无法在 python 中打开 VideoCapture

Question

我从 GitHub 下载了这段代码，用于 OpenCV 项目。第一次一切正常，但之后它就打不开了，它一直在第 8 行向我显示以下错误：

Traceback (most recent call last):
  File "hand.py", line 8, in <module>
    crop_img = img[100:300,100:300]
TypeError: 'NoneType' object is not subscriptable

代码如下：

import cv2
import numpy as np
import math
cap = cv2.VideoCapture(0)
while(cap.isOpened()):
    ret, img = cap.read()
    cv2.rectangle(img,(300,300),(100,100),(0,255,0),0)
    crop_img = img[100:300,100:300]
    grey = cv2.cvtColor(crop_img, cv2.COLOR_BGR2GRAY)
    value = (35, 35)
    blurred = cv2.GaussianBlur(grey, value, 0)
    _, thresh1 = cv2.threshold(blurred, 127, 255,
                               cv2.THRESH_BINARY_INV+cv2.THRESH_OTSU)
    cv2.imshow('Thresholded', thresh1)
    contours, hierarchy = cv2.findContours(thresh1.copy(),cv2.RETR_TREE, \
            cv2.CHAIN_APPROX_NONE)
    max_area = -1
    for i in range(len(contours)):
        cnt=contours[i]
        area = cv2.contourArea(cnt)
        if(area>max_area):
            max_area=area
            ci=i
    cnt=contours[ci]
    x,y,w,h = cv2.boundingRect(cnt)
    cv2.rectangle(crop_img,(x,y),(x+w,y+h),(0,0,255),0)
    hull = cv2.convexHull(cnt)
    drawing = np.zeros(crop_img.shape,np.uint8)
    cv2.drawContours(drawing,[cnt],0,(0,255,0),0)
    cv2.drawContours(drawing,[hull],0,(0,0,255),0)
    hull = cv2.convexHull(cnt,returnPoints = False)
    defects = cv2.convexityDefects(cnt,hull)
    count_defects = 0
    cv2.drawContours(thresh1, contours, -1, (0,255,0), 3)
    for i in range(defects.shape[0]):
        s,e,f,d = defects[i,0]
        start = tuple(cnt[s][0])
        end = tuple(cnt[e][0])
        far = tuple(cnt[f][0])
        a = math.sqrt((end[0] - start[0])**2 + (end[1] - start[1])**2)
        b = math.sqrt((far[0] - start[0])**2 + (far[1] - start[1])**2)
        c = math.sqrt((end[0] - far[0])**2 + (end[1] - far[1])**2)
        angle = math.acos((b**2 + c**2 - a**2)/(2*b*c)) * 57
        if angle <= 90:
            count_defects += 1
            cv2.circle(crop_img,far,1,[0,0,255],-1)
        #dist = cv2.pointPolygonTest(cnt,far,True)
        cv2.line(crop_img,start,end,[0,255,0],2)
        #cv2.circle(crop_img,far,5,[0,0,255],-1)
    if count_defects == 1:
        cv2.putText(img,"this is 2", (50,50), cv2.FONT_HERSHEY_SIMPLEX, 2, 2)
    elif count_defects == 2:
        str = "this is 3 !!!"
        cv2.putText(img, str, (5,50), cv2.FONT_HERSHEY_SIMPLEX, 1, 2)
    elif count_defects == 3:
        cv2.putText(img,"This is 4 :P", (50,50), cv2.FONT_HERSHEY_SIMPLEX, 2, 2)
    elif count_defects == 4:
        cv2.putText(img,"this is 5 !!!", (50,50), cv2.FONT_HERSHEY_SIMPLEX, 2, 2)
    else:
        cv2.putText(img,"this is 0 !!!", (50,50),\
                    cv2.FONT_HERSHEY_SIMPLEX, 2, 2)
    #cv2.imshow('drawing', drawing)
    #cv2.imshow('end', crop_img)
    cv2.imshow('Gesture', img)
    all_img = np.hstack((drawing, crop_img))
    cv2.imshow('Contours', all_img)
    k = cv2.waitKey(10)
    if k == 27:
        break

Answer 1

异常表明 cap.read 将 None 返回给 img。你应该注意这一点：

cap = cv2.VideoCapture(0)
while(cap.isOpened()):
  ret, img = cap.read()
  if not ret: break

文档说：

If no frames has been grabbed (camera has been disconnected, or there are no more frames in video file), the methods return false and the functions return NULL pointer.

因此，如果您使用 USB 网络摄像头，请确保您的 USB 连接稳定。特别是，如果您使用任何 USB hubs/extensions 或您的端口形状不佳。

Answer 2

我找到了解决方案，似乎某些糟糕的视频驱动程序 return 无效的第一帧。我所做的只是检查 ret 并继续，如果它是假的，它工作正常。

无法在 python 中打开 VideoCapture

Can't open VideoCapture in python

python

opencv

nonetype