Apply random brightness jitter at synthesis time

Per-plate HSV V scaling in [0.55, 1.45] for cheap color diversity. Heavy
geometric augmentation belongs in the training transforms (next step) so
each epoch sees a different version, not in synthesis.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

data_gen/generate_synthetic.py

@@ -50,6 +50,13 @@ REGION_MAP = {
 }
 
 
+def random_bright(img: np.ndarray, scale_range=(0.55, 1.45)) -> np.ndarray:
+    """HSV V 채널 곱하기로 plate 전체 밝기 무작위 변경. 색상 다양성용."""
+    hsv = cv2.cvtColor(img, cv2.COLOR_BGR2HSV).astype(np.float32)
+    hsv[:, :, 2] = np.clip(hsv[:, :, 2] * random.uniform(*scale_range), 0, 255)
+    return cv2.cvtColor(hsv.astype(np.uint8), cv2.COLOR_HSV2BGR)
+
+
 class LPGenerator:
     def __init__(self, asset_dir: Path):
         self.asset = Path(asset_dir)
@@ -189,6 +196,7 @@ def main():
     p.add_argument("--types", default="1,2,3,4", help="포함할 type (콤마 구분)")
     p.add_argument("--type_weights", default=None,
                    help="--types 순서대로 콤마 구분 가중치. 미지정 시 한국 도로 분포 기본값 사용.")
+    p.add_argument("--no_bright", action="store_true", help="random_bright 끄기 (디버깅용)")
     p.add_argument("--dict", default=None, help="검증용 dict 경로 (선택)")
     p.add_argument("--seed", type=int, default=42)
     args = p.parse_args()
@@ -240,6 +248,8 @@ def main():
                 plate, top, bot = fn()
                 label = make_label_two_line(plate, top, bot)
                 seen_chars.update(top); seen_chars.update(bot)
+            if not args.no_bright:
+                plate = random_bright(plate)
             fname = f"{i:06d}.jpg"
             cv2.imwrite(str(img_dir / fname), plate)
             records.append((f"images/{fname}", json.dumps(label, ensure_ascii=False)))