如何解决将数据写入pgm文件的错误

Question

当尝试将帧转换为灰度时如何解决错误？

错误消息：cv2.cvtColor（图像，cv2.COLOR_BGR2GRAY）cv2。错误：OpenCV（4.0.0）c：\ projects \ opencv-python \ opencv \ modules \ imgproc \ src \ color.hpp：259：错误：函数'__cdecl cv :: CvtHelper中的（-2：Unspecified error），结构cv :: Set <1，-1，-1>，结构cv :: Set <0,2,5>，2> :: CvtHelper （const class cv :: _ InputArray＆，const class cv :: _ OutputArray＆，int）'>输入图像中无效的通道数：>'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