OpenCV exercises

This is a typical "getting started" example to OpenCV, because it uses simple pixel manipulation in order to make a negative effect in a region of the given image.

To invert the color of a pixel, the formula used is color = 255 - color in each channel. To run this example: ./regions <img_path> <x1> <y1> <x2> <y2> where x1, y1, x2 and y2 the rectangular area to be turned into negative.

#include <iostream>
#include <cstdlib>
#include <opencv2/opencv.hpp>

using namespace cv;
using namespace std;

#define CHECK_VALID_RANGE(x, dimension) if(x < 0 || x >= dimension) \
    {cout << "Invalid value for "#x << endl; exit(1);}

int main(int argc, char** argv){
    if (argc != 2 && argc != 6) {
        cout << "usage: " << endl
             << "\t" << argv[0] << "<img_path>" 
             << " // To get image dimensions" << endl
             << "\t" << argv[0] << "<img_path> <x1> <y1> <x2> <y2>" 
             << " // To negative the rectangular area" << endl;

        exit(1);
    }
    
    Mat image = imread(argv[1], CV_LOAD_IMAGE_COLOR);
    
    if (!image.data)
        cout << "Could not open " << argv[1] << endl;

    int cols = image.cols;
    int rows = image.rows;

    cout << "###### Image dimensions ######" << endl
         << "Columns: "  << cols << endl
         << "Rows: "     << rows << endl
         << "##############################" << endl;

    if (argc == 2) exit(0);

    int x1 = atoi(argv[2]); CHECK_VALID_RANGE(x1, rows);
    int y1 = atoi(argv[3]); CHECK_VALID_RANGE(y1, cols);
    int x2 = atoi(argv[4]); CHECK_VALID_RANGE(x2, rows);
    int y2 = atoi(argv[5]); CHECK_VALID_RANGE(y2, cols);

    namedWindow(argv[1],WINDOW_AUTOSIZE);

    for (int i = min(x1, x2); i < max(x1, x2); ++i) {
        for (int j = min(y1, y2); j < max(y1, y2); ++j) {
            image.at<Vec3b>(i,j)[0] = 255- image.at<Vec3b>(i,j)[0];
            image.at<Vec3b>(i,j)[1] = 255- image.at<Vec3b>(i,j)[1];
            image.at<Vec3b>(i,j)[2] = 255- image.at<Vec3b>(i,j)[2];
        }
    }

    imshow(argv[1], image);  
    waitKey();

    exit(0);
}