different value of thresh - converting Matlab code to OpenCV code

Question

I would like some help regarding passing a code that is in matlab to opencv c ++. I am trying to do some operations with the RGB channels, however, the value of thresh is not being the same - I am sending the same image. Could someone please help me?

MATLAB

im = imread('1.png');

[m,n,p] = size(im);

R=im(:, :, 1);
G=im(:, :, 2);
B=im(:, :, 3);

thresh=0;

for j=1:n
    for i=1:m
        thresh = thresh + double((1.262*G(i,j))-(0.884*R(i,j))-(0.311*B(i,j)));
    end
end

C++

#include <opencv2/opencv.hpp>
#include "opencv2/highgui.hpp"
#include <opencv2/core/mat.hpp>
#include <iostream>
using namespace std;
using namespace cv;


int main(){
    Mat img = imread("1.png", IMREAD_COLOR); 
    int thresh = 0;

    for(int j = 0; j <= img.cols; j++){
        for(int i = 0; i <= img.rows; i++){
            Vec3b color = img.at<Vec3b>(i,j);
            uchar a = color.val[0], b = color.val[1], c = color.val[2];
            thresh += double((1.262*b)-(0.884*c)-(0.311*a));
        }
    }
    
    cout << thresh;
    return 0;
}

Answer 1

First mistake is in the upper values of for loops because you are exceeding the range of image borders.

j <= img.cols should be j < img.cols and

i <= img.rows should be i < img.rows

Second mistake is that you don't make explicit type conversion for your uchar type pixel values

thresh += double((1.262*b)-(0.884*c)-(0.311*a));

should be

thresh += double((1.262*static_cast<double>(b))
          -(0.884*static_cast<double>(c))
          -(0.311*static_cast<double>(a)));

Here is the whole code I tried:

#include <opencv2/opencv.hpp>
#include "opencv2/highgui.hpp"
#include <opencv2/core/mat.hpp>
#include <iostream>

using namespace std;
using namespace cv;


int main()
{
    Mat img = imread("img.jpg", IMREAD_COLOR);
    double thresh = 0.0;

    resize(img,img,Size(100,100));

    for(int j = 0; j < img.cols; j++){
        for(int i = 0; i < img.rows; i++){
            
            
            // 1ST WAY 
            Vec3b color = img.at<Vec3b>(i,j);
            uchar a = color.val[0], b = color.val[1], c = color.val[2];

            thresh += double((1.262*static_cast<double>(b))
                             -(0.884*static_cast<double>(c))
                             -(0.311*static_cast<double>(a)));
            
//            2ND WAY

//            thresh += double((1.262 * (double)img.at<Vec3b>(Point(i,j))[1])
//                    - (0.884*(double)img.at<Vec3b>(Point(i,j))[2])
//                    - (0.311 * (double)img.at<Vec3b>(Point(i,j))[0]));
        }
    }

    cout << thresh << endl;
    return 0;
}