this problem will
  use more memory.

  What we still have to do is to avoid this memory increase for
  cases when we don't use BVH motion steps.

- Reduces number of maximum available textures on pre-kepler cards.

  There is not much we can do here, hardware gets old but we need
  to move forward on more modern hardware..

1 files changed, 23 insertions, 5 deletions

diff --git a/intern/cycles/kernel/geom/geom_curve.h b/intern/cycles/kernel/geom/geom_curve.h
index 9de335403ce..c8749545cdd 100644
--- a/intern/cycles/kernel/geom/geom_curve.h
+++ b/intern/cycles/kernel/geom/geom_curve.h
@@ -229,6 +229,15 @@ ccl_device_forceinline bool bvh_cardinal_curve_intersect(KernelGlobals *kg, Inte
 	float3 P, float3 dir, uint visibility, int object, int curveAddr, float time,int type, uint *lcg_state, float difl, float extmax)
 #endif
 {
+	const bool is_curve_primitive = (type & PRIMITIVE_CURVE);
+
+	if(!is_curve_primitive) {
+		const float2 prim_time = kernel_tex_fetch(__prim_time, curveAddr);
+		if(time < prim_time.x || time > prim_time.y) {
+			return false;
+		}
+	}
+
 	int segment = PRIMITIVE_UNPACK_SEGMENT(type);
 	float epsilon = 0.0f;
 	float r_st, r_en;
@@ -257,7 +266,7 @@ ccl_device_forceinline bool bvh_cardinal_curve_intersect(KernelGlobals *kg, Inte
 
 #ifdef __KERNEL_AVX2__
 		avxf P_curve_0_1, P_curve_2_3;
-		if(type & PRIMITIVE_CURVE) {
+		if(is_curve_primitive) {
 			P_curve_0_1 = _mm256_loadu2_m128(&kg->__curve_keys.data[k0].x, &kg->__curve_keys.data[ka].x);
 			P_curve_2_3 = _mm256_loadu2_m128(&kg->__curve_keys.data[kb].x, &kg->__curve_keys.data[k1].x);
 		}
@@ -268,7 +277,7 @@ ccl_device_forceinline bool bvh_cardinal_curve_intersect(KernelGlobals *kg, Inte
 #else  /* __KERNEL_AVX2__ */
 		ssef P_curve[4];
 
-		if(type & PRIMITIVE_CURVE) {
+		if(is_curve_primitive) {
 			P_curve[0] = load4f(&kg->__curve_keys.data[ka].x);
 			P_curve[1] = load4f(&kg->__curve_keys.data[k0].x);
 			P_curve[2] = load4f(&kg->__curve_keys.data[k1].x);
@@ -363,7 +372,7 @@ ccl_device_forceinline bool bvh_cardinal_curve_intersect(KernelGlobals *kg, Inte
 
 		float4 P_curve[4];
 
-		if(type & PRIMITIVE_CURVE) {
+		if(is_curve_primitive) {
 			P_curve[0] = kernel_tex_fetch(__curve_keys, ka);
 			P_curve[1] = kernel_tex_fetch(__curve_keys, k0);
 			P_curve[2] = kernel_tex_fetch(__curve_keys, k1);
@@ -689,6 +698,15 @@ ccl_device_forceinline bool bvh_curve_intersect(KernelGlobals *kg, Intersection
 #  define dot3(x, y) dot(x, y)
 #endif
 
+	const bool is_curve_primitive = (type & PRIMITIVE_CURVE);
+
+	if(!is_curve_primitive) {
+		const float2 prim_time = kernel_tex_fetch(__prim_time, curveAddr);
+		if(time < prim_time.x || time > prim_time.y) {
+			return false;
+		}
+	}
+
 	int segment = PRIMITIVE_UNPACK_SEGMENT(type);
 	/* curve Intersection check */
 	int flags = kernel_data.curve.curveflags;
@@ -703,7 +721,7 @@ ccl_device_forceinline bool bvh_curve_intersect(KernelGlobals *kg, Intersection
 #ifndef __KERNEL_SSE2__
 	float4 P_curve[2];
 
-	if(type & PRIMITIVE_CURVE) {
+	if(is_curve_primitive) {
 		P_curve[0] = kernel_tex_fetch(__curve_keys, k0);
 		P_curve[1] = kernel_tex_fetch(__curve_keys, k1);
 	}
@@ -738,7 +756,7 @@ ccl_device_forceinline bool bvh_curve_intersect(KernelGlobals *kg, Intersection
 #else
 	ssef P_curve[2];
 	
-	if(type & PRIMITIVE_CURVE) {
+	if(is_curve_primitive) {
 		P_curve[0] = load4f(&kg->__curve_keys.data[k0].x);
 		P_curve[1] = load4f(&kg->__curve_keys.data[k1].x);
 	}