Implement an Armature constraint that mimics the modifier.

The main use one can imagine for this is adding tweak controls to
parts of a model that are already deformed by multiple other major
bones. It is natural to expect such locations to deform as if the
tweaks aren't there by default; however currently there is no easy
way to make a bone follow multiple other bones.

This adds a new constraint that implements the math behind the Armature
modifier, with support for explicit weights, bone envelopes, and dual
quaternion blending. It can also access bones from multiple armatures
at the same time (mainly because it's easier to code it that way.)

This also fixes dquat_to_mat4, which wasn't used anywhere before.

Differential Revision: https://developer.blender.org/D3664

release/scripts/startup/bl_operators/constraint.py

@@ -0,0 +1,76 @@
+# ##### BEGIN GPL LICENSE BLOCK #####
+#
+#  This program is free software; you can redistribute it and/or
+#  modify it under the terms of the GNU General Public License
+#  as published by the Free Software Foundation; either version 2
+#  of the License, or (at your option) any later version.
+#
+#  This program is distributed in the hope that it will be useful,
+#  but WITHOUT ANY WARRANTY; without even the implied warranty of
+#  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#  GNU General Public License for more details.
+#
+#  You should have received a copy of the GNU General Public License
+#  along with this program; if not, write to the Free Software Foundation,
+#  Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+#
+# ##### END GPL LICENSE BLOCK #####
+
+# <pep8-80 compliant>
+
+import bpy
+from bpy.types import (
+    Operator,
+)
+from bpy.props import (
+    IntProperty,
+)
+
+
+class CONSTRAINT_OT_add_target(Operator):
+    """Add a target to the constraint"""
+    bl_idname = "constraint.add_target"
+    bl_label = "Add Target"
+    bl_options = {'UNDO', 'INTERNAL'}
+
+    def execute(self, context):
+        context.constraint.targets.new()
+        return {'FINISHED'}
+
+
+class CONSTRAINT_OT_remove_target(Operator):
+    """Remove the target from the constraint"""
+    bl_idname = "constraint.remove_target"
+    bl_label = "Remove Target"
+    bl_options = {'UNDO', 'INTERNAL'}
+
+    index = IntProperty()
+
+    def execute(self, context):
+        tgts = context.constraint.targets
+        tgts.remove(tgts[self.index])
+        return {'FINISHED'}
+
+
+class CONSTRAINT_OT_normalize_target_weights(Operator):
+    """Normalize weights of all target bones"""
+    bl_idname = "constraint.normalize_target_weights"
+    bl_label = "Normalize Weights"
+    bl_options = {'UNDO', 'INTERNAL'}
+
+    def execute(self, context):
+        tgts = context.constraint.targets
+        total = sum(t.weight for t in tgts)
+
+        if total > 0:
+            for t in tgts:
+                t.weight = t.weight / total
+
+        return {'FINISHED'}
+
+
+classes = (
+    CONSTRAINT_OT_add_target,
+    CONSTRAINT_OT_remove_target,
+    CONSTRAINT_OT_normalize_target_weights,
+)