Cycles: Keep all hair strands

Previously, hair strands of zero length of too few control
points would have been ignored. This is fine for a render
without motion blur. But once motion blur is enabled it is
becoming more tricky to match topology.

Even more, it was causing access (and possibly writes) past
the array boundaries in case when time step 0 ignored some
strands and steps around it did not.

If this is becoming problematic for BVH to do reliable
intersections this is to be solved on the BVH builder side.
The export from Blender to Cycles shouldn't really make
decisions there.

intern/cycles/blender/blender_curves.cpp

@@ -185,9 +185,7 @@ static bool ObtainCacheParticleData(Mesh *mesh,
 						float3 cKey = make_float3(nco[0], nco[1], nco[2]);
 						cKey = transform_point(&itfm, cKey);
 						if(step_no > 0) {
-							float step_length = len(cKey - pcKey);
+							const float step_length = len(cKey - pcKey);
-							if(step_length == 0.0f)
-								continue;
 							curve_length += step_length;
 						}
 						CData->curvekey_co.push_back_slow(cKey);
@@ -336,9 +334,6 @@ static void ExportCurveTrianglePlanes(Mesh *mesh, ParticleCurveData *CData,
 	/* compute and reserve size of arrays */
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
 		for(int curve = CData->psys_firstcurve[sys]; curve < CData->psys_firstcurve[sys] + CData->psys_curvenum[sys]; curve++) {
-			if(CData->curve_keynum[curve] <= 1 || CData->curve_length[curve] == 0.0f)
-				continue;
-
 			numverts += 2 + (CData->curve_keynum[curve] - 1)*2;
 			numtris += (CData->curve_keynum[curve] - 1)*2;
 		}
@@ -349,9 +344,6 @@ static void ExportCurveTrianglePlanes(Mesh *mesh, ParticleCurveData *CData,
 	/* actually export */
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
 		for(int curve = CData->psys_firstcurve[sys]; curve < CData->psys_firstcurve[sys] + CData->psys_curvenum[sys]; curve++) {
-			if(CData->curve_keynum[curve] <= 1 || CData->curve_length[curve] == 0.0f)
-				continue;
-
 			float3 xbasis;
 			float3 v1;
 			float time = 0.0f;
@@ -421,9 +413,6 @@ static void ExportCurveTriangleGeometry(Mesh *mesh,
 	/* compute and reserve size of arrays */
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
 		for(int curve = CData->psys_firstcurve[sys]; curve < CData->psys_firstcurve[sys] + CData->psys_curvenum[sys]; curve++) {
-			if(CData->curve_keynum[curve] <= 1 || CData->curve_length[curve] == 0.0f)
-				continue;
-
 			numverts += (CData->curve_keynum[curve] - 1)*resolution + resolution;
 			numtris += (CData->curve_keynum[curve] - 1)*2*resolution;
 		}
@@ -434,9 +423,6 @@ static void ExportCurveTriangleGeometry(Mesh *mesh,
 	/* actually export */
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
 		for(int curve = CData->psys_firstcurve[sys]; curve < CData->psys_firstcurve[sys] + CData->psys_curvenum[sys]; curve++) {
-			if(CData->curve_keynum[curve] <= 1 || CData->curve_length[curve] == 0.0f)
-				continue;
-
 			float3 firstxbasis = cross(make_float3(1.0f,0.0f,0.0f),CData->curvekey_co[CData->curve_firstkey[curve]+1] - CData->curvekey_co[CData->curve_firstkey[curve]]);
 			if(!is_zero(firstxbasis))
 				firstxbasis = normalize(firstxbasis);
@@ -564,9 +550,6 @@ static void ExportCurveSegments(Scene *scene, Mesh *mesh, ParticleCurveData *CDa
 	/* compute and reserve size of arrays */
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
 		for(int curve = CData->psys_firstcurve[sys]; curve < CData->psys_firstcurve[sys] + CData->psys_curvenum[sys]; curve++) {
-			if(CData->curve_keynum[curve] <= 1 || CData->curve_length[curve] == 0.0f)
-				continue;
-
 			num_keys += CData->curve_keynum[curve];
 			num_curves++;
 		}
@@ -584,19 +567,27 @@ static void ExportCurveSegments(Scene *scene, Mesh *mesh, ParticleCurveData *CDa
 	/* actually export */
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
 		for(int curve = CData->psys_firstcurve[sys]; curve < CData->psys_firstcurve[sys] + CData->psys_curvenum[sys]; curve++) {
-			if(CData->curve_keynum[curve] <= 1 || CData->curve_length[curve] == 0.0f)
-				continue;
-
 			size_t num_curve_keys = 0;
 
