Sketch Retargetting:

Use stroke normal to align roll of the retargetted bones.

Unlike stroke conversion, the final roll is not forced to point exactly to the normal. Rather, It tries to align the X or Z axis of the bones (whichever is closest) to the stroke normal. Forcing the roll can invert knees while this rightly only makes one of the joints rotation axis perpendicular.

source/blender/include/reeb.h

@@ -63,6 +63,7 @@ typedef struct EmbedBucket {
 	float val;
 	int	  nv;
 	float p[3];
+	float *no; /* if present, normal of the bucket */
 } EmbedBucket;
 
 typedef struct ReebNode {

source/blender/src/editarmature_retarget.c

@@ -64,6 +64,7 @@
 #include "BIF_editarmature.h"
 #include "BIF_retarget.h"
 #include "BIF_space.h"
+#include "BIF_toolbox.h"
 
 #include "PIL_time.h"
 
@@ -161,30 +162,52 @@ void getEditBoneRollUpAxis(EditBone *bone, float roll, float up_axis[3])
 	VECCOPY(up_axis, mat[2]);
 }
 
-float getNewBoneRoll(EditBone *bone, float old_up_axis[3], float quat[4])
+float rollBoneByQuatAligned(EditBone *bone, float old_up_axis[3], float quat[4], float qroll[4], float aligned_axis[3])
 {
-	float mat[3][3];
-	float nor[3], up_axis[3], new_up_axis[3], vec[3];
-	float roll;
+	float nor[3], new_up_axis[3], x_axis[3], z_axis[3];
 	
 	VECCOPY(new_up_axis, old_up_axis);
 	QuatMulVecf(quat, new_up_axis);
-
+	
 	VecSubf(nor, bone->tail, bone->head);
 	
-	vec_roll_to_mat3(nor, 0, mat);
-	VECCOPY(up_axis, mat[2]);
+	Crossf(x_axis, nor, aligned_axis);
+	Crossf(z_axis, x_axis, nor);
 	
-	roll = NormalizedVecAngle2(new_up_axis, up_axis);
+	Normalize(new_up_axis);
+	Normalize(x_axis);
+	Normalize(z_axis);
 	
-	Crossf(vec, up_axis, new_up_axis);
-	
-	if (Inpf(vec, nor) < 0)
+	if (Inpf(new_up_axis, x_axis) < 0)
 	{
-		roll = -roll;
+		VecMulf(x_axis, -1);
 	}
 	
-	return roll;
+	if (Inpf(new_up_axis, z_axis) < 0)
+	{
+		VecMulf(z_axis, -1);
+	}
+	
+	if (NormalizedVecAngle2(x_axis, new_up_axis) < NormalizedVecAngle2(z_axis, new_up_axis))
+	{
+		RotationBetweenVectorsToQuat(qroll, new_up_axis, x_axis); /* set roll rotation quat */
+		return rollBoneToVector(bone, x_axis);
+	}
+	else
+	{
+		RotationBetweenVectorsToQuat(qroll, new_up_axis, z_axis); /* set roll rotation quat */
+		return rollBoneToVector(bone, z_axis);
+	}
+}
+
+float rollBoneByQuat(EditBone *bone, float old_up_axis[3], float quat[4])
+{
+	float new_up_axis[3];
+	
+	VECCOPY(new_up_axis, old_up_axis);
+	QuatMulVecf(quat, new_up_axis);
+	
+	return rollBoneToVector(bone, new_up_axis);
 }
 
 /************************************ DESTRUCTORS ******************************************************/
@@ -1617,7 +1640,7 @@ static void repositionControl(RigGraph *rigg, RigControl *ctrl, float head[3], f
 	
 	VecAddf(ctrl->bone->head, head, parent_offset); 
 	VecAddf(ctrl->bone->tail, ctrl->bone->head, tail_offset);
-	ctrl->bone->roll = getNewBoneRoll(ctrl->bone, ctrl->up_axis, qrot);
+	ctrl->bone->roll = rollBoneByQuat(ctrl->bone, ctrl->up_axis, qrot);
 	
 	ctrl->flag |= RIG_CTRL_DONE;
 
@@ -1632,7 +1655,7 @@ static void repositionControl(RigGraph *rigg, RigControl *ctrl, float head[3], f
 
 }
 
-static void repositionBone(RigGraph *rigg, RigEdge *edge, float vec0[3], float vec1[3])
+static void repositionBone(RigGraph *rigg, RigEdge *edge, float vec0[3], float vec1[3], float *up_axis)
 {
 	EditBone *bone;
 	RigControl *ctrl;
@@ -1652,6 +1675,22 @@ static void repositionBone(RigGraph *rigg, RigEdge *edge, float vec0[3], float v
 	
