Doubt MATLAB code for Opencv C++

Question

Good evening, everyone!

I would like some help in relation to passing a code that is in Matlab to opencv c++. I’m trying to do some operations with rgb channels, but Thresh’s value is not the same - I’m getting the same image. Would 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;
}