python3從網絡攝像機解析mjpeg http流

前言
網絡攝像頭的視頻流解析直接使用通過http的Mjpeg是具有邊界幀信息的multipart / x-mixed-replace，而jpeg數據只是以二進制形式發送。因此，實際上不需要關心HTTP協議標頭。所有jpeg幀均以marker開頭，0xff 0xd8並以結尾0xff 0xd9。因此，上面的代碼從http流中提取了此類幀，並將其一一解碼。像下面

...(http)
0xff 0xd8      --|
[jpeg data]      |--this part is extracted and decoded
0xff 0xd9      --|
...(http)
0xff 0xd8      --|
[jpeg data]      |--this part is extracted and decoded
0xff 0xd9      --|
...(http)

如果圖像的獲取是從tcp網絡中傳輸到本地進行解析需要對bytes類型數據進行解碼

在使用OpenCV直接調用網絡攝像頭時可能會出現

Cam not found
這時候就需要下面這種辦法

代碼： 
幀解析

import cv2
cap = cv2.VideoCapture('http://localhost:8080/frame.mjpg')
 
while True:
  ret, frame = cap.read()
  print(frame)
  if ret == True:
    cv2.imshow('Video', frame)
 
    if cv2.waitKey(1) == 27:
      exit(0)

視頻流解析

import cv2
import requests
import numpy as np
 
r = requests.get('http://192.168.1.xx/mjpeg.cgi', auth=('user', 'password'), stream=True)
if(r.status_code == 200):
    bytes = bytes()
    for chunk in r.iter_content(chunk_size=1024):
        bytes += chunk
        a = bytes.find(b'\xff\xd8')
        b = bytes.find(b'\xff\xd9')
        if a != -1 and b != -1:
            jpg = bytes[a:b+2]
            bytes = bytes[b+2:]
            i = cv2.imdecode(np.fromstring(jpg, dtype=np.uint8), cv2.IMREAD_COLOR)
            cv2.imshow('i', i)
            if cv2.waitKey(1) == 27:
                exit(0)
else:
    print("Received unexpected status code {}".format(r.status_code))