代码如下：
#include<opencv2/highgui/highgui.hpp>
#include<stdio.h>
using namespace cv;
using namespace std;
int main()
{
    Mat img = imread("/home/jay/Pictures/python.jpg",CV_LOAD_IMAGE_COLOR);
    unsigned char *input = (unsigned char*)(img.data);
    int i,j,r,g,b;
    for(int i = 0;i < img.rows;i++){
    for(int j = 0;j < img.cols;j++){
            b = input[j + img.step * i ] ;
            g = input[j + img.step * i + 1];
            r = input[j + img.step * i + 2];
            }
        }
    Mat target_gray;
    cvtColor(img , target_gray, CV_RGB2GRAY);
    //这里需要读取target_gray的data所有数据？
    return 0;
}