-			for(int curvekey = CData->curve_firstkey[curve]; curvekey < CData->curve_firstkey[curve] + CData->curve_keynum[curve]; curvekey++) {
+			for(int curvekey = CData->curve_firstkey[curve];
-				float3 ickey_loc = CData->curvekey_co[curvekey];
+			    curvekey < CData->curve_firstkey[curve] + CData->curve_keynum[curve];
-				float time = CData->curvekey_time[curvekey]/CData->curve_length[curve];
+			    curvekey++)
-				float radius = shaperadius(CData->psys_shape[sys], CData->psys_rootradius[sys], CData->psys_tipradius[sys], time);
+			{
-
+				const float3 ickey_loc = CData->curvekey_co[curvekey];
-				if(CData->psys_closetip[sys] && (curvekey == CData->curve_firstkey[curve] + CData->curve_keynum[curve] - 1))
+				const float curve_time = CData->curvekey_time[curvekey];
+				const float curve_length = CData->curve_length[curve];
+				const float time = (curve_length > 0.0f)
+				                           ? curve_time / curve_length
+				                           : 0.0f;
+				float radius = shaperadius(CData->psys_shape[sys],
+				                           CData->psys_rootradius[sys],
+				                           CData->psys_tipradius[sys],
+				                           time);
+				if(CData->psys_closetip[sys] &&
+				   (curvekey == CData->curve_firstkey[curve] + CData->curve_keynum[curve] - 1))
+				{
 					radius = 0.0f;
-
+				}
 				mesh->add_curve_key(ickey_loc, radius);
 				if(attr_intercept)
 					attr_intercept->add(time);
@@ -623,8 +614,10 @@ static void ExportCurveSegments(Scene *scene, Mesh *mesh, ParticleCurveData *CDa
 
 static float4 CurveSegmentMotionCV(ParticleCurveData *CData, int sys, int curve, int curvekey)
 {
-	float3 ickey_loc = CData->curvekey_co[curvekey];
+	const float3 ickey_loc = CData->curvekey_co[curvekey];
-	float time = CData->curvekey_time[curvekey]/CData->curve_length[curve];
+	const float curve_time = CData->curvekey_time[curvekey];
+	const float curve_length = CData->curve_length[curve];
+	float time = (curve_length > 0.0f) ? curve_time / curve_length : 0.0f;
 	float radius = shaperadius(CData->psys_shape[sys], CData->psys_rootradius[sys], CData->psys_tipradius[sys], time);
 
 	if(CData->psys_closetip[sys] && (curvekey == CData->curve_firstkey[curve] + CData->curve_keynum[curve] - 1))
@@ -683,13 +676,7 @@ static void ExportCurveSegmentsMotion(Mesh *mesh, ParticleCurveData *CData, int
 	int num_curves = 0;
 
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
-		if(CData->psys_curvenum[sys] == 0)
-			continue;
-
 		for(int curve = CData->psys_firstcurve[sys]; curve < CData->psys_firstcurve[sys] + CData->psys_curvenum[sys]; curve++) {
-			if(CData->curve_keynum[curve] <= 1 || CData->curve_length[curve] == 0.0f)
-				continue;
-
 			/* Curve lengths may not match! Curves can be clipped. */
 			int curve_key_end = (num_curves+1 < (int)mesh->curve_first_key.size() ? mesh->curve_first_key[num_curves+1] : (int)mesh->curve_keys.size());
 			const int num_center_curve_keys = curve_key_end - mesh->curve_first_key[num_curves];
@@ -715,7 +702,10 @@ static void ExportCurveSegmentsMotion(Mesh *mesh, ParticleCurveData *CData, int
 			else {
 				/* Number of keys has changed. Genereate an interpolated version
 				 * to preserve motion blur. */
-				float step_size = 1.0f / (num_center_curve_keys-1);
+				const float step_size =
+				        num_center_curve_keys > 1
+				                ? 1.0f / (num_center_curve_keys - 1)
+				                : 0.0f;
 				for(int step_index = 0;
 				    step_index < num_center_curve_keys;
 				    ++step_index)
@@ -774,11 +764,10 @@ static void ExportCurveTriangleUV(ParticleCurveData *CData,
 
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
 		for(int curve = CData->psys_firstcurve[sys]; curve < CData->psys_firstcurve[sys] + CData->psys_curvenum[sys]; curve++) {
-			if(CData->curve_keynum[curve] <= 1 || CData->curve_length[curve] == 0.0f)
-				continue;
-
 			for(int curvekey = CData->curve_firstkey[curve]; curvekey < CData->curve_firstkey[curve] + CData->curve_keynum[curve] - 1; curvekey++) {
-				time = CData->curvekey_time[curvekey]/CData->curve_length[curve];
+				const float curve_time = CData->curvekey_time[curvekey];
+				const float curve_length = CData->curve_length[curve];
+				time = (curve_length > 0.0f) ? curve_time / curve_length : 0.0f;
 
 				for(int section = 0; section < resol; section++) {
 					uvdata[vertexindex] = CData->curve_uv[curve];
@@ -819,9 +808,6 @@ static void ExportCurveTriangleVcol(ParticleCurveData *CData,
 
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
 		for(int curve = CData->psys_firstcurve[sys]; curve < CData->psys_firstcurve[sys] + CData->psys_curvenum[sys]; curve++) {
-			if(CData->curve_keynum[curve] <= 1 || CData->curve_length[curve] == 0.0f)
-				continue;
-
 			for(int curvekey = CData->curve_firstkey[curve]; curvekey < CData->curve_firstkey[curve] + CData->curve_keynum[curve] - 1; curvekey++) {
 				for(int section = 0; section < resol; section++) {
 					/* Encode vertex color using the sRGB curve. */