python3从网络摄像机解析mjpeg http流

前言
网络摄像头的视频流解析直接使用通过http的Mjpeg是具有边界帧信息的multipart / x-mixed-replace，而jpeg数据只是以二进制形式发送。因此，实际上不需要关心HTTP协议标头。所有jpeg帧均以marker开头，0xff 0xd8并以结尾0xff 0xd9。因此，上面的代码从http流中提取了此类帧，并将其一一解码。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)

如果图像的获取是从tcp网络中传输到本地进行解析需要对bytes类型数据进行解码

在使用OpenCV直接调用网络摄像头时可能会出现

Cam not found
这时候就需要下面这种办法

代码： 
帧解析

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)

视频流解析

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'xffxd8')
        b = bytes.find(b'xffxd9')
        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))