feat(qc): loop_qc tool + loop-qc agent — measure loop-seam smoothness

Pose gap / velocity gap / root drift at the LoopMode.Linear wrap; exit-code
gated. Skill updated: loop QC is workflow step 5, with the Blender-side
fixer playbook.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/loop_qc.py

@@ -0,0 +1,123 @@
+"""loop_qc.py — measure how cleanly a clip loops.
+
+The game loops dance clips with Godot LoopMode.Linear: after the last keyframe it
+wraps straight back to frame 0. A loop is smooth only if, at that seam, both the
+POSE and the MOTION are continuous:
+
+  1. pose gap     — per-bone local-rotation difference (deg) between last & first frame.
+                    Large gap => the character visibly snaps to a new pose on wrap.
+  2. velocity gap — per-bone angular velocity just BEFORE the seam vs just AFTER it.
+                    Mismatch => motion direction jerks even if the pose matches.
+  3. root drift   — pelvis world-position difference end vs start. Nonzero => the body
+                    teleports sideways/vertically on wrap (only matters for in-place clips).
+
+Run headless:
+  blender --background --python tools/loop_qc.py -- --src <clip.glb> [--name <Clip>] \
+          [--pose-deg 5] [--vel-deg 3] [--drift-cm 3]
+
+Exit code 0 = SMOOTH (within thresholds), 1 = NOT SMOOTH, 2 = error. So it can gate a
+pipeline step. Prints a ranked list of the worst-offending bones to say WHERE the pop is.
+"""
+import bpy, sys, math, argparse
+from mathutils import Quaternion
+
+def parse():
+    argv = sys.argv[sys.argv.index("--")+1:] if "--" in sys.argv else []
+    p = argparse.ArgumentParser()
+    p.add_argument("--src", required=True)
+    p.add_argument("--name", default=None, help="clip/action name; default = first action")
+    p.add_argument("--pose-deg", type=float, default=5.0, help="max per-bone pose gap (deg)")
+    p.add_argument("--vel-deg", type=float, default=3.0, help="max per-bone velocity gap (deg/frame)")
+    p.add_argument("--drift-cm", type=float, default=3.0, help="max pelvis drift (cm)")
+    p.add_argument("--top", type=int, default=8, help="how many worst bones to list")
+    return p.parse_args(argv)
+
+def load(src, name):
+    bpy.ops.wm.read_factory_settings(use_empty=True)
+    bpy.ops.import_scene.gltf(filepath=src)
+    arm = next(o for o in bpy.data.objects if o.type == 'ARMATURE')
+    if name:
+        act = bpy.data.actions.get(name)
+        if act is None:
+            acts = [a.name for a in bpy.data.actions]
+            raise SystemExit(f"[loop_qc] no action '{name}'; have {acts}")
+        arm.animation_data.action = act
+    else:
+        act = arm.animation_data.action
+    return arm, act
+
+def local_quats(arm, frame):
+    bpy.context.scene.frame_set(frame)
+    out = {}
+    for pb in arm.pose.bones:
+        q = pb.matrix_basis.to_quaternion().normalized()
+        out[pb.name] = q
+    return out
+
+def ang(q1, q2):
+    d = abs(max(-1.0, min(1.0, q1.dot(q2))))
+    return math.degrees(2.0 * math.acos(d))
+
+def main():
+    a = parse()
+    arm, act = load(a.src, a.name)
+    f0, f1 = int(act.frame_range[0]), int(act.frame_range[1])
+    scene = bpy.context.scene
+
+    q_first = local_quats(arm, f0)
+    q_last  = local_quats(arm, f1)
+    q_first1 = local_quats(arm, f0 + 1)   # for velocity just after the seam
+    q_last1  = local_quats(arm, f1 - 1)   # for velocity just before the seam
+
+    bones = [b for b in q_first if b in q_last]
+
+    # 1. pose gap
+    pose = sorted(((ang(q_last[b], q_first[b]), b) for b in bones), reverse=True)
+    pose_max, pose_bone = pose[0]
+    pose_mean = sum(v for v, _ in pose) / len(pose)
+
+    # 2. velocity gap: v_before = last-1 -> last ; v_after = first -> first+1
+    vel = []
+    for b in bones:
+        v_before = ang(q_last1[b], q_last[b])
+        v_after  = ang(q_first[b], q_first1[b])
+        vel.append((abs(v_before - v_after), b, v_before, v_after))
+    vel.sort(reverse=True)
+    vel_max, vel_bone = vel[0][0], vel[0][1]
+    vel_mean = sum(v for v, *_ in vel) / len(vel)
+
+    # 3. root drift (pelvis world pos)
+    root = 'pelvis' if 'pelvis' in arm.pose.bones else arm.pose.bones[0].name
+    scene.frame_set(f0); p0 = (arm.matrix_world @ arm.pose.bones[root].head).copy()
+    scene.frame_set(f1); p1 = (arm.matrix_world @ arm.pose.bones[root].head).copy()
+    drift_cm = (p1 - p0).length * 100.0
+
+    ok_pose = pose_max <= a.pose_deg
+    ok_vel  = vel_max <= a.vel_deg
+    ok_drift = drift_cm <= a.drift_cm
+    smooth = ok_pose and ok_vel and ok_drift
+
+    print("=== LOOP QC ===")
+    print(f"clip={act.name}  frames={f0}..{f1}")
+    print(f"[{'PASS' if ok_pose else 'FAIL'}] pose gap   : max {pose_max:6.2f}° (limit {a.pose_deg})  mean {pose_mean:.2f}°  worst={pose_bone}")
+    print(f"[{'PASS' if ok_vel else 'FAIL'}] velocity gap: max {vel_max:6.2f}°/f (limit {a.vel_deg})  mean {vel_mean:.2f}  worst={vel_bone}")
+    print(f"[{'PASS' if ok_drift else 'FAIL'}] root drift  : {drift_cm:6.2f} cm (limit {a.drift_cm})  bone={root}")
+    print(f"\nVERDICT: {'SMOOTH ✓' if smooth else 'NOT SMOOTH ✗ — will pop on loop'}")
+
+    print(f"\nworst {a.top} bones by pose gap (deg end→start):")
+    for v, b in pose[:a.top]:
+        print(f"  {v:7.2f}  {b}")
+    print(f"\nworst {a.top} bones by velocity mismatch (deg/frame across seam):")
+    for v, b, vb, va in vel[:a.top]:
+        print(f"  {v:7.2f}  {b}  (before {vb:.2f} / after {va:.2f})")
+
+    sys.exit(0 if smooth else 1)
+
+if __name__ == "__main__":
+    try:
+        main()
+    except SystemExit:
+        raise
+    except Exception as e:
+        print(f"[loop_qc] ERROR: {e}")
+        sys.exit(2)