diff --git a/ppocr/postprocess/db_postprocess.py b/ppocr/postprocess/db_postprocess.py
index 0792cde0..b04c8bc3 100644
--- a/ppocr/postprocess/db_postprocess.py
+++ b/ppocr/postprocess/db_postprocess.py
@@ -24,6 +24,7 @@ import string
 import cv2
 from shapely.geometry import Polygon
 import pyclipper
+from copy import deepcopy
 
 
 class DBPostProcess(object):
@@ -39,13 +40,15 @@ class DBPostProcess(object):
         self.min_size = 3
         self.dilation_kernel = np.array([[1, 1], [1, 1]])
 
-    def boxes_from_bitmap(self, pred, _bitmap, dest_width, dest_height):
-        '''
-        _bitmap: single map with shape (1, H, W),
-                whose values are binarized as {0, 1}
-        '''
-
-        bitmap = _bitmap
+    def boxes_from_bitmap(self, pred, mask):
+        """
+        Get boxes from the binarized image predicted by DB.
+        :param pred: the binarized image predicted by DB.
+        :param mask: new 'pred' after threshold filtering.
+        :return: (boxes, the score of each boxes)
+        """
+        dest_height, dest_width = pred.shape[-2:]
+        bitmap = deepcopy(mask)
         height, width = bitmap.shape
 
         outs = cv2.findContours((bitmap * 255).astype(np.uint8), cv2.RETR_LIST,
@@ -87,6 +90,11 @@ class DBPostProcess(object):
         return boxes, scores
 
     def unclip(self, box):
+        """
+        Shrink or expand the boxaccording to 'unclip_ratio'
+        :param box: The predicted box.
+        :return: uncliped box
+        """
         unclip_ratio = self.unclip_ratio
         poly = Polygon(box)
         distance = poly.area * unclip_ratio / poly.length
@@ -96,6 +104,11 @@ class DBPostProcess(object):
         return expanded
 
     def get_mini_boxes(self, contour):
+        """
+        Get boxes from the contour or box.
+        :param contour: The predicted contour.
+        :return: The predicted box.
+        """
         bounding_box = cv2.minAreaRect(contour)
         points = sorted(list(cv2.boxPoints(bounding_box)), key=lambda x: x[0])
 
@@ -119,6 +132,12 @@ class DBPostProcess(object):
         return box, min(bounding_box[1])
 
     def box_score_fast(self, bitmap, _box):
+        """
+        Calculate the score of box.
+        :param bitmap: The binarized image predicted by DB.
+        :param _box: The predicted box
+        :return: score
+        """
         h, w = bitmap.shape[:2]
         box = _box.copy()
         xmin = np.clip(np.floor(box[:, 0].min()).astype(np.int), 0, w - 1)
@@ -137,13 +156,14 @@ class DBPostProcess(object):
 
         pred = pred[:, 0, :, :]
         segmentation = pred > self.thresh
-
         boxes_batch = []
         for batch_index in range(pred.shape[0]):
-            height, width = pred.shape[-2:]
 
-            mask = cv2.dilate(np.array(segmentation[batch_index]).astype(np.uint8), self.dilation_kernel)
-            tmp_boxes, tmp_scores = self.boxes_from_bitmap(pred[batch_index], mask, width, height)
+            mask = cv2.dilate(
+                np.array(segmentation[batch_index]).astype(np.uint8),
+                self.dilation_kernel)
+            tmp_boxes, tmp_scores = self.boxes_from_bitmap(pred[batch_index],
+                                                           mask)
 
             boxes = []
             for k in range(len(tmp_boxes)):