yolov5检测视频流的原理、detect.py解读

基于yolov5的谷物视频检测专栏链接：点我

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前言

yolov5可以检测视频，实验室为该实验配备了一个工业相机，并想使用该相机进行实时检测，为了能使用该相机，今天去看了下yolov5的源代码。记录下自己的体会。

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一、先从readme开始

detect.py runs inference on a variety of sources, downloading models automatically from the latest YOLOv5 release and saving results to runs/detect.

$ python detect.py --source 0  # webcam
                            file.jpg  # image 
                            file.mp4  # video
                            path/  # directory
                            path/*.jpg  # glob
                            'https://youtu.be/NUsoVlDFqZg'  # YouTube video
                            'rtsp://example.com/media.mp4'  # RTSP, RTMP, HTTP stream

上面的语句是yolov5的readme文件的教程。传送门

可以看出，调用摄像头使用的是–source 参数，到detect.py去查看该参数是如何使用的。

很明显是调用的是LoadStream()函数。

二、查看LoadStream()具体实现

class LoadStreams:  # multiple IP or RTSP cameras
    def __init__(self, sources='streams.txt', img_size=640, stride=32):
        self.mode = 'stream'
        self.img_size = img_size
        self.stride = stride
		# 如果参数为源的列表，则读取为一个列表
        if os.path.isfile(sources):
            with open(sources, 'r') as f:
                sources = [x.strip() for x in f.read().strip().splitlines() if len(x.strip())]
        else:
            sources = [sources]

        n = len(sources)
        self.imgs = [None] * n
        self.sources = [clean_str(x) for x in sources]  # clean source names for later
        # 遍历列表
        for i, s in enumerate(sources):
            # Start the thread to read frames from the video stream
            print(f'{i + 1}/{n}: {s}... ', end='')
            url = eval(s) if s.isnumeric() else s
            if 'youtube.com/' in url or 'youtu.be/' in url:  # if source is YouTube video
                check_requirements(('pafy', 'youtube_dl'))
                import pafy
                url = pafy.new(url).getbest(preftype="mp4").url
            # 使用opencv读取帧
            cap = cv2.VideoCapture(url)
            assert cap.isOpened(), f'Failed to open {s}'
            # 帧的宽高
            w = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
            h = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))
            self.fps = cap.get(cv2.CAP_PROP_FPS) % 100
			
            _, self.imgs[i] = cap.read()  # guarantee first frame
            # 使用多线程读取视频帧
            thread = Thread(target=self.update, args=([i, cap]), daemon=True)
            print(f' success ({w}x{h} at {self.fps:.2f} FPS).')
            thread.start()
        print('')  # newline

        # check for common shapes
        s = np.stack([letterbox(x, self.img_size, stride=self.stride)[0].shape for x in self.imgs], 0)  # shapes
        self.rect = np.unique(s, axis=0).shape[0] == 1  # rect inference if all shapes equal
        if not self.rect:
            print('WARNING: Different stream shapes detected. For optimal performance supply similarly-shaped streams.')

    def update(self, index, cap):
        # Read next stream frame in a daemon thread
        n = 0
        # 只要cap没有关闭，就是一直读取视频的帧
        while cap.isOpened():
            n += 1
            # _, self.imgs[index] = cap.read()
            # 从视频文件或捕获设备抓取下一帧。
            cap.grab()
            if n == 4:  # read every 4th frame
                # cap.retrieve() 解码并安返回抓取的视频帧
                success, im = cap.retrieve()
                self.imgs[index] = im if success else self.imgs[index] * 0
                n = 0
            time.sleep(1 / self.fps)  # wait time
	# 作为可迭代对象，必须实现下面两个方法
    def __iter__(self):
        self.count = -1
        return self
	
    def __next__(self):
        self.count += 1
        # 复制图片列表
        img0 = self.imgs.copy()
        # 按q退出
        if cv2.waitKey(1) == ord('q'):  # q to quit
            cv2.destroyAllWindows()
            raise StopIteration

        # Letterbox
        img = [letterbox(x, self.img_size, auto=self.rect, stride=self.stride)[0] for x in img0]

        # Stack
        img = np.stack(img, 0)

        # Convert
        img = img[:, :, :, ::-1].transpose(0, 3, 1, 2)  # BGR to RGB, to bsx3x416x416
        img = np.ascontiguousarray(img)

        return self.sources, img, img0, None

    def __len__(self):
        return 0  # 1E12 frames = 32 streams at 30 FPS for 30 years

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总结

对视频处理，工业相机需要使用第三方工具包来调用。