python - Python OpenCV : Mutithreading with opecv video streaming

Question

我想与opencv视频流一起运行一个多线程。如果在视频不断流传输的过程中检测到对象3秒钟，我想激活GPIO。我尝试使用多线程(join方法)，但视频在线程调用期间暂停，因为它具有time.sleep()。有什么方法可以持续并行地运行线程流视频？下面是具有相同方式的代码。如果我删除连接，那么time.sleep根本不起作用。

import threading 
import time
import numpy as np
import cv2

def print_hello():
    print("Hello")
    time.sleep(3)
    print ("World")

t1 = threading.Thread(target=print_hello)  

cap = cv2.VideoCapture(0)

while(True):
    # Capture frame-by-frame
    ret, frame = cap.read()

    # Our operations on the frame come here
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # Display the resulting frame
    cv2.imshow('frame',gray)

    t1.start()
    t1.join()

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

# When everything done, release the capture
cap.release()
cv2.destroyAllWindows()

Answer 1

.join()等待线程结束并阻塞代码-因此它不发送任何内容以在循环内运行它，但是您应该在循环后或程序结束时运行它
另一个问题是循环中的.start()，因为.start()只能运行一次线程，因此在循环中多次使用它会产生错误。
您可以在循环之前启动线程，并在线程内部运行一些循环以使其始终运行。

import threading 
import time
import numpy as np
import cv2

# --- functions ---

running = True

def print_hello():
    while running:
        print("Hello World")
        time.sleep(3)

# --- main ---

t1 = threading.Thread(target=print_hello)  
t1.start()

# --- loop ---

cap = cv2.VideoCapture(0)

while True:
    # Capture frame-by-frame
    ret, frame = cap.read()

    # Our operations on the frame come here
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # Display the resulting frame
    cv2.imshow('frame',gray)

    if cv2.waitKey(1) & 0xFF == ord('q'):
        break

# --- after loop ---

running = False # to stop loop in thread
t1.join()

cap.release()
cv2.destroyAllWindows()

如果必须在循环中启动线程，则还必须在循环中创建新线程。
在此示例中，我使用键 t来启动新线程-否则，它将在每个循环中创建新线程，因此它将在短时间内创建数百个线程，因此没有任何意义。

import threading 
import time
import numpy as np
import cv2

# --- functions ---

def print_hello():
    print("Hello")
    time.sleep(3)
    print("World")

# --- main ---

all_threads = []

# --- loop ---

cap = cv2.VideoCapture(0)

while True:
    # Capture frame-by-frame
    ret, frame = cap.read()

    # Our operations on the frame come here
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # Display the resulting frame
    cv2.imshow('frame',gray)

    key = cv2.waitKey(1) & 0xFF
    if key == ord('q'):
        break
    if key == ord('t'):
        t = threading.Thread(target=print_hello)  
        t.start()
        all_threads.append(t)

# --- after loop ---

for t in all_threads:
    t.join()

cap.release()
cv2.destroyAllWindows()

但是，即使多次按 t，您也可以同时创建多个线程，它们将一起工作。如果您不需要它，则必须控制威胁是否仍在起作用，并仅在不再起作用时才创建新威胁-使用 is_alive()-这样可以使其变得更加复杂。

import threading 
import time
import numpy as np
import cv2

# --- functions ---

def print_hello():
    print("Hello")
    time.sleep(3)
    print("World")

# --- main ---

t = None

# --- loop ---

cap = cv2.VideoCapture(0)

while True:
    # Capture frame-by-frame
    ret, frame = cap.read()

    # Our operations on the frame come here
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)

    # Display the resulting frame
    cv2.imshow('frame',gray)

    key = cv2.waitKey(1) & 0xFF
    if key == ord('q'):
        break

    if key == ord('t'):
        if t is None or not t.is_alive():
            t = threading.Thread(target=print_hello)  
            t.start()
        else:
            print('previous thread is still running')
            
# --- after loop ---

if t is not None:
    t.join()

cap.release()
cv2.destroyAllWindows()