OpenCV Pixel Access, at, ptr, data, iteration (example)

I once wrote the following article.
http://study.marearts.com/2014/04/opencv-study-mat-point-access-method.html
This article is a sample code.


..

#include "opencv2/opencv.hpp"

using namespace cv;
using namespace std;

int main(int, char)
{
    namedWindow("img", 1);
    Mat img = imread("anapji.jpg");
    
    if (img.depth() == CV_8U)
        printf("8bit unsigend\n");

    img.

    /*
    cout << img.elemSize() << endl;
    cout << img.channels() << endl;
    cout << img.rows << endl;
    cout << img.step << endl;
    */

    //using data
    //for (int i = img.rows / 10 * 7; i < img.rows / 10 * 8; i++) {

    //for (int i = 0; i < img.rows ; i++) {

    for (int i = img.rows / 10 * 7; i < img.rows / 10 * 8; i++) {
        for (int j = 0; j < img.cols; j++) {


            unsigned char r, g, b;

            b = img.data[i * img.step + j * img.elemSize() + 0];
            g = img.data[i * img.step + j * img.elemSize() + 1];
            r = img.data[i * img.step + j * img.elemSize() + 2];

            img.data[i * img.step + j * img.elemSize() + 0] = unsigned char(255 - b);
            img.data[i * img.step + j * img.elemSize() + 1] = unsigned char(255 - g);
            img.data[i * img.step + j * img.elemSize() + 2] = unsigned char(255 - r);

        }
    }




    

    //using at
    for (int i = img.rows / 10 * 2; i < img.rows / 10 * 3; ++i)
    {
        for (int j = 0; j < img.cols; ++j)
        {
            //Vec3b means 'uchar 3ch'
            unsigned char b = img.at< cv::Vec3b>(i, j)[0];
            unsigned char g = img.at< cv::Vec3b>(i, j)[1];
            unsigned char r = img.at< cv::Vec3b>(i, j)[2];

            //printf("%d %d %d\n", b, g, r);

            img.at< cv::Vec3b>(i, j)[0] = unsigned char(255 - b); //b
            img.at< cv::Vec3b>(i, j)[1] = unsigned char(255 - g); //g
            img.at< cv::Vec3b>(i, j)[2] = unsigned char(255 - r); //r
        }
    }


    
    //using ptr
    for (int i = img.rows / 10 * 5; i < img.rows / 10 * 6; i++) {

        cv::Vec3b* ptr = img.ptr< cv::Vec3b >(i);

        for (int j = 0; j < img.cols; j++) {

            unsigned char b1 = (ptr[j][0]);
            unsigned char g1 = (ptr[j][1]); //note!!
            unsigned char r1 = (ptr[j][2]);


            cv::Vec3b bgr = ptr[j];
            unsigned char b2 = (bgr[0]);
            unsigned char g2 = (bgr[1]); //note!!
            unsigned char r2 = (bgr[2]);

            

            ptr[j] = cv::Vec3b(255 - b1, 255 - g1, 255 - r1);

        }
    }



    

    //using iteration
    cv::MatIterator_< cv::Vec3b> itd = img.begin< cv::Vec3b>(), itd_end = img.end< cv::Vec3b>();

    for (int i = 0; itd != itd_end; ++itd, ++i) {

        cv::Vec3b bgr = (*itd);

        (*itd)[0] = 255 - bgr[0];
        (*itd)[1] = 255 - bgr[1];
        (*itd)[2] = 255 - bgr[2];
    }

    

    imshow("img", img); //show
    waitKey(0);

    destroyAllWindows();

    return 0;
}

..

input

output