python - 函数 'cv::cvtColor'和Cap.read()中的src.empty()无法读取视频帧

Question

Python 3.8，Opencv 4.4.0
我已经正确加载了正在运行的视频文件，我认为Cap.read()无法读取视频帧。我正在研究口罩检测项目，并想在预先录制的戴口罩视频中进行测试。
完整代码

# import packages
import cv2
from tensorflow.keras.models import load_model
from tensorflow.keras.preprocessing.image import load_img, img_to_array
import numpy as np

model = load_model('model_02.h5')
img_width, img_hight = 200, 200

face_cascade = cv2.CascadeClassifier('haarcascade_frontalface_default.xml')  
cap = cv2.VideoCapture('wearingmask.mp4') # for video

img_count_full = 0
 
#parameters for text
font = cv2.FONT_HERSHEY_SIMPLEX 
org = (1, 1)
class_lable=' '      
fontScale = 1
# Blue color in BGR 
color = (255, 0, 0) 
thickness = 2 #1

while True:
    img_count_full += 1
     
    response, color_img = cap.read()
     
    if response == False:
        break       
gray_img = cv2.cvtColor(color_img, cv2.COLOR_BGR2GRAY)
faces = face_cascade.detectMultiScale(gray_img, 1.1, 6) 

完全错误

error                                     Traceback (most recent call last)
<ipython-input-8-d7656fcb48d9> in <module>
     39         break
     40 
---> 41 gray_img = cv2.cvtColor(color_img, cv2.COLOR_BGR2GRAY)
     42 faces = face_cascade.detectMultiScale(gray_img, 1.1, 6)

error: OpenCV(4.4.0) C:\Users\appveyor\AppData\Local\Temp\1\pip-req-build-9d_dfo3_\opencv\modules\imgproc\src\color.cpp:182: error: (-215:Assertion failed) !_src.empty() in function 'cv::cvtColor'

Answer 1

这很自我解释。您代码的核心逻辑基本上是遍历视频中的图像，直到没有更多图像为止，然后将cvtColor应用于最后一个图像。
当while为response时，您的False循环中断，即您已经到达视频文件的末尾。但是，当response为False时，相应的color_img实际上是None。要实际获得最后一个有效图像，您需要执行以下操作:

while True:
    img_count_full += 1
     
    response, frame = cap.read()
    if response == False:
        break
    color_img = frame

gray_img = cv2.cvtColor(color_img, cv2.COLOR_BGR2GRAY)