Fix occasional wrong normal for curves with minimum width.

This caused a couple of fireflies in koro_final.blend. The wrong normal would
cause the shading point to be set as backfacing, which triggered another bug
with hair BSDFs on the backface of hair curves. That one is not fixed yet but
there's a comment in the code about it now.

2 files changed, 40 insertions, 27 deletions

diff --git a/intern/cycles/kernel/geom/geom_curve.h b/intern/cycles/kernel/geom/geom_curve.h
index e57bcd894a6..57fa0388842 100644
--- a/intern/cycles/kernel/geom/geom_curve.h
+++ b/intern/cycles/kernel/geom/geom_curve.h
@@ -438,6 +438,8 @@ ccl_device_inline bool bvh_cardinal_curve_intersect(KernelGlobals *kg, Intersect
 			* dP* is reversed if necessary.*/
 			float t = isect->t;
 			float u = 0.0f;
+			float gd = 0.0f;
+
 			if(flags & CURVE_KN_RIBBONS) {
 				float3 tg = (p_en - p_st);
 				float w = tg.x * tg.x + tg.y * tg.y;
@@ -508,7 +510,7 @@ ccl_device_inline bool bvh_cardinal_curve_intersect(KernelGlobals *kg, Intersect
 				}
 				/* --- */
 				float3 tg = (p_en - p_st) / l;
-				float gd = (or2 - or1) / l;
+				gd = (or2 - or1) / l;
 				float difz = -dot(p_st,tg);
 				float cyla = 1.0f - (tg.z * tg.z * (1 + gd*gd));
 				float halfb = (-p_st.z - tg.z*(difz + gd*(difz*gd + or1)));
@@ -576,8 +578,8 @@ ccl_device_inline bool bvh_cardinal_curve_intersect(KernelGlobals *kg, Intersect
 				isect->object = object;
 				isect->type = type;
 				isect->u = u;
-				isect->v = 0.0f;
-				/*isect->v = 1.0f - coverage; */
+				isect->v = gd;
+				/*isect->transparency = 1.0f - coverage; */
 				isect->t = t;
 				hit = true;
 			}
@@ -812,13 +814,10 @@ ccl_device_inline bool bvh_curve_intersect(KernelGlobals *kg, Intersection *isec
 				isect->object = object;
 				isect->type = type;
 				isect->u = z*invl;
-				isect->v = td/(4*a*a);
-				/*isect->v = 1.0f - adjradius;*/
+				isect->v = gd;
+				/*isect->transparency = 1.0f - adjradius;*/
 				isect->t = t;
 
-				if(backface) 
-					isect->u = -isect->u;
-				
 				return true;
 			}
 		}
@@ -901,35 +900,41 @@ ccl_device_inline float3 bvh_curve_refine(KernelGlobals *kg, ShaderData *sd, con
 			motion_cardinal_curve_keys(kg, sd->object, sd->prim, sd->time, ka, k0, k1, kb, P_curve);
 		}
 
-		float l = 1.0f;
-		tg = normalize_len(float4_to_float3(P_curve[2] - P_curve[1]), &l);
-		float r1 = P_curve[1].w;
-		float r2 = P_curve[2].w;
-		float gd = ((r2 - r1)/l);
-		
-		P = P + D*t;
-
 		float3 p[4];
 		p[0] = float4_to_float3(P_curve[0]);
 		p[1] = float4_to_float3(P_curve[1]);
 		p[2] = float4_to_float3(P_curve[2]);
 		p[3] = float4_to_float3(P_curve[3]);
 
+		P = P + D*t;
+
 #ifdef __UV__
 		sd->u = isect->u;
 		sd->v = 0.0f;
 #endif
 	
-		tg = normalize(curvetangent(isect->u, p[0], p[1], p[2], p[3]));
-
-		if(kernel_data.curve.curveflags & CURVE_KN_RIBBONS)
+		if(kernel_data.curve.curveflags & CURVE_KN_RIBBONS) {
+			tg = normalize(curvetangent(isect->u, p[0], p[1], p[2], p[3]));
 			sd->Ng = normalize(-(D - tg * (dot(tg, D))));
+		}
 		else {
+			/* direction from inside to surface of curve */
 			float3 p_curr = curvepoint(isect->u, p[0], p[1], p[2], p[3]);	
 			sd->Ng = normalize(P - p_curr);
-			sd->Ng = sd->Ng - gd * tg;
-			sd->Ng = normalize(sd->Ng);
+
+			/* adjustment for changing radius */
+			float gd = isect->v;
+
+			if(gd != 0.0f) {
+				tg = normalize(curvetangent(isect->u, p[0], p[1], p[2], p[3]));
+				sd->Ng = sd->Ng - gd * tg;
+				sd->Ng = normalize(sd->Ng);
+			}
 		}
+
+		/* todo: sometimes the normal is still so that this is detected as
+		 * backfacing even if cull backfaces is enabled */
+
 		sd->N = sd->Ng;
 	}
 	else {
@@ -945,9 +950,6 @@ ccl_device_inline float3 bvh_curve_refine(KernelGlobals *kg, ShaderData *sd, con
 
 		float l = 1.0f;
 		tg = normalize_len(float4_to_float3(P_curve[1] - P_curve[0]), &l);
-		float r1 = P_curve[0].w;
-		float r2 = P_curve[1].w;
-		float gd = ((r2 - r1)/l);
 		
 		P = P + D*t;
 
@@ -963,9 +965,14 @@ ccl_device_inline float3 bvh_curve_refine(KernelGlobals *kg, ShaderData *sd, con
 			sd->Ng = normalize(sd->Ng);
 		}
 		else {
+			float gd = isect->v;
+
+			/* direction from inside to surface of curve */
 			sd->Ng = (dif - tg * sd->u * l) / (P_curve[0].w + sd->u * l * gd);
+
+			/* adjustment for changing radius */
 			if (gd != 0.0f) {
-				sd->Ng = sd->Ng - gd * tg ;
+				sd->Ng = sd->Ng - gd * tg;
 				sd->Ng = normalize(sd->Ng);
 			}
 		}
@@ -980,7 +987,7 @@ ccl_device_inline float3 bvh_curve_refine(KernelGlobals *kg, ShaderData *sd, con
 #endif
 
 	/*add fading parameter for minimum pixel width with transparency bsdf*/
-	/*sd->curve_transparency = isect->v;*/
+	/*sd->curve_transparency = isect->transparency;*/
 	/*sd->curve_radius = sd->u * gd * l + r1;*/
 
 	if(isect->object != OBJECT_NONE) {
diff --git a/intern/cycles/kernel/svm/svm_closure.h b/intern/cycles/kernel/svm/svm_closure.h
index caae65487c4..fe3fe697bca 100644
--- a/intern/cycles/kernel/svm/svm_closure.h
+++ b/intern/cycles/kernel/svm/svm_closure.h
@@ -336,8 +336,14 @@ ccl_device void svm_node_closure_bsdf(KernelGlobals *kg, ShaderData *sd, float *
 			
 			if(sd->flag & SD_BACKFACING && sd->type & PRIMITIVE_ALL_CURVE) {
 				ShaderClosure *sc = svm_node_closure_get_bsdf(sd, mix_weight);
+
 				if(sc) {
-					sc->weight = make_float3(1.0f,1.0f,1.0f);
+					/* todo: giving a fixed weight here will cause issues when
+					 * mixing multiple BSDFS. energey 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 */
+					sc->weight = make_float3(1.0f, 1.0f, 1.0f);
 					sc->N = N;
 					sd->flag |= bsdf_transparent_setup(sc);
 				}