Cycles: Improved robustness of hair motion blur.

In some instances, the number of control vertices of a hair could change mid-frame.
Cycles would then be unable to calculate proper motion blur for those hairs. This adds
interpolated CVs to fill in for the missing data. While this will not necessarily result in
a fully accurate reconstruction of the guide hair, it preserves motion blur instead of disabling it.

Reviewers: #cycles, sergey

Reviewed By: #cycles, sergey

Subscribers: sergey, brecht, #cycles

Tags: #cycles

Differential Revision: https://developer.blender.org/D3695

1 files changed, 67 insertions, 22 deletions

diff --git a/intern/cycles/blender/blender_curves.cpp b/intern/cycles/blender/blender_curves.cpp
index 5fd3455061d..1d51aca062b 100644
--- a/intern/cycles/blender/blender_curves.cpp
+++ b/intern/cycles/blender/blender_curves.cpp
@@ -633,6 +633,41 @@ 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];
+	float time = CData->curvekey_time[curvekey]/CData->curve_length[curve];
+	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;
+
+	/* curve motion keys store both position and radius in float4 */
+	float4 mP = float3_to_float4(ickey_loc);
+	mP.w = radius;
+	return mP;
+}
+
+static float4 LerpCurveSegmentMotionCV(ParticleCurveData *CData, int sys, int curve, float step)
+{
+	step = clamp(step, 0.0f, 1.0f);
+	float curve_key_f = step * (CData->curve_keynum[curve] - 1);
+	int curvekey = (int)floorf(curve_key_f);
+	float remainder = curve_key_f - curvekey;
+	if(remainder == 0.0f) {
+		return CurveSegmentMotionCV(CData, sys, curve, curvekey);
+	}
+	int curvekey2 = curvekey + 1;
+	if(curvekey2 >= (CData->curve_keynum[curve] - 1)) {
+		curvekey2 = (CData->curve_keynum[curve] - 1);
+		curvekey = curvekey2 - 1;
+	}
+
+	float4 mP = CurveSegmentMotionCV(CData, sys, curve, curvekey);
+	float4 mP2 = CurveSegmentMotionCV(CData, sys, curve, curvekey2);
+	return lerp(mP, mP2, remainder);
+}
+
 static void ExportCurveSegmentsMotion(Mesh *mesh, ParticleCurveData *CData, int motion_step)
 {
 	VLOG(1) << "Exporting curve motion segments for mesh " << mesh->name
@@ -654,6 +689,7 @@ static void ExportCurveSegmentsMotion(Mesh *mesh, ParticleCurveData *CData, int
 	float4 *mP = attr_mP->data_float4() + motion_step*numkeys;
 	bool have_motion = false;
 	int i = 0;
+	int num_curves = 0;
 
 	for(int sys = 0; sys < CData->psys_firstcurve.size(); sys++) {
 		if(CData->psys_curvenum[sys] == 0)
@@ -663,30 +699,39 @@ static void ExportCurveSegmentsMotion(Mesh *mesh, ParticleCurveData *CData, int
 			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]; curvekey++) {
-				if(i < mesh->curve_keys.size()) {
-					float3 ickey_loc = CData->curvekey_co[curvekey];
-					float time = CData->curvekey_time[curvekey]/CData->curve_length[curve];
-					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;
-
-					/* curve motion keys store both position and radius in float4 */
-					mP[i] = float3_to_float4(ickey_loc);
-					mP[i].w = radius;
-
-					/* unlike mesh coordinates, these tend to be slightly different
-					 * between frames due to particle transforms into/out of object
-					 * space, so we use an epsilon to detect actual changes */
-					float4 curve_key = float3_to_float4(mesh->curve_keys[i]);
-					curve_key.w = mesh->curve_radius[i];
-					if(len_squared(mP[i] - curve_key) > 1e-5f*1e-5f)
-						have_motion = true;
+			/* 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());
+			int center_curve_len = curve_key_end - mesh->curve_first_key[num_curves];
+			int diff = CData->curve_keynum[curve] - center_curve_len;
+
+			if(diff == 0) {
+				for(int curvekey = CData->curve_firstkey[curve]; curvekey < CData->curve_firstkey[curve] + CData->curve_keynum[curve]; curvekey++) {
+					if(i < mesh->curve_keys.size()) {
+						mP[i] = CurveSegmentMotionCV(CData, sys, curve, curvekey);
+						if(!have_motion) {
+							/* unlike mesh coordinates, these tend to be slightly different
+							 * between frames due to particle transforms into/out of object
+							 * space, so we use an epsilon to detect actual changes */
+							float4 curve_key = float3_to_float4(mesh->curve_keys[i]);
+							curve_key.w = mesh->curve_radius[i];
+							if(len_squared(mP[i] - curve_key) > 1e-5f*1e-5f)
+								have_motion = true;
+						}
+					}
+					i++;
 				}
-
-				i++;
 			}
+			else {
+				/* Number of keys has changed. Genereate an interpolated version to preserve motion blur. */
+				float step = 0;
+				float step_size = 1.0f / (center_curve_len-1);
+				for(; i < curve_key_end; i++) {
+					mP[i] = LerpCurveSegmentMotionCV(CData, sys, curve, step);
+					step = i * step_size;
+				}
+				have_motion = true;
+			}
+			num_curves++;
 		}
 	}