fix: T-posing skinned models — autorig produced an orphan skin with no weights

The model animated its skeleton but rendered a permanent T-pose: the mesh
wasn't skin-bound. Root cause was in autorig's binding, introduced while
chasing an earlier "skins:0" export:

- The earlier "skins:0" was actually caused by bone-heat weighting failing
  (0 weighted verts → empty skin gets dropped), NOT by the Armature modifier.
- The "fix" then stripped the Armature modifier and used parent_type=ARMATURE.
  That makes the glTF exporter emit a skin OBJECT but with no node.skin
  reference and no per-vertex JOINTS/WEIGHTS — an orphan skin. The mesh then
  renders its bind pose (T-pose) forever while the skeleton animates unseen.

Fix: keep the standard ARMATURE_AUTO result (Armature modifier + vertex
groups). With the nearest-bone fallback ensuring real weights, the exporter
now writes a COMPLETE skin (verified: meshnode.skin=0, JOINTS/WEIGHTS present).

Also:
- skinned_player_model: _ensure_meshes_bound() re-binds any skinned mesh whose
  skeleton NodePath doesn't resolve at load — graceful degradation instead of
  a silent T-pose for imperfect GLBs.
- smoke test now asserts the spawned player's mesh is bound to its skeleton,
  so this class of bug fails the test instead of shipping. 30/30 pass.
- regenerated miku_test.glb with the corrected pipeline.

Co-Authored-By: Claude Fable 5 <[email protected]>

characters/skinned_player_model.gd

@@ -88,6 +88,8 @@ func load_model(path: String) -> void:
 
 	if not skeleton:
 		push_warning("SkinnedPlayerModel: no skeleton in '%s'" % path)
+	else:
+		_ensure_meshes_bound(scene)
 	if animation_player:
 		_index_animations()
 	else:
@@ -103,6 +105,19 @@ func load_model(path: String) -> void:
 	_play_clip("Idle")
 
 
+## Make sure every skinned MeshInstance3D is actually driven by the skeleton.
+## A correctly-exported GLB binds automatically, but if one imports with a skin
+## resource whose `skeleton` NodePath doesn't resolve, the mesh renders its bind
+## pose (a permanent T-pose) while the skeleton animates invisibly. This repairs
+## that at load time so a bad export degrades gracefully instead of T-posing.
+func _ensure_meshes_bound(scene: Node) -> void:
+	for mi in scene.find_children("*", "MeshInstance3D", true, false):
+		if mi.skin == null:
+			continue  # not a skinned mesh
+		if mi.skeleton.is_empty() or mi.get_node_or_null(mi.skeleton) != skeleton:
+			mi.skeleton = mi.get_path_to(skeleton)
+
+
 ## Map canonical clip names to whatever actually shipped in the GLB and set
 ## loop modes (glTF has no loop flag, so we set it here).
 func _index_animations() -> void:

debug/spawn_smoke_test.gd

@@ -79,6 +79,14 @@ func _test_spawn_with_skin(skin_id: String, expect_skinned: bool) -> void:
 		if _check(skinned != null, "SkinnedModel created for GLB skin"):
 			_check(skinned.loaded, "GLB model loaded")
 			_check(skinned.skeleton != null, "skeleton found in GLB")
+			# The mesh must be bound to the skeleton, else it renders its bind
+			# pose (T-pose) while the skeleton animates invisibly.
+			var bound := false
+			if skinned.skeleton:
+				for mi in skinned.find_children("*", "MeshInstance3D", true, false):
+					if mi.skin != null and mi.get_node_or_null(mi.skeleton) == skinned.skeleton:
+						bound = true
+			_check(bound, "skinned mesh is bound to the skeleton (won't T-pose)")
 			if skinned.animation_player:
 				_check(skinned.animation_player.is_playing(), "animation playing")
 				print("  clips resolved: ", skinned._resolved_clips)

tools/autorig.py

@@ -269,13 +269,18 @@ def rigid_nearest_bone_weights(mesh, arm):
 
 
 def bind_mesh_to_armature(mesh, arm):
-    """Bind mesh to armature robustly and in a form the glTF exporter skins.
+    """Bind mesh to armature so the glTF exporter writes a COMPLETE skin
+    (skin object + node.skin reference + per-vertex JOINTS/WEIGHTS).
 
     1. Try Blender automatic (bone-heat) weights for smooth deformation.
-    2. If that assigns (almost) nothing, fall back to rigid nearest-bone.
-    3. Present the result as a parent_type='ARMATURE' relationship with NO
-       Armature modifier — the Blender 5.x glTF exporter only emits a skin
-       for that exact configuration (a lingering modifier yields skins:0).
+    2. If that assigns (almost) nothing — common on layered hair/clothing
+       meshes where bone-heat fails — fall back to rigid nearest-bone.
+
+    Keep the standard ARMATURE_AUTO result: an Armature modifier plus vertex
+    groups (parent_type stays OBJECT). That is exactly what the exporter needs
+    to write the vertex weights. (An earlier version stripped the modifier and
+    used parent_type='ARMATURE'; that produced an ORPHAN skin with no weights,
+    so the mesh rendered its bind pose — a permanent T-pose — in game.)
     """
     bpy.ops.object.select_all(action="DESELECT")
     mesh.select_set(True)
@@ -291,14 +296,6 @@ def bind_mesh_to_armature(mesh, arm):
         weighted = rigid_nearest_bone_weights(mesh, arm)
     print(f"Bound mesh: {weighted}/{total} verts weighted")
 
-    # Normalize to the exporter-friendly form: parent_type=ARMATURE, no modifier.
-    for m in list(mesh.modifiers):
-        if m.type == "ARMATURE":
-            mesh.modifiers.remove(m)
-    if mesh.parent != arm:
-        mesh.parent = arm
-    mesh.parent_type = "ARMATURE"
-
 
 def main():
     clear_scene()