This is the first exercise using OpenCV image manipulation. In this example, we'll be reading and processing a region of a given image in order to create a negative effect on it. To achive this goal, the following fomula is used to invert the color of the pixels: pixel_color = 255 - pixel_color.

As the image used is in grayscale, we will be aplying it for one channel only. The region is seleted by a couple of for-loops that go through an area within the image, inverting its pixels, as shown below.

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

using namespace cv;
using namespace std;

int main()
{
  Mat image;
  int pxl;

  image= imread("img/camera__.jpg",CV_LOAD_IMAGE_GRAYSCALE);
  if(!image.data)
    cout << "couldn't load image" << endl;

  namedWindow("window",WINDOW_AUTOSIZE);

  for(int i=100;i<200;i++){
    for(int j=100;j<200;j++){
      pxl = image.at<uchar>(i,j);
      image.at<uchar>(i,j) = 255 - pxl;
    }
  }
  imwrite("negative.jpg", image);
  imshow("window", image); 

  waitKey();

  return 0;
}