Color image histogram in OpenCV

histogram.cpp

void drawHistogram(cv::Mat src, cv::Mat& histImage)
  {
      using namespace cv;
      using namespace std;

      /// Separate the image in 3 places ( B, G and R )
        vector<Mat> bgr_planes;
        split( src, bgr_planes );

        /// Establish the number of bins
        int histSize = 256;

        /// Set the ranges ( for B,G,R) )
        float range[] = { 0, 256 } ;
        const float* histRange = { range };

        bool uniform = true; bool accumulate = false;

        Mat b_hist, g_hist, r_hist;

        /// Compute the histograms:
        calcHist( &bgr_planes[0], 1, 0, Mat(), b_hist, 1, &histSize, &histRange, uniform, accumulate );
        calcHist( &bgr_planes[1], 1, 0, Mat(), g_hist, 1, &histSize, &histRange, uniform, accumulate );
        calcHist( &bgr_planes[2], 1, 0, Mat(), r_hist, 1, &histSize, &histRange, uniform, accumulate );

        // Draw the histograms for B, G and R
        int hist_w = 512; int hist_h = 200;
        int bin_w = cvRound( (double) hist_w/histSize );

        histImage.create( hist_h, hist_w, CV_8UC4);
        histImage = Scalar::all(0);

        /// Normalize the result to [ 0, histImage.rows ]
        normalize(b_hist, b_hist, 0, histImage.rows, NORM_MINMAX, -1, Mat() );
        normalize(g_hist, g_hist, 0, histImage.rows, NORM_MINMAX, -1, Mat() );
        normalize(r_hist, r_hist, 0, histImage.rows, NORM_MINMAX, -1, Mat() );

        /// Draw for each channel
        for( int i = 1; i < histSize; i++ )
        {
            line( histImage, Point( bin_w*(i-1), hist_h - cvRound(b_hist.at<float>(i-1)) ) ,
                             Point( bin_w*(i), hist_h - cvRound(b_hist.at<float>(i)) ),
                             Scalar( 255, 0, 0, 255), 2, CV_AA, 0  );
            line( histImage, Point( bin_w*(i-1), hist_h - cvRound(g_hist.at<float>(i-1)) ) ,
                             Point( bin_w*(i), hist_h - cvRound(g_hist.at<float>(i)) ),
                             Scalar( 0, 255, 0, 255), 2, CV_AA, 0  );
            line( histImage, Point( bin_w*(i-1), hist_h - cvRound(r_hist.at<float>(i-1)) ) ,
                             Point( bin_w*(i), hist_h - cvRound(r_hist.at<float>(i)) ),
                             Scalar( 0, 0, 255, 255), 2, CV_AA, 0  );
        }
  }