获取摄像头最新帧

1. grab()函数【没奏效】

# 跳过无需处理的帧
grabbed = capture.grab()
if not grabbed:
	print("跳过帧失败")
	await send_message(websocket, "跳过帧失败 或 视频流已结束")
	url = await websocket.recv()
	break

# 获取最新的帧
retrieved, frame = capture.retrieve()
if not retrieved:
    print("检测画面失败")
    await send_message(websocket, "检测画面失败 或 视频流已结束")
    url = await websocket.recv()
    break

2. 多线程【没奏效】

import cv2
import threading
import sys


class RTSCapture(cv2.VideoCapture):
    _cur_frame = None
    _reading = False
    schemes = ["rtsp://","rtmp://"]
    @staticmethod
    def create(url, *schemes):
        rtscap = RTSCapture(url)
        rtscap.frame_receiver = threading.Thread(target=rtscap.recv_frame, daemon=True)
        rtscap.schemes.extend(schemes)
        if isinstance(url, str) and url.startswith(tuple(rtscap.schemes)):
            rtscap._reading = True
        elif isinstance(url, int):
            pass
        return rtscap

    def isStarted(self):
        ok = self.isOpened()
        if ok and self._reading:
            ok = self.frame_receiver.is_alive()
        return ok

    def recv_frame(self):
        while self._reading and self.isOpened():
            ok, frame = self.read()
            if not ok: break
            self._cur_frame = frame
        self._reading = False

    def read2(self):
        frame = self._cur_frame
        self._cur_frame = None
        return frame is not None, frame

    def start_read(self):
        self.frame_receiver.start()
        self.read_latest_frame = self.read2 if self._reading else self.read

    def stop_read(self):
        self._reading = False
        if self.frame_receiver.is_alive(): self.frame_receiver.join()


if __name__ == '__main__':
    if len(sys.argv) < 2:
        print("usage:")
        print("need rtsp://xxx")
        sys.exit()

    rtscap = RTSCapture.create(sys.argv[1])
    rtscap.start_read()

    while rtscap.isStarted():
        ok, frame = rtscap.read_latest_frame()
        if cv2.waitKey(100) & 0xFF == ord('q'):
            break
        if not ok:
            continue


        # inhere
        cv2.imshow("cam", frame)


    rtscap.stop_read()
    rtscap.release()
    cv2.destroyAllWindows()

3. 重新定义VideoCapture类。【奏效】

import cv2
import queue
import threading


class VideoCapture:
    def __init__(self, name):
        self.cap = cv2.VideoCapture(name)
        self.q = queue.Queue()
        t = threading.Thread(target=self._reader)
        t.daemon = True
        t.start()

    def _reader(self):
        while True:
            ret, frame = self.cap.read()
            if not ret:
                break
            if not self.q.empty():
                try:
                    self.q.get_nowait()
                except queue.Empty:
                    pass
            self.q.put(frame)

    def read(self):
        return self.q.get()

    def release(self):
        self.cap.release()
        self.q = queue.Queue()

    def get_resolution(self):
        return self.cap.get(cv2.CAP_PROP_FRAME_WIDTH),self.cap.get(cv2.CAP_PROP_FRAME_HEIGHT)

url = "xxx"
while True:
    frame = capture.read()
    if frame is None or len(frame) == 0:
		print("画面检测失败 或 视频流已结束")
        break
    width, height = capture.get_resolution()
    frame = cv2.cvtColor(frame, cv2.COLOR_BGR2RGB)
    frame = Image.fromarray(np.uint8(frame))
    # …… 算法