使用 opencv 线程捕获空白图像 其他实验：

问题描述

我正在尝试将从笔记本电脑相机捕获的帧写入循环缓冲区。我想创建一个线程来处理这个任务（以异步方式）。当我尝试使用 get_last() 方法存储最后一帧并显示它时，显示黑屏。我花了很多时间找出原因，但一切都在脉络中。恕我直言，这段代码应该有一个小问题，但看不到它。任何帮助将不胜感激。完整的工作代码如下所示：

import cv2
import sys
import time
import threading

class CamBuffer(threading.Thread):
    def __init__(self,stream=0):
        self.current = 0
        self.max_size = 5
        self.buffer = [None] * self.max_size
        self.capture = cv2.VideoCapture(stream)
        if not self.capture.isOpened():
            print("Could not open video")
            sys.exit()

        print("Opened video")
        self.running = True
        super().__init__()

    def run(self):
        print('running thread')
        while self.running:
            print('capturing frame ',self.current)
            _,frame = self.capture.read()
            if _:
                print('saving frame ',self.current)
                self.buffer[self.current] = frame
                self.current = self.current + 1
                if (self.current >= self.max_size):
                    self.current = 0
        self.capture.release()
        print('stopped thread')

    def terminate(self):
        print('terminating thread')
        self.running = False

    def get_last(self):
        current = 0
        if self.current > 0:
            current = self.current - 1

        print('get_last()',current)
        return self.buffer[current]
        

if __name__ == "__main__":
    print('Frame buffer test')
    stream = 0
    cb = CamBuffer(stream)
    cb.start()
    time.sleep(1.25)

    frame = cb.get_last()
    if frame is not None:
        print('showing frame')
        cv2.imshow('Frame',frame)

    time.sleep(3)
    cb.terminate()
    cv2.destroyAllWindows()
    print('Frame buffer test [Done]')

解决方法

对我有用的最简单的解决方案就是在 imshow 之后添加 cv.waitKey(1)：

 if frame is not None:
        print('showing frame')
        cv2.imshow('Frame',frame)
        cv2.waitKey(1):

我认为这是 Opencv highgui 模块的一个问题——它是一个 GUI，也有自己的线程，有时它可能会挂起应用程序。尝试过快更新 Highgui 窗口/cv2.imshow() 时出现灰屏，我在 C++ 中也遇到过。我猜是关于锁定 GUI 线程和图像的内存。

顺便说一句，您还可以在捕获线程中添加等待（如果您不需要最大帧率）。

PS. 在意识到可以用那条线解决之前，我尝试了其他可能的问题：lock.acquire/release、全局缓冲区、发送缓冲区容器作为参数。然而，全局也显示灰色屏幕（尽管在捕获过程中它显示了捕获的图像）。当我添加 cv2.waitKey(...) 时，它终于使用了作为参数发送的缓冲区，我意识到它可能已经足够了。

其他实验：

import cv2
import sys
import time
import threading

max_size = 5
buffer = [None] * max_size
frCopyGlobal = None
buff = [None] * max_size #call as param


class CamBuffer(threading.Thread):
    def __init__(self,buff,stream=0):
        self.current = 0
        self.max_size = 5
        self.buffer = [None] * self.max_size
        self.capture = cv2.VideoCapture(stream,apiPreference=cv2.CAP_DSHOW)
        if not self.capture.isOpened():
            print("Could not open video")
            sys.exit()

        print("Opened video")
        self.running = True
        super().__init__()

    def run(self):
        print('running thread')
        
        while self.running:        
            #lock.acquire()
            print('capturing frame ',self.current)
            _,frame = self.capture.read()
            cv2.imshow("F",frame)
            if _:
                #print('saving frame ',self.current%self.max_size,self.current)
                print('saving frame ',self.current)                
                frCopy = frame.copy()
                self.buffer[self.current] = frCopy
                
                frCopyGlobal = frame.copy()
                buffer[self.current] = frCopyGlobal                
                
                buff[self.current] = frame.copy()
                
                #self.buffer[self.current%self.max_size] = frame
                #cv2.imshow("FBUFF",self.buffer[self.current%self.max_size])
                cv2.imshow("FBUFF",self.buffer[self.current])
                cv2.imshow("GLOBAL BUFF",buffer[self.current])
                cv2.imshow("Param BUFF",buff[self.current])
                self.current = self.current + 1
                if (self.current >= self.max_size):
                    self.current = 0
            cv2.waitKey(66)
            #lock.release()
        self.capture.release()
        print('stopped thread')

    def terminate(self):
        print('terminating thread')
        self.running = False

    def get_last(self):
        current = 0
        if self.current > 0:
            current = self.current - 1

        print('get_last()',current)
        for i in self.buffer:
          cv2.imshow("THREAD: "+str(i),i)
        for i in buffer:
          cv2.imshow("GLOBAL: "+str(i),i)
        return self.buffer[current]
        

if __name__ == "__main__":
    lock = threading.Lock() #threading.Lock lock();
    print('Frame buffer test')
    stream = 0    
    cb = CamBuffer(buff,stream) # The buffer must be in common memory in order to read it from the calling thread
    cb.start()
    time.sleep(1.0)
    for i in range(10):
      try:
        cv2.imshow("SAMPLE?",buff[1])
      finally:
              pass
      cv2.waitKey(15)
    time.sleep(1.25)
    #cb.join()
    #cb.sleep(1.)
    #cb.terminate()
    #frame = cb.get_last()
    cb.running = False
    ###lock.acquire()
    #frame = cb.buffer[0]
    frame = buff[0]    
    #frame = buff[0]    
    frame = cb.buffer[0]
    if frame is not None:
        print('Showing frame from Thread')
        cv2.imshow('PARAMETER BUFFER?',frame)
    cv2.waitKey(500)
    #frame = buffer[0] #Global
    #if frame is not None:
    #    print('showing frame From GLOBAL')
    #    cv2.imshow('GLOBAL FRAME',frame)
        
    ###lock.release()
    time.sleep(3)
    cb.terminate()    
    cv2.destroyAllWindows()
    
    print('Frame buffer test [Done]')
    cb.join()
    
    #exit(0)