Equalization of Histogram of an Image with & without OpenCV using C++

Code

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

using namespace std;
using namespace cv;

vector<int> histcal(const Mat image) {
  vector<int> arr(256, 0);
  for (int y = 0; y <= image.rows; y++)
    for (int x = 0; x <= image.cols; x++)
      arr[(int)(image.at<uchar>(y, x))]++;
  return arr;
}

void showhist(vector<int> arr, string name) {
  int maxi = 0;
  for (int i = 0; i < 256; i++)
    maxi = max(maxi, arr[i]);

  float factor = 400 / (float)maxi;
  Mat image = imread("contrast.jpg", CV_LOAD_IMAGE_GRAYSCALE);
  Mat himage = Mat::zeros(500, 400, image.type());
  Scalar col = Scalar(255, 255, 255);
  for (int i = 0; i < 256; i++) {
    int xshift = 50;
    int yfp = 450;  // first point
    int ysp = 450 - factor * arr[i];
    line(himage, Point(xshift + i, yfp), Point(xshift + i, ysp), col);
  }
  namedWindow(name.c_str(), WINDOW_NORMAL);
  imshow(name.c_str(), himage);
}

vector<int> mapping(vector<int> arr) {
  vector<int> carr(arr);
  for (int i = 1; i < 256; i++)
    carr[i] += carr[i - 1];
  vector<int> marr(256, 0);
  for (int i = 0; i < 256; i++) {
    marr[i] = (float(carr[i]) / carr[255] * 255) + 0.5;
  }

  return marr;
}

vector<int> eqhist(const vector<int> arr) {
  vector<int> marr = mapping(arr);
  vector<int> t(256, 0);
  for (int i = 0; i < 256; i++)
    t[marr[i]] += arr[i];
  return t;
}

Mat pixelmapper(const Mat img, vector<int> mapi) {
  Mat nimage = Mat(img.rows, img.cols, img.type());
  for (int y = 0; y < img.rows; y++)
    for (int x = 0; x < img.cols; x++)
      nimage.at<uchar>(y, x) = saturate_cast<uchar>(mapi[img.at<uchar>(y, x)]);
  return nimage;
}

int main() {
  // My cpp code
  Mat gimage;  // original image in grey
  gimage = imread("contrast.jpg", CV_LOAD_IMAGE_GRAYSCALE);
  vector<int> h = histcal(gimage);
  vector<int> e = eqhist(h);
  Mat nimage = pixelmapper(gimage, mapping(h));

  // Opencv Function used
  Mat eimage;
  equalizeHist(gimage, eimage);
  vector<int> ho = histcal(eimage);

  namedWindow("gimage", WINDOW_NORMAL);
  imshow("gimage", gimage);
  namedWindow("eqimage", WINDOW_NORMAL);
  imshow("eqimage", nimage);

  showhist(h, "original_histogram");
  showhist(e, "equalized_histogram");

  namedWindow("eqimageopencv", WINDOW_NORMAL);
  imshow("eqimageopencv", eimage);
  showhist(ho, "opencvhist");

  waitKey(0);
  return 0;
}

Compile & Run

[duke@duke-pc b9]$ g++ b9.cpp `pkg-config --cflags --libs opencv`

[duke@duke-pc b9]$ ./a.out