 	RotationBetweenVectorsToQuat(qrot, v1, v2);
 	
+	VECCOPY(bone->head, vec0);
+	VECCOPY(bone->tail, vec1);
+	
+	if (up_axis != NULL)
+	{
+		float qroll[4];
+
+		bone->roll = rollBoneByQuatAligned(bone, edge->up_axis, qrot, qroll, up_axis);
+		
+		QuatMul(qrot, qrot, qroll);
+	}
+	else
+	{
+		bone->roll = rollBoneByQuat(bone, edge->up_axis, qrot);
+	}
+
 	for (ctrl = rigg->controls.first; ctrl; ctrl = ctrl->next)
 	{
 		if (ctrl->link == bone)
@@ -1659,10 +1698,6 @@ static void repositionBone(RigGraph *rigg, RigEdge *edge, float vec0[3], float v
 			repositionControl(rigg, ctrl, vec0, vec1, qrot, resize);
 		}
 	}
-	
-	VECCOPY(bone->head, vec0);
-	VECCOPY(bone->tail, vec1);
-	bone->roll = getNewBoneRoll(bone, edge->up_axis, qrot);
 }
 
 static RetargetMode detectArcRetargetMode(RigArc *arc);
@@ -2219,10 +2254,12 @@ static void retargetArctoArcAggresive(RigGraph *rigg, RigArc *iarc, RigNode *ino
 		 edge;
 		 edge = edge->next, i++)
 	{
+		float *no = NULL;
 		if (i < nb_joints)
 		{
 			bucket = peekBucket(&iter, best_positions[i]);
 			vec1 = bucket->p;
+			no = bucket->no;
 		}
 		else
 		{
@@ -2231,7 +2268,7 @@ static void retargetArctoArcAggresive(RigGraph *rigg, RigArc *iarc, RigNode *ino
 		
 		if (edge->bone)
 		{
-			repositionBone(rigg, edge, vec0, vec1);
+			repositionBone(rigg, edge, vec0, vec1, no);
 		}
 		
 		vec0 = vec1;
@@ -2297,7 +2334,7 @@ static void retargetArctoArcLength(RigGraph *rigg, RigArc *iarc, RigNode *inode_
 	for (edge = iarc->edges.first; edge; edge = edge->next)
 	{
 		float new_bone_length = edge->length / iarc->length * embedding_length;
-
+		float *no = NULL;
 		float length = 0;
 
 		while (bucket && new_bone_length > length)
@@ -2306,17 +2343,19 @@ static void retargetArctoArcLength(RigGraph *rigg, RigArc *iarc, RigNode *inode_
 			bucket = nextBucket(&iter);
 			previous_vec = vec1;
 			vec1 = bucket->p;
+			no = bucket->no;
 		}
 		
 		if (bucket == NULL)
 		{
 			vec1 = node_end->p;
+			no = NULL;
 		}
 
 		/* no need to move virtual edges (space between unconnected bones) */		
 		if (edge->bone)
 		{
-			repositionBone(rigg, edge, vec0, vec1);
+			repositionBone(rigg, edge, vec0, vec1, no);
 		}
 		
 		vec0 = vec1;
@@ -2359,11 +2398,11 @@ void *exec_retargetArctoArc(void *param)
 
 		if (testFlipArc(iarc, inode_start))
 		{
-			repositionBone(rigg, edge, earc->tail->p, earc->head->p);
+			repositionBone(rigg, edge, earc->tail->p, earc->head->p, NULL);
 		}
 		else
 		{
-			repositionBone(rigg, edge, earc->head->p, earc->tail->p);
+			repositionBone(rigg, edge, earc->head->p, earc->tail->p, NULL);
 		}
 	}
 	else
@@ -2774,6 +2813,12 @@ void BIF_retargetArc(ReebArc *earc)
 	
 	template = armatureSelectedToGraph(ob, arm);
 	
+	if (template->arcs.first == NULL)
+	{
+		error("No deforming bones selected");
+		return;
+	}
+	
 	rigg = cloneRigGraph(template);
 	
 	iarc = rigg->arcs.first;

source/blender/src/editarmature_sketch.c

@@ -380,6 +380,7 @@ ReebArc *strokeToArc(SK_Stroke *stk)
 	for (i = 0; i < arc->bcount; i++)
 	{
 		VECCOPY(arc->buckets[i].p, stk->points[i + 1].p);
+		arc->buckets[i].no = stk->points[i + 1].no;
 	}
 	
 	return arc;