answer_22.cpp

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


// histogram transform
cv::Mat histogram_transform(cv::Mat img, int m0, int s0){
  // get height and width
  int width = img.cols;
  int height = img.rows;
  int channel = img.channels();

  // histogram transformation hyper-parameters
  double m, s;
  double sum = 0., squared_sum = 0.;
  double val;

  // output image
  cv::Mat out = cv::Mat::zeros(height, width, CV_8UC3);

  // get sum
  for (int y = 0; y < height; y++){
    for (int x = 0; x < width; x++){
      for (int c = 0; c < 3; c++){
        val = (float)img.at<cv::Vec3b>(y, x)[c];
        sum += val;
        squared_sum += (val * val);
      }
    }
  }

  // get standard deviation
  m = sum / (height * width * channel);
  s = sqrt(squared_sum / (height * width * channel) - m * m);


  // histogram transformation
  for (int y = 0; y < height; y++){
    for ( int x = 0; x < width; x++){
      for ( int c = 0; c < 3; c++){
        val = img.at<cv::Vec3b>(y, x)[c];
        
        out.at<cv::Vec3b>(y, x)[c] = (uchar)(s0 / s * (val - m) + m0);
      }
    }
  }

  return out;
}


int main(int argc, const char* argv[]){
  // read image
  cv::Mat img = cv::imread("imori_dark.jpg", cv::IMREAD_COLOR);

  // histogram transform
  cv::Mat out = histogram_transform(img, 128, 52);
  
  //cv::imwrite("out.jpg", out);
  cv::imshow("answer", out);
  cv::waitKey(0);
  cv::destroyAllWindows();
 
  return 0;
}