opencv 图像row、col坐标对应关系

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行列与坐标系对应关系 
行rows：Y 
列cols：X 
注意！注意！注意！ 
在Mat类型变量访问时下标是反着写的，即：按照(y, x)的关系形式访问，下面通过代码展示来说明这一点

#include "opencv2/core.hpp"
#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"

#include <iostream>

using namespace cv;
using namespace std;

int main()
{
    Mat mat_src = Mat::eye(3, 4, CV_8UC1);

    cout << "mat_src :" << endl;
    cout << mat_src    << endl;

    cout << endl;
    cout << "Rows : " << mat_src.rows << endl;
    cout << "Cols : " << mat_src.cols << endl;

    //注: mat_src.at<float>(y, x), 下标关系为: y-x
    mat_src.at<float>(0, 2) = 2; 
    mat_src.at<float>(2, 0) = 4;

    cout << endl;
    cout << "mat_src :" << endl;
    cout << mat_src    << endl;

    return 0;
}