webカメラを用いたプログラムを実行したい！

前提・実現したいこと

関数上でメインとなるプログラムを作成して、if name=="main":上で関数を呼び、プログラムを実行したいです。

発生している問題・エラーメッセージ

Traceback (most recent call last):
  File "##.py", line 47, in <module>
    camera()
  File "##.py", line 13, in camera
    frame = cv2.resize(frame, (int(frame.shape[1]/4), int(frame.shape[0]/4)))
AttributeError: 'NoneType' object has no attribute 'shape'

該当のソースコード

python
1-*- coding:utf-8 -*-
2
3import cv2
4
5   def camera():
6       cap = cv2.VideoCapture(0)
7       before = None
8       while True:
9           ret, frame = cap.read()
10           frame = cv2.resize(frame, (int(frame.shape[1]/4), int(frame.shape[0]/4)))
11           cv2.imshow('Raw Frame', frame)
12           gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
13           if before is None:
14               before = gray.copy().astype('float')
15               continue
16           cv2.accumulateWeighted(gray, before, 0.5)
17           mdframe = cv2.absdiff(gray, cv2.convertScaleAbs(before))
18           cv2.imshow('MotionDetected Frame', mdframe)
19           thresh = cv2.threshold(mdframe, 3, 255, cv2.THRESH_BINARY)[1]
20           image, contours, hierarchy = cv2.findContours(thresh.copy(), cv2.RETR_EXTERNAL, cv2.CHAIN_APPROX_SIMPLE)
21           max_area = 0
22           target = contours[0]
23           for cnt in contours:
24               area = cv2.contourArea(cnt)
25               if max_area < area and area < 10000 and area > 1000:
26                   max_area = area;
27                   target = cnt
28
29               if max_area <= 1000:
30                       areaframe = frame
31                       cv2.putText(areaframe, 'not detected', (0,50), cv2.FONT_HERSHEY_PLAIN, 3, (0, 255,0), 3, cv2.LINE_AA)
32
33               else:
34                   x,y,w,h = cv2.boundingRect(target)
35                   areaframe = cv2.rectangle(frame,(x,y),(x+w,y+h),(0,255,0),2)
36                   cv2.imshow('MotionDetected Area Frame', areaframe)
37                   k = cv2.waitKey(1)
38                   if k == 27:
39                       break
40                   cap.release()
41                   cv2.destroyAllWindows()
42
43
44if __name__=="__main__":
45    camera()