参考资料：

python opencv写视频——cv2.VideoWriter()_cv2.cv.videowriter(_翟羽嚄的博客-CSDN博客

import cv2 as cv
import numpy as np

#1. 打开原始视频
video_in = cv.VideoCapture("../SampleVideos/Unity2D.mp4")
video_width = int(video_in.get(cv.CAP_PROP_FRAME_WIDTH))
video_height = int(video_in.get(cv.CAP_PROP_FRAME_HEIGHT))
video_fps = int(video_in.get(cv.CAP_PROP_FPS))
print("Original Video Resolution: (", video_width, ",", video_height, ") FPS:", video_fps)


#2. 创建VideoWriter对象
#   cv.VideoWriter(filename, fourcc, fps, frameSize[, isColor])
#   filename: 要保存的文件的路径
#   fourcc： 指定编码器
#   fps： 要保存的视频的帧率
#   frameSize： 要保存的文件的画面尺寸
#   isColor： 指示是黑白画面还是彩色的画面
#参考资料：https://blog.csdn.net/mao_hui_fei/article/details/107573021
video_out = cv.VideoWriter("out.avi", cv.VideoWriter_fourcc(*'XVID'), video_fps, (800,600), True)

#循环处理每一帧图像
while True:
    ret,frame = video_in.read()
    if ret == False:
        break;
    frame = cv.resize(frame, (800, 600))
    video_out.write(frame)
    cv.imshow('TestVideoOut', frame)
    key = cv.waitKey(10)
    if key == 27:
        break;

#释放资源
video_in.release()
video_out.release()