opencv-python：视频读写

最近尝试跑YOLOv3的效果，希望读入视频、逐帧处理、显示并保存视频，这里结合代码简单记录一下方法~

def test_video(model, video_path, video_out_path):
    cap = cv2.VideoCapture(video_path)
    fourcc = cv2.VideoWriter_fourcc(*'MJPG')
    out = cv2.VideoWriter(video_out_path, fourcc, fps=20.0, frameSize=(1920, 1080))
    pre_time = 0
    while(True):
        # time
        curr_time = time.time()
        print(curr_time-pre_time)
        pre_time = curr_time
        # process
        ret, frame = cap.read()
        frame_pil = Image.fromarray(frame)  # opencv to PIL.Image
        frame_pil = test_image(model, image=frame_pil, conf_thres=0.5)
        frame = np.asarray(frame_pil)   # PIL.Image to opencv
        cv2.namedWindow('frame', cv2.WINDOW_NORMAL)
        cv2.imshow('frame', frame)
        # save
        out.write(frame)
        if cv2.waitKey(1) == 113:
            break
    cap.release()
    out.release()

• 视频读取：cap = cv2.VideoCapture(video_path)，按照video_path路径读取视频，比如本例中输入的video_path = " /xxxxx/video_name.mp4"；然后，在循环中，就可以用“ret, frame = cap.read()”来逐帧获取“return，frame”了。（return为True/False，代表是否正常读取）
• 视频显示：cv2.imshow(‘frame’, frame)，将处理结果逐帧显示
• 视频保存：fourcc = cv2.VideoWriter_fourcc(*‘MJPG’) 指定视频编解码的4字节代码；out = cv2.VideoWriter(video_out_path, fourcc, fps=20.0, frameSize=(1920, 1080)) 中设置保存路径、fourcc、fps、frameSize。

注意，frameSize必须与frame匹配（width，height），fourcc也需要设置正确（之前用的DIVX就没有成功），否则容易造成视频损坏无法读取。