我试图从视频中裁剪图像，但它没有显示结果。我知道TypeError是发生，因为没有在crop crop_img的内容，但因为我是一个初学者在opencv，所以我不知道它的解决方案。下面的代码我从使用opencv与树莓派和网络摄像头的项目复制来跟随这一行。现在我正试图在我的Windows 10上进行测试，所以可能会出现一些接口问题。
代码：-

# -*- coding: utf-8 -*-

import numpy as np

import cv2

video_capture = cv2.VideoCapture(-1)
video_capture.set(3, 160)
video_capture.set(4, 120)

while(True):
    # Capture the frames

    ret, frame = video_capture.read()
    #print("image recieved");
    #cv2.imshow('frame',ret)

    # Crop the image

    crop_img = frame[60:120, 0:160]

    cv2.imshow("cropped", crop_img)


    # Convert to grayscale

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

    # Gaussian blur

    blur = cv2.GaussianBlur(gray,(5,5),0)

    cv2.imshow('show', blur)

    # Color thresholding

    ret,thresh = cv2.threshold(blur,60,255,cv2.THRESH_BINARY_INV)

    # Find the contours of the frame

    contours,hierarchy = cv2.findContours(thresh.copy(), 1, cv2.CHAIN_APPROX_NONE)


    # Find the biggest contour (if detected)

    if len(contours) > 0:

        c = max(contours, key=cv2.contourArea)
        M = cv2.moments(c)
        cx = int(M['m10']/M['m00'])
        cy = int(M['m01']/M['m00'])
        cv2.line(crop_img,(cx,0),(cx,720),(255,0,0),1)
        cv2.line(crop_img,(0,cy),(1280,cy),(255,0,0),1)
        cv2.drawContours(crop_img, contours, -1, (0,255,0), 1)

        if cx >= 120:
            print ("Turn Left!")

        if cx < 120 and cx > 50:
            print ("On Track!")

        if cx <= 50:
            print ("Turn Right")
        else:
            print ("I don't see the line")
    #Display the resulting frame

    cv2.imshow('frame',crop_img)
    if cv2.waitKey(1) & 0xFF == ord('q'):
        break\

错误：-

runfile('C:/Users/HP/OneDrive/Desktop/Sultan/Veggitech/Transport robot with Voice Control/Code/Test Code/line_follower_opencv.py', wdir='C:/Users/HP/OneDrive/Desktop/Sultan/Veggitech/Transport robot with Voice Control/Code/Test Code')
Traceback (most recent call last):

  File "<ipython-input-2-583afe4334df>", line 1, in <module>
    runfile('C:/Users/HP/OneDrive/Desktop/Sultan/Veggitech/Transport robot with Voice Control/Code/Test Code/line_follower_opencv.py', wdir='C:/Users/HP/OneDrive/Desktop/Sultan/Veggitech/Transport robot with Voice Control/Code/Test Code')

  File "C:\Users\HP\Anaconda3\lib\site-packages\spyder_kernels\customize\spydercustomize.py", line 827, in runfile
    execfile(filename, namespace)

  File "C:\Users\HP\Anaconda3\lib\site-packages\spyder_kernels\customize\spydercustomize.py", line 110, in execfile
    exec(compile(f.read(), filename, 'exec'), namespace)

  File "C:/Users/HP/OneDrive/Desktop/Sultan/Veggitech/Transport robot with Voice Control/Code/Test Code/line_follower_opencv.py", line 25, in <module>
    crop_img = frame[60:120, 0:160]

TypeError: 'NoneType' object is not subscriptable