opencv histogram comparison

foreword

The image histogram is explained in detail in , and image histogram comparison is a means of measuring the similarity of two images. To compare two histograms (H1 and H2), first we have to choose a metric (d(H1,H2)) to represent how well the two histograms match. OpenCV provides 4 different metrics to calculate matches:
Correlation (HISTCMP_CORREL):


N is N is the total number of histogram bins.

Chi-Square ( HISTCMP_CHISQR)

Intersection ( method=HISTCMP_INTERSECT)

Bhattacharyya distance ( HISTCMP_BHATTACHARYYA)

opencv function support

Function prototype:

CV_EXPORTS_W double compareHist( InputArray H1, InputArray H2, int method );

Parameter description:
H1: The first to compare the histogram.
H2: The second wants a histogram, the same size as H1.
method : The method of comparison, see #HistCompMethods, for Correlation and Intersection methods, the higher the metric, the more accurate the match. Square and Bhattacharyya distance are the opposite.

Code example:

    cv::Mat image = cv::imread("D:\\QtProject\\Opencv_Example\\Hist\\Hist.png", cv::IMREAD_GRAYSCALE);
    if (image.empty()) {
    
    
      cout << "Cannot load image" << endl;
      return;
    }
    imshow("image",image);

    const int bins[1] = {
    
     256 };
    float hranges[2] = {
    
     0,255 };
    const float* ranges[1] = {
    
     hranges };

    Mat hist;
    // 计算直方图
    calcHist(&image, 1, 0, Mat(), hist, 1, bins, ranges);
    normalize(hist, hist, 0, 1, NORM_MINMAX, -1, Mat()); //将数据规皈依到0和1之间

    cv::Mat imageCvert;
    image.convertTo(imageCvert, -1, 1, 50);
    imshow( "imageCvert", imageCvert);
    Mat histCompare;
    calcHist(&imageCvert, 1, 0, Mat(), histCompare, 1, bins, ranges);
    normalize(histCompare, histCompare, 0, 1, NORM_MINMAX, -1, Mat()); //将数据规皈依到0和1之间


   double CORREL = compareHist( hist, histCompare, HISTCMP_CORREL );
   double CHISQR = compareHist( hist, histCompare, HISTCMP_CHISQR );
   double INTERSECT = compareHist( hist, histCompare, HISTCMP_INTERSECT );
   double BHATTACHARYYA = compareHist( hist, histCompare, HISTCMP_BHATTACHARYYA );
   cout << "HISTCMP_CORREL:" << CORREL<<endl
        <<  "HISTCMP_CHISQR:"  << CHISQR << endl
        << "HISTCMP_INTERSECT:" << INTERSECT << endl
        << "HISTCMP_BHATTACHARYYA:" << BHATTACHARYYA << endl;

Two images compared:

run results: