doc/python_api/examples/bpy.types.PoseBone.bbone_segment_matrix.py

@@ -0,0 +1,34 @@
+"""
+This example shows how to use B-Bone segment matrices to emulate deformation
+produced by the Armature modifier or constraint when assigned to the given bone
+(without Preserve Volume). The coordinates are processed in armature Pose space:
+"""
+
+def bbone_deform_matrix(pose_bone, point):
+  index, blend_next = pose_bone.bbone_segment_index(point)
+
+  rest1 = pose_bone.bbone_segment_matrix(index, rest=True)
+  pose1 = pose_bone.bbone_segment_matrix(index, rest=False)
+  deform1 = pose1 @ rest1.inverted()
+
+  # bbone_segment_index ensures that index + 1 is always valid
+  rest2 = pose_bone.bbone_segment_matrix(index + 1, rest=True)
+  pose2 = pose_bone.bbone_segment_matrix(index + 1, rest=False)
+  deform2 = pose2 @ rest2.inverted()
+
+  deform = deform1 * (1 - blend_next) + deform2 * blend_next
+
+  return pose_bone.matrix @ deform @ pose_bone.bone.matrix_local.inverted()
+
+# Armature modifier deforming vertices:
+mesh = bpy.data.objects["Mesh"]
+pose_bone = bpy.data.objects["Armature"].pose.bones["Bone"]
+
+for vertex in mesh.data.vertices:
+  vertex.co = bbone_deform_matrix(pose_bone, vertex.co) @ vertex.co
+
+# Armature constraint modifying an object transform:
+empty = bpy.data.objects["Empty"]
+matrix = empty.matrix_world
+
+empty.matrix_world = bbone_deform_matrix(pose_bone, matrix.translation) @ matrix

source/blender/blenkernel/BKE_armature.h

@@ -539,10 +539,29 @@ void BKE_pchan_bbone_segments_cache_copy(struct bPoseChannel *pchan,
 
 /**
  * Calculate index and blend factor for the two B-Bone segment nodes
- * affecting the point at 0 <= pos <= 1.
+ * affecting the specified point along the bone.
+ *
+ * \param pchan: Pose channel.
+ * \param head_tail: head-tail position along the bone (auto-clamped between 0 and 1).
+ * \param r_index: OUTPUT index of the first segment joint affecting the point.
+ * \param r_blend_next: OUTPUT blend factor between the first and the second segment in [0..1]
+ */
+void BKE_pchan_bbone_deform_clamp_segment_index(const struct bPoseChannel *pchan,
+                                                float head_tail,
+                                                int *r_index,
+                                                float *r_blend_next);
+
+/**
+ * Calculate index and blend factor for the two B-Bone segment nodes
+ * affecting the specified point in object (pose) space.
+ *
+ * \param pchan: Pose channel.
+ * \param co: Pose space coordinates of the point being deformed.
+ * \param r_index: OUTPUT index of the first segment joint affecting the point.
+ * \param r_blend_next: OUTPUT blend factor between the first and the second segment in [0..1]
  */
 void BKE_pchan_bbone_deform_segment_index(const struct bPoseChannel *pchan,
-                                          float pos,
+                                          const float *co,
                                           int *r_index,
                                           float *r_blend_next);
 

source/blender/blenkernel/intern/armature.cc

@@ -1565,20 +1565,20 @@ void BKE_pchan_bbone_segments_cache_copy(bPoseChannel *pchan, bPoseChannel *pcha
   }
 }
 
-void BKE_pchan_bbone_deform_segment_index(const bPoseChannel *pchan,
-                                          float pos,
-                                          int *r_index,
-                                          float *r_blend_next)
+void BKE_pchan_bbone_deform_clamp_segment_index(const bPoseChannel *pchan,
+                                                float head_tail,
+                                                int *r_index,
+                                                float *r_blend_next)
 {
   int segments = pchan->bone->segments;
 
-  CLAMP(pos, 0.0f, 1.0f);
+  CLAMP(head_tail, 0.0f, 1.0f);
 
   /* Calculate the indices of the 2 affecting b_bone segments.
    * Integer part is the first segment's index.
    * Integer part plus 1 is the second segment's index.
    * Fractional part is the blend factor. */
-  float pre_blend = pos * float(segments);
+  float pre_blend = head_tail * float(segments);
 
   int index = int(floorf(pre_blend));
   CLAMP(index, 0, segments - 1);
@@ -1590,6 +1590,22 @@ void BKE_pchan_bbone_deform_segment_index(const bPoseChannel *pchan,
   *r_blend_next = blend;
 }
 
+void BKE_pchan_bbone_deform_segment_index(const bPoseChannel *pchan,
+                                          const float *co,
+                                          int *r_index,
+                                          float *r_blend_next)
+{
+  const Mat4 *mats = pchan->runtime.bbone_deform_mats;
+  const float(*mat)[4] = mats[0].mat;
+
+  /* Transform co to bone space and get its y component. */
+  const float y = mat[0][1] * co[0] + mat[1][1] * co[1] + mat[2][1] * co[2] + mat[3][1];
+
+  /* Calculate the indices of the 2 affecting b_bone segments. */
+  BKE_pchan_bbone_deform_clamp_segment_index(
+      pchan, y / pchan->bone->length, r_index, r_blend_next);
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */

source/blender/blenkernel/intern/armature_deform.cc

@@ -106,15 +106,11 @@ static void b_bone_deform(const bPoseChannel *pchan,
 {
   const DualQuat *quats = pchan->runtime.bbone_dual_quats;
   const Mat4 *mats = pchan->runtime.bbone_deform_mats;
-  const float(*mat)[4] = mats[0].mat;
-  float blend, y;
+  float blend;
   int index;
 
