opencv_mat_traverse.cpp

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string>

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

void color_reduce1(const cv::Mat& src_mat, int div, cv::Mat* dst_mat) {
    *dst_mat = src_mat.clone();
    for (int r = 0; r < src_mat.rows; ++r) {
        for (int c = 0; c < src_mat.cols; ++c) {
            dst_mat->at<cv::Vec3b>(r, c)[0] = 
                    src_mat.at<cv::Vec3b>(r, c)[0] / div * div + div / 2;
            dst_mat->at<cv::Vec3b>(r, c)[1] = 
                    src_mat.at<cv::Vec3b>(r, c)[1] / div * div + div / 2;
            dst_mat->at<cv::Vec3b>(r, c)[2] = 
                    src_mat.at<cv::Vec3b>(r, c)[2] / div * div + div / 2;
        }
    }
}

void color_reduce2(const cv::Mat& src_mat, int div, cv::Mat* dst_mat) {
    *dst_mat = src_mat.clone();
    int num_rows = src_mat.rows;
    int num_elems_per_row = src_mat.cols * src_mat.channels();
    for (int i = 0; i < num_rows; ++i) {
        const uchar* in_data = src_mat.ptr<uchar>(i);
        uchar* out_data = dst_mat->ptr<uchar>(i);
        for (int j = 0; j < num_elems_per_row; ++j) {
            out_data[j] = in_data[j] / div * div + div / 2;
        }
    }
}

int main(int argc, char** argv) {
    srand(time(NULL));
    if (argc != 2) {
        std::cerr << "Usage: color_reduce <img_dir>\n";
        return 1;
    }
    cv::Mat img = cv::imread(argv[1]);
    cv::Mat img_out1;
    cv::Mat img_out2;
  
    int div = 64;
    clock_t start, end;
    start = clock();
    for (int i = 0; i < 1000; ++i) {
        color_reduce1(img, div, &img_out1);
    }
    end = clock();
    std::cout << (1.0 * end - start) / CLK_TCK << std::endl;

    start = clock();
    for (int i = 0; i < 1000; ++i) {
        color_reduce2(img, div, &img_out2);
    }
    end = clock();
    std::cout << (1.0 * end - start) / CLK_TCK << std::endl;

    cv::imshow("img", img);
    cv::imshow("img_out1", img_out1);
    cv::imshow("img_out2", img_out2);
    cv::waitKey(0);

    return 0;
}