提示：本文中视频拼接指的是将多张图像按空间合并在一张图像上，而不是将多张图像按时间顺序拼接成一个多帧片段。

C++版拼接视频不如Python版简洁。

Python版：Python OpenCV【视频合并：多个图像拼接在一张图像】_IT.Husky的博客-CSDN博客

为了实现C++版拼接视频，首先复习一下OpenCV相关知识。

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一、OpenCV知识点

1.OpenCV裁剪矩形区域赋值 

Mat img;//读取图像
 
cv::Rect rect(x, y, WIDTH, HEIGHT);//定义坐标（x,y）为矩形的左上角起点，宽高为WIDTH和HEIGHT的矩形
 
Mat tmp = img(rect);//将img上矩形框住的部分赋给tmp

2.OpenCV将Mat粘贴到指定位置 

cv::Mat src;//读取图像

cv::Mat image;//定义一个存放图像的Mat

cv::Rect rect(0, 0, src.cols, src.rows);//定义坐标(0,0)为矩形的左上角起点，宽高为src.cols, src.rows的矩形

src.copyTo(image(rect));//把src的内容复制到image指定的Rect框中

二、程序样例

1.程序源码

代码如下（带注释）：

#include <iostream>
#include <opencv.hpp>
using namespace std;

int main()
{
    //读视频
	cv::VideoCapture videoLeftUp("kll.mp4");
	cv::VideoCapture videoRightUp("kll.mp4");
	cv::VideoCapture videoLeftDown("kll.mp4");
	cv::VideoCapture videoRightDown("kll.mp4");
	int fps = videoLeftUp.get(5);
	cv::Size size = cv::Size(videoLeftUp.get(3), videoLeftUp.get(4));
	//写视频
    cv::VideoWriter writer("kllHeBing.mp4", CV_FOURCC('m', 'p', '4', 'v'), fps, size, true);
	cv::Mat frameLeftUp, frameRightUp, frameLeftDown, frameRightDown;
	//读视频帧
    while (videoLeftUp.read(frameLeftUp) && videoRightUp.read(frameRightUp) &&
		videoLeftDown.read(frameLeftDown) && videoRightDown.read(frameRightDown))
	{
        //调整帧尺寸
		cv::Mat frameLeftUpNew, frameRightUpNew, frameLeftDownNew, frameRightDownNew;
		resize(frameLeftUp, frameLeftUpNew, cv::Size(frameLeftUp.cols / 2, frameLeftUp.rows / 2));
		resize(frameRightUp, frameRightUpNew, cv::Size(frameRightUp.cols / 2, frameRightUp.rows / 2));
		resize(frameLeftDown, frameLeftDownNew, cv::Size(frameLeftDown.cols / 2, frameLeftDown.rows / 2));
		resize(frameRightDown, frameRightDownNew, cv::Size(frameRightDown.cols / 2, frameRightDown.rows / 2));
		//新建Mat 用于图像合并
		cv::Mat frameOut = cv::Mat::zeros(frameLeftUp.rows, frameLeftUp.cols, frameLeftUp.type());
        //定义图像放的位置
		cv::Rect rectLeftUp = cvRect(0, 0, frameLeftUpNew.cols, frameLeftUpNew.rows);
		cv::Rect rectRightUp = cvRect(frameRightUpNew.cols, 0, frameRightUpNew.cols, frameRightUpNew.rows);
		cv::Rect rectLeftDown = cvRect(0, frameLeftDownNew.rows, frameLeftDownNew.cols, frameLeftDown.rows);
		cv::Rect rectRightDown = cvRect(frameRightDownNew.cols, frameRightDownNew.rows, frameRightDownNew.cols, frameRightDownNew.rows);
		//将调整尺寸的帧图像放置新建Mat的指定位置
		frameLeftUpNew.copyTo(frameOut(rectLeftUp));
		frameRightUpNew.copyTo(frameOut(rectRightUp));
		frameLeftDownNew.copyTo(frameOut(rectLeftDown));
		frameRightDownNew.copyTo(frameOut(rectRightDown));
		/*cv::imshow("frameOut.jpg", frameOut);
		cv::waitKey(0);*/
        //写入帧图像
		writer.write(frameOut);
	}
    //释放内存
	writer.release();
	videoLeftUp.release();
	videoRightUp.release();
	videoLeftDown.release();
	videoRightDown.release();

	system("pause");
	return 0;
}

2.运行结果

源视频图像首帧：

拼接视频图像首帧：

留作业：请尝试以下合并结果↓