-  /* Transform co to bone space and get its y component. */
-  y = mat[0][1] * co[0] + mat[1][1] * co[1] + mat[2][1] * co[2] + mat[3][1];
-
   /* Calculate the indices of the 2 affecting b_bone segments. */
-  BKE_pchan_bbone_deform_segment_index(pchan, y / pchan->bone->length, &index, &blend);
+  BKE_pchan_bbone_deform_segment_index(pchan, co, &index, &blend);
 
   pchan_deform_accumulate(
       &quats[index], mats[index + 1].mat, co, weight * (1.0f - blend), vec, dq, defmat);

source/blender/blenkernel/intern/constraint.cc

@@ -729,7 +729,7 @@ static void constraint_target_to_mat4(Object *ob,
         int index;
 
         /* figure out which segment(s) the headtail value falls in */
-        BKE_pchan_bbone_deform_segment_index(pchan, headtail, &index, &fac);
+        BKE_pchan_bbone_deform_clamp_segment_index(pchan, headtail, &index, &fac);
 
         /* apply full transformation of the segment if requested */
         if (full_bbone) {
@@ -2631,18 +2631,12 @@ static void armdef_accumulate_bone(bConstraintTarget *ct,
   if (bone->segments > 1 && bone->segments == pchan->runtime.bbone_segments) {
     Mat4 *b_bone_mats = pchan->runtime.bbone_deform_mats;
     Mat4 *b_bone_rest_mats = pchan->runtime.bbone_rest_mats;
-    float(*iamat)[4] = b_bone_mats[0].mat;
     float basemat[4][4];
 
-    /* The target is a B-Bone:
-     * FIRST: find the segment (see b_bone_deform in `armature.cc`)
-     * Need to transform co back to bone-space, only need y. */
-    float y = iamat[0][1] * co[0] + iamat[1][1] * co[1] + iamat[2][1] * co[2] + iamat[3][1];
-
     /* Blend the matrix. */
     int index;
     float blend;
-    BKE_pchan_bbone_deform_segment_index(pchan, y / bone->length, &index, &blend);
+    BKE_pchan_bbone_deform_segment_index(pchan, co, &index, &blend);
 
     if (r_sum_dq != nullptr) {
       /* Compute the object space rest matrix of the segment. */

source/blender/makesrna/intern/rna_pose_api.cc

@@ -47,6 +47,24 @@ static float rna_PoseBone_do_envelope(bPoseChannel *chan, const float vec[3])
                             bone->dist * scale);
 }
 
+static void rna_PoseBone_bbone_segment_index(
+    bPoseChannel *pchan, ReportList *reports, const float pt[3], int *r_index, float *r_blend_next)
+{
+  if (!pchan->bone || pchan->bone->segments <= 1) {
+    BKE_reportf(reports, RPT_ERROR, "Bone '%s' is not a B-Bone!", pchan->name);
+    return;
+  }
+  if (pchan->runtime.bbone_segments != pchan->bone->segments) {
+    BKE_reportf(reports,
+                RPT_ERROR,
+                "Bone '%s' has out of date B-Bone segment data - depsgraph update required!",
+                pchan->name);
+    return;
+  }
+
+  BKE_pchan_bbone_deform_segment_index(pchan, pt, r_index, r_blend_next);
+}
+
 static void rna_PoseBone_bbone_segment_matrix(
     bPoseChannel *pchan, ReportList *reports, float mat_ret[16], int index, bool rest)
 {
@@ -55,7 +73,10 @@ static void rna_PoseBone_bbone_segment_matrix(
     return;
   }
   if (pchan->runtime.bbone_segments != pchan->bone->segments) {
-    BKE_reportf(reports, RPT_ERROR, "Bone '%s' has out of date B-Bone segment data!", pchan->name);
+    BKE_reportf(reports,
+                RPT_ERROR,
+                "Bone '%s' has out of date B-Bone segment data - depsgraph update required!",
+                pchan->name);
     return;
   }
   if (index < 0 || index > pchan->runtime.bbone_segments) {
@@ -271,6 +292,30 @@ void RNA_api_pose_channel(StructRNA *srna)
       func, "factor", 0, -FLT_MAX, FLT_MAX, "Factor", "Envelope factor", -FLT_MAX, FLT_MAX);
   RNA_def_function_return(func, parm);
 
+  /* B-Bone segment index from point */
+  func = RNA_def_function(srna, "bbone_segment_index", "rna_PoseBone_bbone_segment_index");
+  RNA_def_function_flag(func, FUNC_USE_REPORTS);
+  RNA_def_function_ui_description(
+      func, "Retrieve the index and blend factor of the B-Bone segments based on vertex position");
+  parm = RNA_def_float_vector_xyz(func,
+                                  "point",
+                                  3,
+                                  nullptr,
+                                  -FLT_MAX,
+                                  FLT_MAX,
+                                  "Point",
+                                  "Vertex position in armature pose space",
+                                  -FLT_MAX,
+                                  FLT_MAX);
+  RNA_def_parameter_flags(parm, PropertyFlag(0), PARM_REQUIRED);
+  /* outputs */
+  parm = RNA_def_property(func, "index", PROP_INT, PROP_NONE);
+  RNA_def_property_ui_text(parm, "", "The index of the first segment joint affecting the point");
+  RNA_def_function_output(func, parm);
+  parm = RNA_def_property(func, "blend_next", PROP_FLOAT, PROP_NONE);
+  RNA_def_property_ui_text(parm, "", "The blend factor between the given and the following joint");
+  RNA_def_function_output(func, parm);
+
   /* B-Bone segment matrices */
   func = RNA_def_function(srna, "bbone_segment_matrix", "rna_PoseBone_bbone_segment_matrix");
   RNA_def_function_ui_description(