fix(skinned_mesh): resolve issues with bone transformations (#217)

Fixed bone transformation issues in skinned meshes, including location and orientation. Made adjustments to ensure transformations are correctly applied and multiplied.

fix: #213

addon/i3dio/node_classes/skinned_mesh.py

@@ -23,27 +23,36 @@ def __init__(self, id_: int, bone_object: bpy.types.Bone,
 
     @property
     def _transform_for_conversion(self) -> mathutils.Matrix:
+        conversion_matrix: mathutils.Matrix = self.i3d.conversion_matrix
 
         if self.blender_object.parent is None:
             # The bone is parented to the armature directly, and therefore should just use the matrix_local which is in
             # relation to the armature anyway.
-            bone_transform = self.blender_object.matrix_local
+            bone_transform = conversion_matrix @ self.blender_object.matrix_local @ conversion_matrix.inverted()
+
+            # Blender bones are visually pointing along the Z-axis, but internally they use the Y-axis. This creates a
+            # discrepancy when converting to GE's expected orientation. To resolve this, apply a -90-degree rotation
+            # around the X-axis. The translation is extracted first to avoid altering the
+            # bone's position during rotation.
+            rot_fix = mathutils.Matrix.Rotation(math.radians(-90.0), 4, 'X')
+            translation = bone_transform.to_translation()
+            bone_transform = rot_fix @ bone_transform.to_3x3().to_4x4()
+            bone_transform.translation = translation
+
             if self.i3d.settings['collapse_armatures']:
-                bone_transform = self.parent.blender_object.matrix_local @ bone_transform
+                # collapse_armatures deletes the armature object in the I3D,
+                # so we need to mutliply the armature matrix into the root bone
+                armature_obj = self.parent.blender_object
+                armature_matrix = conversion_matrix @ armature_obj.matrix_local @ conversion_matrix.inverted()
+
+                bone_transform = armature_matrix @ bone_transform
         else:
-            # To find the transform of the bone, we take the inverse of its parents transform in armature space and
+            # To find the transform of child bone, we take the inverse of its parents transform in armature space and
             # multiply that with the bones transform in armature space. The new 4x4 matrix gives the position and
             # rotation in relation to the parent bone (of the head, that is)
             bone_transform = self.blender_object.parent.matrix_local.inverted() @ self.blender_object.matrix_local
 
-        # Blender bones are visually pointing along the Z-axis, but internally they are using Y. To get around this
-        # discrepancy the local matrix has a 90 deg rotation around the X-axis. To make the bone have the expected
-        # orientation in GE, rotate it -90 deg on around X.
-        bone_transform = bone_transform @ mathutils.Matrix.Rotation(math.radians(-90.0), 4, 'X')
-
-        conversion_matrix = self.i3d.conversion_matrix @ bone_transform @ self.i3d.conversion_matrix.inverted()
-
-        return conversion_matrix
+        return bone_transform
 
 
 class SkinnedMeshRootNode(TransformGroupNode):