无法启动 python 多处理。处理两次

Question

Below given is my code. I am trying to scan barcodes and display it using OpenCV. The program works well but there is a huge lag in fps when grabbing frames from drone camera as RTMP stream. Due to the same I am trying to use multi processing method.

import pandas as pd
import cv2
import numpy as np
from pyzbar.pyzbar import decode
from pyzbar.pyzbar import ZBarSymbol
import time 
import multiprocessing 
global frame
def barcode(frame):
    for barcode in decode(frame, symbols=[ZBarSymbol.CODE128]):
        myData = barcode.data.decode('utf-8')
        pts = np.array([barcode.polygon],np.int32)
        pts = pts.reshape((-1,1,2))
        cv2.polylines(frame, [pts], True, (255,0,255),5)
        pts2 = barcode.rect
        akash = []
        akash.append(myData)
        cv2.putText(frame, myData, (pts2[0], pts2[1]), cv2.FONT_HERSHEY_SIMPLEX, 0.9, (255,99,71), 2)

p1 = multiprocessing.Process(target = barcode)        

cv2.namedWindow("Result", cv2.WINDOW_NORMAL)

vid = cv2.VideoCapture(0)
if __name__ == '__main__':
    while(True):
        ret, frame = vid.read()
        if frame is not None:
            p1.start()   
            cv2.imshow('Result',frame)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            break
vid.release()
vid.destroyAllWindows()

and the error is

AssertionError                            Traceback (most recent call last)
<ipython-input-1-df50d7c70cda> in <module>
     27         ret, frame = vid.read()
     28         if frame is not None:
---> 29             p1.start()
     30             cv2.imshow('Result',frame)
     31         if cv2.waitKey(1) & 0xFF == ord('q'):

C:\ProgramData\Anaconda3\lib\multiprocessing\process.py in start(self)
    113         '''
    114         self._check_closed()
--> 115         assert self._popen is None, 'cannot start a process twice'
    116         assert self._parent_pid == os.getpid(), \
    117                'can only start a process object created by current process'

AssertionError: cannot start a process twice

Answer 1

Try not to create processes inside loops. The best way to use processes is to create n processes outside and then, with the help of Queues , access and push data.

• In the following code, I created 5 processes which would run infinitely and try to fetch data from inQ queue.
• Then do all the processing that you were doing.
• After that, I'm pushing it to outQ queue, which we'll use later to show the results.
• In the main, I am simply reading the data from the opencv vid object and the pushing to the inQ which our Processes will use to fetch frame.
• Next, I'm just fetching the results. This way appears better to me as we don't have to create processes in every iteration as well as we have multiple processes ready to process the data at all times. You can also set the buffer limit for the queue if you want. Also, with live streams, try to have a skipFrame parameter to skip a few frames. That would boost up the fps.

import cv2
import numpy as np
from pyzbar.pyzbar import decode
from pyzbar.pyzbar import ZBarSymbol
import time 
from multiprocessing import Process, Queue

inQ = Queue()
outQ = Queue()


def barcode():
    global inQ
    global outQ
    try:
        print("Solving..")
        frame = inQ.get()
        for barcode in decode(frame, symbols=[ZBarSymbol.CODE128]):
            myData = barcode.data.decode('utf-8')
            pts = np.array([barcode.polygon],np.int32)
            pts = pts.reshape((-1,1,2))
            cv2.polylines(frame, [pts], True, (255,0,255),5)
            pts2 = barcode.rect
            akash = []
            akash.append(myData)
            cv2.putText(frame, myData, (pts2[0], pts2[1]), cv2.FONT_HERSHEY_SIMPLEX, 0.9, (255,99,71), 2)

            outQ.put(frame)
    except Exception as e:
        print(e)


for _ in range(5):  # configure yourself      
    Process(target = barcode).start()        

cv2.namedWindow("Result", cv2.WINDOW_NORMAL)


if __name__ == '__main__':
    print("Inside main")
    vid = cv2.VideoCapture(0)
    while vid.isOpened():
        print("While...")
        ret, frame = vid.read()
        if ret:
            try:
                inQ.put(frame)
            except Exception as e:
                print(e)
        try:
            output = outQ.get()
            cv2.imshow("Result", output)
        except Exception as e:
            print(e)

    vid.release()
    vid.destroyAllWindows()