error: (-215) !empty() in function detectMultiScale

Question

I had the same issue.

I didn't need to download anything else to solve this. CV2 had everything I needed.

Instead of trying to figure out where the .xml files are and hard coding the values, I used a property given by cv2.

From OP

face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')
eye_cascade = cv2.CascadeClassifier('haarcascade_eye.xml')

Becomes

face_cascade = cv2.CascadeClassifier(cv2.data.haarcascades + 'haarcascade_frontalface_default.xml')
eye_cascade = cv2.CascadeClassifier(cv2.data.haarcascades + 'haarcascade_eye.xml')

The XML or file is missing or the path to it is incorrect or the create_capture path is incorrect.

The paths in the opencv sample look like this:

cascade_fn = args.get('--cascade', "../../data/haarcascades/haarcascade_frontalface_alt.xml")
nested_fn  = args.get('--nested-cascade', "../../data/haarcascades/haarcascade_eye.xml")

cam = create_capture(video_src, fallback='synth:bg=../data/lena.jpg:noise=0.05')

I ran the same code. There are two things to note here. 1. Give the entire path of the .xml files. 2. Give a key press event instruction at the end.

Add this block of code at the end and run your file, worked for me:

k = cv2.waitKey(0)
if k == 27:         # wait for ESC key to exit
    cv2.destroyAllWindows()
elif k == ord('s'): # wait for 's' key to save and exit
    cv2.imwrite('messigray.png',img)
    cv2.destroyAllWindows()

For example, my code looked like

import numpy as np
import cv2

face_cascade = cv2.CascadeClassifier('C:\\opencv\\build\\etc\\haarcascades\\haarcascade_frontalface_default.xml')
eye_cascade = cv2.CascadeClassifier('C:\\opencv\\build\\etc\\haarcascades\\haarcascade_eye.xml')

img = cv2.imread('lena.jpg')

gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)

faces = face_cascade.detectMultiScale(gray, 1.3, 5)
#faces = face_cascade.detectMultiScale(gray)

for (x,y,w,h) in faces:
    cv2.rectangle(img,(x,y),(x+w,y+h),(255,0,0),2)
    roi_gray = gray[y:y+h, x:x+w]
    roi_color = img[y:y+h, x:x+w]
    eyes = eye_cascade.detectMultiScale(roi_gray)
    for (ex,ey,ew,eh) in eyes:
        cv2.rectangle(roi_color,(ex,ey),(ex+ew,ey+eh),(0,255,0),2)

cv2.imshow('img',img)

k = cv2.waitKey(0)
if k == 27:         # wait for ESC key to exit
    cv2.destroyAllWindows()
elif k == ord('s'): # wait for 's' key to save and exit
    cv2.imwrite('messigray.png',img)
    cv2.destroyAllWindows()

My output looked like this:

The XML file is missing, you can get the file from the GitHub repository and place it in the same directory as your project. Link to the folder on GitHub is here. Just download the file named haarcascade_frontalface_default.xml. Actually, the file exists on your system. Just go to the site-packages folder of your python installation folder and check the cv2/data folder for the file

Use the entire file path and use "\\" instead of "\" in the xml file path.

The file path should be as follows:

face_cascade = cv2.CascadeClassifier('C:\\opencv\\build\\etc\\haarcascades\\haarcascade_frontalface_default.xml')

instead of:

cascade_fn = args.get('--cascade', "..\..\data\haarcascades\haarcascade_frontalface_alt.xml")