Using opencv to inverse Image.

#include <cv.hpp>

using namespace cv;

int main( int argc, char** argv )
{
  Mat image;
  int height, width;
  // load lena picture and convert to grayscale
  image = imread("resource/lena.png", IMREAD_GRAYSCALE);
  // store height and width
  height = image.rows;
  width = image.cols;
  // show image
  imshow("lena1", image);
  // inverse image
  for(int i = 0; i < height; i++) {
      for(int j = 0; j < width; j++) {
          image.at<uchar>(i, j) = 255 - image.at<uchar>(i,j);
      }
  }
  // show image again to compare
  imshow("lena2", image);

  waitKey(0);

  return 0;
}

Result: