void cv::equalizeHist( InputArray _src, OutputArray _dst )
{CV_Assert( _src.type() == CV_8UC1 );if (_src.empty())return;CV_OCL_RUN(_src.dims() <= 2 && _dst.isUMat(),ocl_equalizeHist(_src, _dst))Mat src = _src.getMat();_dst.create( src.size(), src.type() );Mat dst = _dst.getMat();Mutex histogramLockInstance;const int hist_sz = EqualizeHistCalcHist_Invoker::HIST_SZ;int hist[hist_sz] = {0,};int lut[hist_sz];EqualizeHistCalcHist_Invoker calcBody(src, hist, &histogramLockInstance);EqualizeHistLut_Invoker      lutBody(src, dst, lut);cv::Range heightRange(0, src.rows);if(EqualizeHistCalcHist_Invoker::isWorthParallel(src))parallel_for_(heightRange, calcBody);elsecalcBody(heightRange);int i = 0;while (!hist[i]) ++i;int total = (int)src.total();if (hist[i] == total){dst.setTo(i);return;}float scale = (hist_sz - 1.f)/(total - hist[i]);int sum = 0;for (lut[i++] = 0; i < hist_sz; ++i){sum += hist[i];lut[i] = saturate_cast<uchar>(sum * scale);}if(EqualizeHistLut_Invoker::isWorthParallel(src))parallel_for_(heightRange, lutBody);elselutBody(heightRange);
}

void cv::calcHist(     const Mat *      images,int              nimages,const int *      channels,InputArray      mask,OutputArray      hist,int              dims,const int *      histSize,const float **  ranges,bool              uniform = true,bool              accumulate = false )     

double cv::compareHist (     InputArray      H1,InputArray      H2,int                 method
)  

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>using namespace cv;
using namespace std;int main( int, char** argv )
{Mat src, dst;const char* source_window = "Source image";const char* equalized_window = "Equalized Image";// Load imagesrc = imread( argv[1], 1 );if( src.empty() ){ cout<<"Usage: ./Histogram_Demo <path_to_image>"<<endl;return -1;}// Convert to grayscale
  cvtColor( src, src, COLOR_BGR2GRAY );// Apply Histogram Equalization
  equalizeHist( src, dst );// Display results
  namedWindow( source_window, WINDOW_AUTOSIZE );namedWindow( equalized_window, WINDOW_AUTOSIZE );imshow( source_window, src );imshow( equalized_window, dst );// Wait until user exits the programwaitKey(0);return 0;}

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui/highgui.hpp"
#include "opencv2/imgproc/imgproc.hpp"
#include <iostream>
#include <stdio.h>using namespace std;
using namespace cv;/*** @function main*/
int main( int argc, char** argv )
{Mat src_base, hsv_base;Mat src_test1, hsv_test1;Mat src_test2, hsv_test2;Mat hsv_half_down;/// Load three images with different environment settingsif( argc < 4 ){printf("** Error. Usage: ./compareHist_Demo <image_settings0> <image_setting1> <image_settings2>\n");return -1;}src_base = imread( argv[1], 1 );src_test1 = imread( argv[2], 1 );src_test2 = imread( argv[3], 1 );/// Convert to HSV
    cvtColor( src_base, hsv_base, COLOR_BGR2HSV );cvtColor( src_test1, hsv_test1, COLOR_BGR2HSV );cvtColor( src_test2, hsv_test2, COLOR_BGR2HSV );hsv_half_down = hsv_base( Range( hsv_base.rows/2, hsv_base.rows - 1 ), Range( 0, hsv_base.cols - 1 ) );/// Using 50 bins for hue and 60 for saturationint h_bins = 50; int s_bins = 60;int histSize[] = { h_bins, s_bins };// hue varies from 0 to 179, saturation from 0 to 255float h_ranges[] = { 0, 180 };float s_ranges[] = { 0, 256 };const float* ranges[] = { h_ranges, s_ranges };// Use the o-th and 1-st channelsint channels[] = { 0, 1 };/// Histograms
    MatND hist_base;MatND hist_half_down;MatND hist_test1;MatND hist_test2;/// Calculate the histograms for the HSV imagescalcHist( &hsv_base, 1, channels, Mat(), hist_base, 2, histSize, ranges, true, false );normalize( hist_base, hist_base, 0, 1, NORM_MINMAX, -1, Mat() );calcHist( &hsv_half_down, 1, channels, Mat(), hist_half_down, 2, histSize, ranges, true, false );normalize( hist_half_down, hist_half_down, 0, 1, NORM_MINMAX, -1, Mat() );calcHist( &hsv_test1, 1, channels, Mat(), hist_test1, 2, histSize, ranges, true, false );normalize( hist_test1, hist_test1, 0, 1, NORM_MINMAX, -1, Mat() );calcHist( &hsv_test2, 1, channels, Mat(), hist_test2, 2, histSize, ranges, true, false );normalize( hist_test2, hist_test2, 0, 1, NORM_MINMAX, -1, Mat() );/// Apply the histogram comparison methodsfor( int i = 0; i < 4; i++ ){int compare_method = i;double base_base = compareHist( hist_base, hist_base, compare_method );double base_half = compareHist( hist_base, hist_half_down, compare_method );double base_test1 = compareHist( hist_base, hist_test1, compare_method );double base_test2 = compareHist( hist_base, hist_test2, compare_method );printf( " Method [%d] Perfect, Base-Half, Base-Test(1), Base-Test(2) : %f, %f, %f, %f \n", i, base_base, base_half , base_test1, base_test2 );}printf( "Done \n" );return 0;
}