[OpenCV] Image sub-sampling

#include <iostream>
#include <string>
#include <vector>

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

#include "dlib/string.h"


std::vector<std::string> labels{ "dolphin", "ibis" };
int main() {

    std::string filename;
    cv::Mat cropped_image;

    // for each category of objects
    for (int class_id = 0; class_id < labels.size(); class_id++) {
        // each category has 50 samples
        for (int sample_id = 1; sample_id < 51; sample_id++) {
            filename = "./data/" + labels[class_id] + "/image_" + dlib::pad_int_with_zeros(sample_id, 4) + ".jpg";
            cv::Mat img = cv::imread(filename);
            cv::Mat img_cpoy = img.clone();
            cv::Size s = img.size();
            int rows = s.height;
            int cols = s.width;

            /*!
                The main procedure starts here.
                1. The dimension(width and height) at each level is divided by 2^N at level N
                2. The number of blocks divided are 2^(N^2), i.e. [1, 4, 16,...]
            !*/
            int width = cols / 2;
            int height = rows / 2;
            // for level = 0 to 2
            for (int j = 0; j < 3; j++) {

                // dimension
                width = cols / std::pow(2, j);
                height = rows / std::pow(2, j);

                // number of blocks
                for (int i = 0; i < std::pow(2, 2 * j); i++) {
                    int X = (i % static_cast<int>(std::pow(2, j)))* width;
                    int Y = (i / static_cast<int>(std::pow(2, j)))* height;
                    cropped_image = img(cv::Rect(X, Y, width, height));
                    cv::rectangle(img_cpoy, cv::Rect(X, Y, width, height), cv::Scalar(0, 0, 255), 2);

                    cv::imshow("cropped", cropped_image);
                    cv::imshow("area", img_cpoy);
                    cv::waitKey(0);
                }
            }
        }
    }
    return 0;
}