diff --git a/mv-and-ip/car_plate.py b/mv-and-ip/car_plate.py
index c477de2..475ef90 100644
--- a/mv-and-ip/car_plate.py
+++ b/mv-and-ip/car_plate.py
@@ -47,8 +47,12 @@ def _uniform_car_plate(img: cv.typing.MatLike) -> cv.typing.MatLike:
 
 @dataclass
 class CarPlateHsvBoundary:
+    """HSV boundary for car plate color detection."""
+
     lower_bound: cv.typing.MatLike
+    """Lower bound of HSV range for car plate color detection."""
     upper_bound: cv.typing.MatLike
+    """Upper bound of HSV range for car plate color detection."""
     need_revert: bool
     """是否取反黑白颜色，因为蓝牌和黄牌的操作正好是反的"""
 
@@ -65,7 +69,10 @@ CAR_PLATE_HSV_BOUNDARIES: tuple[CarPlateHsvBoundary, ...] = (
 
 @dataclass
 class CarPlateMask:
+    """Car plate mask result."""
+
     mask: cv.typing.MatLike
+    """The masked image in U8 format."""
     need_revert: bool
     """是否对颜色取反，与CarPlateHsvBoundary中的同名字段含义一致"""
 
@@ -73,7 +80,13 @@ class CarPlateMask:
 def _batchly_mask_car_plate(
     hsv: cv.typing.MatLike,
 ) -> typing.Iterator[CarPlateMask]:
-    """ """
+    """
+    Iterate over each car plate HSV boundary and apply mask to the given HSV image.
+
+    :param hsv: The HSV image to apply mask.
+    :return: An iterator of CarPlateMask.
+    """
+
     for boundary in CAR_PLATE_HSV_BOUNDARIES:
         # 以给定HSV范围检测符合该颜色的位置
         mask = cv.inRange(hsv, boundary.lower_bound, boundary.upper_bound)
@@ -90,6 +103,8 @@ def _batchly_mask_car_plate(
 
 @dataclass
 class CarPlateRegion:
+    """Car plate region result."""
+
     x: int
     y: int
     w: int
@@ -99,34 +114,46 @@ class CarPlateRegion:
 
 
 MIN_AREA: float = 3000
+"""Minimum area for car plate region."""
 MIN_ASPECT_RATIO: float = 1.5
+"""Minimum aspect ratio for car plate region."""
 MAX_ASPECT_RATIO: float = 6.0
+"""Maximum aspect ratio for car plate region."""
 BEST_ASPECT_RATIO: float = 3.5
+"""Best aspect ratio for car plate region."""
 
 
 def _analyse_car_plate_connection(
-    mask: CarPlateMask,
+    masks: typing.Iterator[CarPlateMask],
 ) -> typing.Optional[CarPlateRegion]:
-    # 连通域分析，筛选最符合车牌长宽比的区域
-    num_labels, labels, stats, _ = cv.connectedComponentsWithStats(
-        mask.mask, connectivity=8
-    )
+    """
+    Analyse car plate connection in given masks.
+
+    :param masks: An iterator of CarPlateMask to analyse.
+    :return: The car plate region if succeed, otherwise None.
+    """
 
     best: typing.Optional[CarPlateRegion] = None
     best_score = 0
 
-    for i in range(1, num_labels):
-        x, y, w, h, area = stats[i]
-        # 检查面积
-        if area < MIN_AREA:
-            continue
-        # 标准车牌宽高比约 3:1 ~ 5:1
-        ratio = w / (h + 1e-5)
-        if ratio >= MIN_ASPECT_RATIO and ratio <= MAX_ASPECT_RATIO:
-            score = area * (1 - abs(ratio - BEST_ASPECT_RATIO) / BEST_ASPECT_RATIO)
-            if score > best_score:
-                best_score = score
-                best = CarPlateRegion(x, y, w, h, mask.need_revert)
+    for mask in masks:
+        # 连通域分析，筛选最符合车牌长宽比的区域
+        num_labels, labels, stats, _ = cv.connectedComponentsWithStats(
+            mask.mask, connectivity=8
+        )
+
+        for i in range(1, num_labels):
+            x, y, w, h, area = stats[i]
+            # 检查面积
+            if area < MIN_AREA:
+                continue
+            # 标准车牌宽高比约 3:1 ~ 5:1
+            ratio = w / (h + 1e-5)
+            if ratio >= MIN_ASPECT_RATIO and ratio <= MAX_ASPECT_RATIO:
+                score = area * (1 - abs(ratio - BEST_ASPECT_RATIO) / BEST_ASPECT_RATIO)
+                if score > best_score:
+                    best_score = score
+                    best = CarPlateRegion(x, y, w, h, mask.need_revert)
 
     return best
 
@@ -138,20 +165,15 @@ def extract_car_plate(img: cv.typing.MatLike) -> typing.Optional[cv.typing.MatLi
     :param img: The image containing car plate in BGR format.
     :return: The image of binary car plate in U8 format if succeed, otherwise None.
     """
+    # 统一图片大小
     img = _uniform_car_plate(img)
 
     # 转换到HSV空间
     hsv = cv.cvtColor(img, cv.COLOR_BGR2HSV)
 
     # 利用车牌颜色在 HSV 空间定位车牌
-    candidate: typing.Optional[CarPlateRegion] = None
-    for mask in _batchly_mask_car_plate(hsv):
-        # 连通域分析，筛选最符合车牌长宽比的区域作为车牌
-        candidate = _analyse_car_plate_connection(mask)
-        # 找到任意一个就退出
-        if candidate is not None:
-            break
-
+    masks = _batchly_mask_car_plate(hsv)
+    candidate = _analyse_car_plate_connection(masks)
     if candidate is None:
         logging.error("Can not find any car plate.")
         return None