我想使用opencv和python从连接到我的计算机的2个网络摄像头捕获图像 . 这是我写的代码：

#to take snapshot from two webcams simultaneously
    import cv2
    import cv2.cv as cv
    import numpy as np 
    left = cv2.VideoCapture(1)  #capturing video from device port 1 aka webcam1
    right = cv2.VideoCapture(2) #capturing video from device port 2 aka webcam2
    capture_left=cv.CaptureFromCAM(1)   
    capture_right =cv.CaptureFromCAM(2)
    while(True):
       ret,frameL = left.read()
       ret1,frameR = right.read()
       rgb_left = cv2.cvtColor(frameL,0)
       rgb_right = cv2.cvtColor(frameR,0)
       cv2.imshow('frameL',rgb_left)
       cv2.imshow('frameR',rgb_right)
       k=cv2.waitKey(0)
       if k == 27:         # wait for ESC key to break
           break
        elif k == 32: # wait for spacebar to click snapshot
           il=cv.QueryFrame(capture_left)
           cv.SaveImage("defaultL.jpg",il)
           ir=cv.QueryFrame(capture_right)
           cv.SaveImage("defaultR.jpg",ir)

    left.release()
    right.release()
    cv2.destroyAllWindows()

但在执行时这是错误：OpenCV错误：断言失败（scn == 3 || scn == 4）在cv :: cvtColor中，文件........ \ opencv \ modules \ imgproc \ src \ color .cpp，第3648行回溯（最近一次调用最后一次）：文件“C：\ Users \ Administrator \ Desktop \ capture.py”，第15行，在rgb_right = cv2.cvtColor（frameR，0）cv2.error：... ..... \ opencv \ modules \ imgproc \ src \ color.cpp：3648：错误：（-215）scn == 3 ||函数cv :: cvtColor中的scn == 4

[完成时间为1.5秒，退出代码为1]

python opencv