错误:(-215:Assertion failed):empty() in function 'cv::CascadeClassifier::detectMultiScale'

Question

I am trying out Face-detection using "Open-CV" by python. And the program is throwing me an error that I couldn't understand as a python beginner. The Error from Program:

Traceback (most recent call last):
  File "c:\Users\(User's name)\Documents\Python\Face-dection\Facedectection.py", line 6, in <module>
    gray_img=cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
cv2.error: OpenCV(4.5.1) C:\Users\appveyor\AppData\Local\Temp\1\pip-req-build-wvn_it83\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor' 

The code:

import cv2

face_cascade=cv2.CascadeClassifier("haarcascade_frontalface_default.xml")

img = cv2.imread("photo.jpg")
gray_img=cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)

faces=face_cascade.detectMultiScale(gray_img, scaleFactor=1.05,minNeighbors=5)

for x, y, w, h in faces:
    img=cv2.rectangle(img,(x,y),(x+w,y+h),(0,255,0),3)

resized=cv2.resize(img,(int(img.shape[1]/3)),(int(img.shape[0]/3)))

cv2.imshow("Deteced-face",resized)
cv2.waitKey(0)
cv2.destroyAllWindows()

Note: Photo and the.xml file is placed in the same folder

Answer 1

You need to add cv2.data.haarcascades before referencing to the .xml file.

#[...]

face_cascade=cv2.CascadeClassifier(cv2.data.haarcascades + "haarcascade_frontalface_default.xml") #Note the change

#[...]

Also there's one more problem in your code, have a look at this block of code:

resized=cv2.resize(img,(int(img.shape[1]/3)),(int(img.shape[0]/3)))

You're supposed to pass an image and it's new width and height (in a tuple format) as the arguments but you're passing the image and the width and height as separate integers and not in a tuple format.

This is how it should look like:

resized=cv2.resize(img,(int(img.shape[1]/3), int(img.shape[0]/3))) 
'''
Notice I have removed an extra bracket after
cv2.resize(img,(int(img.shape[1]/3)) -> cv2.resize(img,(int(img.shape[1]/3)...
and also I have removed an extra bracket before
(int(img.shape[0]/3))) -> int(img.shape[0]/3)))
'''

This should work:

import cv2

face_cascade=cv2.CascadeClassifier(cv2.data.haarcascades + "haarcascade_frontalface_default.xml") #Note the change

img = cv2.imread("photo.jpg")
gray_img=cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)

faces=face_cascade.detectMultiScale(gray_img, scaleFactor=1.05,minNeighbors=5)

for x, y, w, h in faces:
    img=cv2.rectangle(img,(x,y),(x+w,y+h),(0,255,0),3)

resized=cv2.resize(img,(int(img.shape[1]/3), int(img.shape[0]/3))) 

cv2.imshow("Deteced-face",resized)
cv2.waitKey(0)
cv2.destroyAllWindows()