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尝试将帧转换为灰度时如何修复错误？

错误信息：cv2.cvtColor(image, cv2.COLOR_BGR2GRAY) cv2.error: OpenCV(4.0.0) c:\projects\opencv-python\opencv\modules\imgproc\src\color.hpp:259: error: (- 2：未指定的错误）函数'__cdecl cv::CvtHelper,struct cv::Set<1,-1,-1>,struct cv::Set<0,2,5>,2>::CvtHelper(const class cv::_InputArray &,const class cv::_OutputArray &,int)' > 输入图像中的通道数无效：> 'VScn::contains(scn)' > 其中 > 'scn' 为 1

class OpenCVCapture(object):
def __init__(self, device_id=0):
    """Create an OpenCV capture object associated with the provided webcam
    device ID.
    """
    # Open the camera.
    self._camera = cv2.VideoCapture(device_id)
    if not self._camera.isOpened():
        self._camera.open()
    # Start a thread to continuously capture frames.
    # This must be done because different layers of buffering in the webcam
    # and OS drivers will cause you to retrieve old frames if they aren't 
    # continuously read.
    self._capture_frame = None
    # Use a lock to prevent access concurrent access to the camera.
    self._capture_lock = threading.Lock()
    self._capture_thread = threading.Thread(target=self._grab_frames)
    self._capture_thread.daemon = True
    self._capture_thread.start()

def _grab_frames(self):
    while True:
        retval, frame = self._camera.read()
        frame = cv2.cvtColor(frame, cv2.COLOR_RGB2GRAY)
        with self._capture_lock:
            self._capture_frame = None
            if retval:
                self._capture_frame = frame
        time.sleep(1.0/CAPTURE_HZ)

def read(self):
    """Read a single frame from the camera and return the data as an OpenCV
    image (which is a numpy array).
    """
    frame = None
    with self._capture_lock:
        frame = self._capture_frame

    # If there are problems, keep retrying until an image can be read.
    while frame is None:
        time.sleep(0)
        with self._capture_lock:
            frame = self._capture_frame
    # Save captured image for debugging.
    cv2.imwrite(config.DEBUG_IMAGE, frame)
    # Return the capture image data.
    return frame
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