无法在opencv中更改输入视频分辨率

import cv2
import subprocess

def change_resolution(input_file, output_file, new_width, new_height):
    # 使用OpenCV读取视频
    cap = cv2.VideoCapture(input_file)

    # 获取原始视频的宽度和高度
    width = int(cap.get(cv2.CAP_PROP_FRAME_WIDTH))
    height = int(cap.get(cv2.CAP_PROP_FRAME_HEIGHT))

    # 创建一个VideoWriter对象，用于写入新的视频文件
    fourcc = cv2.VideoWriter_fourcc(*'XVID')
    out = cv2.VideoWriter(output_file, fourcc, 30.0, (new_width, new_height))

    # 使用FFmpeg库来更改视频分辨率
    cmd = ['ffmpeg', '-i', input_file, '-vf', 'scale={}:{}'.format(new_width, new_height), '-c:v', 'libx264', '-crf', '23', output_file]
    subprocess.call(cmd)

    # 使用OpenCV读取新的视频文件，并显示
    cap = cv2.VideoCapture(output_file)
    while(cap.isOpened()):
        ret, frame = cap.read()
        if ret == True:
            cv2.imshow('Frame', frame)
            if cv2.waitKey(25) & 0xFF == ord('q'):
                break
        else:
            break

    # 清理资源
    cap.release()
    out.release()
    cv2.destroyAllWindows()

# 调用函数来更改视频分辨率
change_resolution('input.mp4', 'output.mp4', 640, 480)