使用detectMultiscale模块时的OpenCV错误

Question

如果尝试使用detectMultiscale模块，我会报错...

import cv2

def take_photo():
    cap = cv2.VideoCapture(0)
    ret, frame = cap.read()
    image = cv2.cvtColor(frame, cv2.IMREAD_COLOR)
    return image

image = take_photo()
gray_image = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)

haar_face_cascade = cv2.CascadeClassifier('data/haarcascade_frontalface_alt.xml')
faces = haar_face_cascade.detectMultiScale(gray_image, scaleFactor=1.1, minNeighbors=5)
print('Faces found: ', len(faces))

这是我得到的错误：

cv2.error: OpenCV(4.1.0) C:\projects\opencv-python\opencv\modules\objdetect\src\cascadedetect.cpp:1658: error: (-215:Assertion failed) !empty() in function 'cv::CascadeClassifier::detectMultiScale'