#videoname是视频的名字，videoPath_root是视频的文件夹路径，pngPath_root是抽出来的帧存储的文件夹路径
def getVideoPng(videoname, videoPath_root, pngPath_root):
    if not os.path.exists(pngPath_root):
        os.mkdir(pngPath_root)


    videopath = os.path.join(videoPath_root, videoname)
    
    vidcap = cv2.VideoCapture(videopath)
    fps = int(vidcap.get(cv2.CAP_PROP_FPS))
    num_frames = vidcap.get(cv2.CAP_PROP_FRAME_COUNT)
    print(num_frames)
    print(fps)
    n = 0
    while n < num_frames:
        success, image = vidcap.read()
        pngpath = os.path.join(pngPath_root, videoname.split(".")[0], "{}.png".format(j))
        if not os.path.exists(os.path.join(pngPath_root, videoname.split(".")[0])):
            os.mkdir(os.path.join(pngPath_root, videoname.split(".")[0]))
        print(n)
        imag = cv2.imwrite(pngpath, image)
        n+=1
        
    print("done for {}".format(videoname))