python - 通过这段代码，我想收集脸部样本，但它给了错误

Question

import cv2
import numpy 
face_classifier = cv2.CascadeClassifier('C:/Users/my pc/AppData/Local/Programs/Python/Python38/Lib/site-packages/cv2/data/haarcascade_frontalface_default.xml')

def face_extractor(img): # to extract face feature
    gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
    faces= face_classifier.detectMultiScale(gray,1,3,5)

    if faces is():     
        #if there is no face on screen
        return None

    for(x,y,w,h) in faces:  # if face present, crop face,return it
        cropped_face = img[y:y+h,x:x+w] 

    return cropped_face    

cap= cv2.VideoCapture(0,cv2.CAP_DSHOW)  # to open camera
count = 0 


while True:
    ret ,frame = cap.read()
    if face_extractor(frame) is not None:
        count += 1
        face = cv2.resize(face_extractor(frame),(200,200))
        face = cv2.cvtColor(face,cv2.COLOR_BGR2GRAY)
        file_name_path = 'C:/Users/my pc/faces/user' + str(count)+'.jpg'
        cv2.imwrite(file_name_path,face)  

        cv2.putText(face,str(count),(50,50),cv2.FONT_HERSHEY_COMPLEX,1,(0,255,0),2)  #count how mnay pics clicked
        cv2.imshow('face cropper',face)

    else:
        print('face not found')
        pass

    if cv2.waitKey1 == 13 or count == 100:
        break

cap.release()
cv2.destroyAllWindows()

print('collecting samples complete')

下面的 是错误

ace.py:9: SyntaxWarning: "is" with a literal. Did you mean "=="?
  if faces is():
Traceback (most recent call last):
  File "face.py", line 24, in <module>
    if face_extractor(frame) is not None:
  File "face.py", line 7, in face_extractor
    faces= face_classifier.detectMultiScale(gray,1,3,5)
cv2.error: OpenCV(4.4.0) C:\Users\appveyor\AppData\Local\Temp\1\pip-req-build-j8nxabm_\opencv\modules\objdetect\src\cascadedetect.cpp:1689: error: (-215:Assertion failed) !empty() in function 'cv::CascadeClassifier::detectMultiScale'

Answer 1

错误消息显示if faces is():有问题，因为Python中没有is()。detectMultiScale()提供带有对象的列表，您可以检查此列表是否为空:

if len(faces) == 0:
    return None

或更可读

if not faces:
    return None