How to write a pixel and read it without changing in OpenCV

Question

I am trying to make an image which holds the ascii values of a array of characters. Each pixel must contain ascii value of each characters in the text. I wrote the image using imwrite , but when i try to access the image using imread the value of the pixels is different.

My program is:

#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <opencv2/highgui/highgui.hpp>
#include<iostream>

using namespace cv;
using namespace std;

void makeimage()
{
    Mat img(100,100,CV_8UC3);
    for(int i=0;i<100;++i)
    {
        for(int j=0;j<100;++j)
        {
            img.at<Vec3b>(i,j)[0]='a';
            img.at<Vec3b>(i,j)[1]='b';
            img.at<Vec3b>(i,j)[2]='c';
        }
    }
    imwrite("image.jpg",img);
}
void readimage()
{
    Mat image=imread("image.jpg",IMREAD_UNCHANGED);

    for(int i=0;i<100;++i)
    {
        for(int j=0;j<100;++j)
        {
            cout<<(char)image.at<Vec3b>(i,j)[0 ];
            cout<<(char)image.at<Vec3b>(i,j)[1 ];
            cout<<(char)image.at<Vec3b>(i,j)[2 ]<<" ";
        }
    }

}

int main(int argc, char *argv[])
{
    makeimage();
    readimage();
    waitKey(0);
    return 0;
}

The output is shown in the image

Answer 1

You are saving your image in jpeg , a lossy format that compresses the image and thus modifies the values.

Save your image in a lossless format, like png , and it will work as expected.

...
imwrite("image.png",img);
...
Mat image=imread("image.png",IMREAD_UNCHANGED);