Add a new B-Bone Custom Handle type that uses the handle bone direction.

Both original handle types are based on location, and Absolute uses it
in a weird way: the Start handle uses the head, while End uses the tail.

This makes controlling the shape of the B-Bone via control bone rotation
really non-intuitive, especially if trying to add a single control for
the tangent in the middle of a B-Bone chain.

To remedy this, add a new custom handle type that uses the orientation
of the control bone, while completely ignoring location. It is even
possible to control both ends of one B-Bone with the same handle bone,
resulting in an S shape.

Reviewers: brecht

Differential Revision: https://developer.blender.org/D3769

source/blender/blenkernel/intern/armature.c

@@ -468,6 +468,7 @@ void b_bone_spline_setup(bPoseChannel *pchan, int rest, Mat4 result_array[MAX_BB
 	Bone *bone = pchan->bone;
 	BBoneSplineParameters param;
 	float imat[4][4], posemat[4][4];
+	float delta[3];
 
 	memset(&param, 0, sizeof(param));
 
@@ -518,11 +519,21 @@ void b_bone_spline_setup(bPoseChannel *pchan, int rest, Mat4 result_array[MAX_BB
 				done = true;
 			}
 			else {
-				float delta[3];
 				sub_v3_v3v3(delta, prev->pose_head, prev->bone->arm_head);
 				sub_v3_v3v3(h1, pchan->pose_head, delta);
 			}
 		}
+		else if (bone->bbone_prev_type == BBONE_HANDLE_TANGENT) {
+			/* Use bone direction by offsetting so that its tail meets current bone's head */
+			if (rest) {
+				sub_v3_v3v3(delta, prev->bone->arm_tail, prev->bone->arm_head);
+				sub_v3_v3v3(h1, bone->arm_head, delta);
+			}
+			else {
+				sub_v3_v3v3(delta, prev->pose_tail, prev->pose_head);
+				sub_v3_v3v3(h1, pchan->pose_head, delta);
+			}
+		}
 		else {
 			/* Apply special handling for smoothly joining B-Bone chains */
 			param.prev_bbone = (prev->bone->segments > 1);
@@ -556,11 +567,21 @@ void b_bone_spline_setup(bPoseChannel *pchan, int rest, Mat4 result_array[MAX_BB
 				done = true;
 			}
 			else {
-				float delta[3];
 				sub_v3_v3v3(delta, next->pose_head, next->bone->arm_head);
 				add_v3_v3v3(h2, pchan->pose_tail, delta);
 			}
 		}
+		else if (bone->bbone_next_type == BBONE_HANDLE_TANGENT) {
+			/* Use bone direction by offsetting so that its head meets current bone's tail */
+			if (rest) {
+				sub_v3_v3v3(delta, next->bone->arm_tail, next->bone->arm_head);
+				add_v3_v3v3(h2, bone->arm_tail, delta);
+			}
+			else {
+				sub_v3_v3v3(delta, next->pose_tail, next->pose_head);
+				add_v3_v3v3(h2, pchan->pose_tail, delta);
+			}
+		}
 		else {
 			/* Apply special handling for smoothly joining B-Bone chains */
 			param.next_bbone = (next->bone->segments > 1);

source/blender/draw/intern/draw_armature.c

@@ -940,6 +940,7 @@ static void ebone_spline_preview(EditBone *ebone, float result_array[MAX_BBONE_S
 	BBoneSplineParameters param;
 	EditBone *prev, *next;
 	float imat[4][4], bonemat[4][4];
+	float tmp[3];
 
 	memset(&param, 0, sizeof(param));
 
@@ -979,6 +980,11 @@ static void ebone_spline_preview(EditBone *ebone, float result_array[MAX_BBONE_S
 			if (ebone->bbone_prev_type == BBONE_HANDLE_RELATIVE) {
 				zero_v3(param.prev_h);
 			}
+			else if (ebone->bbone_prev_type == BBONE_HANDLE_TANGENT) {
+				sub_v3_v3v3(tmp, prev->tail, prev->head);
+				sub_v3_v3v3(tmp, ebone->head, tmp);
+				mul_v3_m4v3(param.prev_h, imat, tmp);
+			}
 			else {
 				param.prev_bbone = (prev->segments > 1);
 
@@ -997,6 +1003,11 @@ static void ebone_spline_preview(EditBone *ebone, float result_array[MAX_BBONE_S
 			if (ebone->bbone_next_type == BBONE_HANDLE_RELATIVE) {
 				copy_v3_fl3(param.next_h, 0.0f, param.length, 0.0);
 			}
+			else if (ebone->bbone_next_type == BBONE_HANDLE_TANGENT) {
+				sub_v3_v3v3(tmp, next->tail, next->head);
+				add_v3_v3v3(tmp, ebone->tail, tmp);
+				mul_v3_m4v3(param.next_h, imat, tmp);
+			}
 			else {
 				param.next_bbone = (next->segments > 1);
 

source/blender/makesdna/DNA_armature_types.h

@@ -221,6 +221,7 @@ typedef enum eBone_BBoneHandleType {
 	BBONE_HANDLE_AUTO = 0,	/* Default mode based on parents & children. */
 	BBONE_HANDLE_ABSOLUTE,	/* Custom handle in absolute position mode. */
 	BBONE_HANDLE_RELATIVE,	/* Custom handle in relative position mode. */
+	BBONE_HANDLE_TANGENT,	/* Custom handle in tangent mode (use direction, not location). */
 } eBone_BBoneHandleType;
 
 #define MAXBONENAME 64

source/blender/makesrna/intern/rna_armature.c

@@ -655,6 +655,7 @@ static void rna_def_bone_common(StructRNA *srna, int editbone)
 		{BBONE_HANDLE_AUTO, "AUTO", 0, "Automatic", "Use connected parent and children to compute the handle"},
 		{BBONE_HANDLE_ABSOLUTE, "ABSOLUTE", 0, "Absolute", "Use the position of the specified bone to compute the handle"},
 		{BBONE_HANDLE_RELATIVE, "RELATIVE", 0, "Relative", "Use the offset of the specified bone from rest pose to compute the handle"},
+		{BBONE_HANDLE_TANGENT, "TANGENT", 0, "Tangent", "Use the orientation of the specified bone to compute the handle, ignoring the location"},
 		{0, NULL, 0, NULL, NULL}
 	};