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my code is written for streaming via UDP with send and rec as below
send.py

import socket
import numpy as np
import cv2 as cv
import sys
addr = ("127.0.0.1", 65534)
buf = 512
width = 640
height = 480
cap = cv.VideoCapture('test.mp4')
cap.set(3, width)
cap.set(4, height)
code = 'start'
code = ('start' + (buf - len(code)) * 'a').encode('utf-8')
np.set_printoptions(threshold=sys.maxsize)
lenght=bytearray(3)
encode_param = [int(cv.IMWRITE_JPEG_QUALITY), 90]
s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
frame=np.array([[[255, 255, 216], # i use a small array instead of frame from test video to debug
  [255, 255, 215],
  [255, 255, 210],
  [255, 255, 210],
  [255, 255 ,209],
  [255, 255, 209],
  [255, 255 ,213],
  [255, 255, 213],
  [255, 255, 208],
  [255, 255 ,208]]])
print('original')
print(frame) 
result, frame = cv.imencode('.jpg', frame, encode_param)
print('frame after decode')
print(frame)
print('length')
print(len(frame))             
frame2 = cv.imdecode(frame, cv.IMREAD_COLOR)#
print('frame2 test decode before sending')
print(frame2)
lenght[0]=len(frame)//(256**2)
lenght[1]=((len(frame)%(256**2))//256)
lenght[2]=((len(frame)%(256**2))%256)
data=lenght+bytearray(frame) 
for i in range(0, len(data), buf):
 s.sendto(data[i:i+buf], addr)
cv.imshow('send', frame2) 

rec.py

import socket
import numpy as np
import cv2 as cv
import sys
np.set_printoptions(threshold=sys.maxsize)
addr = ("127.0.0.1", 65534)
buf = 512
width = 640
height = 480
code = b'start'
num_of_chunks = width * height * 3 / buf
data = b""
if __name__ == '__main__':
    s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    s.bind(addr)
    while True:
        while len(data)<3:
         data+= s.recvfrom(buf)[0]
        length=data[0]*(256**2)+data[1]*256+data[2]
        data = data[3:]
        #print("msg_size: {}".format(length))
        while len(data) < length:
            data+= s.recvfrom(buf)[0]
            #data.append(chunk)
        frame_data = data[:length]
        data = data[length:]  
        frame=np.zeros((len(frame_data),1),dtype=int)
        for j in range(len(frame_data)):
         frame[j]=frame_data[j]
        print('frame collected before decode')
        print(frame)
        print('length')        
        print(len(frame))
        frame2 = cv.imdecode(frame, cv.IMREAD_COLOR)
        cv.imshow('recv', frame2)
        if cv.waitKey(1) & 0xFF == ord('q'):
            break

    s.close()
    cv.destroyAllWindows()

Now is the problem, the rec.py gets frame2 after decode is None. I thought my transmission is problematic so data is lost but i check "frame collected before decode"in rec.py and "'frame after encode" printed in send.py. they are identical, it means the data sent and collected properly. any opinion why decode gives None in my case?