Mat convert to int array to byte array

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It is useful to deal with OpenCV's documentation before asking such basic questions. What you need is cv::Mat::convertTo with CV_32S and CV_8U as rtype and if you want to convert 2D matrix to 1D array use cv::Mat::reshape. You can handle Mat as C array with cv::Mat::ptr

// example for 1 channel Mat
cv::Mat img, imgconv;
img.convertTo(imgconv, CV_32S);

if(imgconv.isContinuous() {
    imgconv = imgconv.reshape(1, 1);
}

for(int i = 0; i < imgconv.rows; ++i) {
    int *ptrarray = imgconv.ptr<int>(i);
    for(int ii = 0; ii < imgconv.cols; ++ii) {
         int x = ptrarray[ii];
    }
}