using opencv histogram cv::calcHist()

The example in opencv distribution is not clear.
Here I added some more to make it clear.

#include "opencv2/imgproc/imgproc.hpp"

#include "opencv2/highgui/highgui.hpp"

#include <iostream>

using namespace cv;

using namespace std;

int _brightness = 100;

int _contrast   = 100;

Mat image;

/* brightness/contrast callback function */

static void updateBrightnessContrast( int /*arg*/, void* )

{

    int brightness = _brightness - 100;

    int contrast = _contrast - 100;

    /*

     * The algorithm is by Werner D. Streidt

     * (http://visca.com/ffactory/archives/5-99/msg00021.html)

     */

    double a, b;

    if( contrast > 0 )

    {

        double delta = 127.*contrast/100;

        a = 255./(255. - delta*2);

        b = a*(brightness - delta);

    }

    else

    {

        double delta = -128.*contrast/100;

        a = (256.-delta*2)/255.;

        b = a*brightness + delta;

    }

    Mat dst, hist;

    image.convertTo(dst, CV_8U, a, b);

    imshow("image", dst);

    int nImages = 1;

    int channels[1] = {0};

    const int histDim = 1;

    int histSize[histDim] = {64};

    float hist_range[] = {0,256};

    const float *ranges[] = { hist_range };

    calcHist(&dst, nImages, channels, Mat(), 

             hist, histDim/*dimension of the histogram*/, 

             histSize, ranges);

    Mat histImage = Mat::ones(200, 320, CV_8U)*255;

    normalize(hist, hist, 0, histImage.rows, CV_MINMAX, CV_32F);

    histImage = Scalar::all(255);

    int binW = cvRound((double)histImage.cols/histSize[0]);

    for( int i = 0; i < histSize[0]; i++ )

        rectangle( histImage, Point(i*binW, histImage.rows),

                   Point((i+1)*binW, histImage.rows - cvRound(hist.at<float>(i))),

                   Scalar::all(0), -1, 8, 0 );

    imshow("histogram", histImage);

}

static void help()

{

    std::cout << "\nThis program demonstrates the use of calcHist() -- histogram creation.\n"

              << "Usage: \n" << "demhist [image_name -- Defaults to baboon.jpg]" << std::endl;

}

const char* keys =

{

    "{1| |baboon.jpg|input image file}"

};

int main( int argc, const char** argv )

{

    help();

    CommandLineParser parser(argc, argv, keys);

    string inputImage = parser.get<string>("1");

    // Load the source image. HighGUI use.

    image = imread( inputImage, 0 );

    if(image.empty())

    {

        std::cerr << "Cannot read image file: " << inputImage << std::endl;

        return -1;

    }

    namedWindow("image", 0);

    namedWindow("histogram", 0);

    createTrackbar("brightness", "image", &_brightness, 200, updateBrightnessContrast);

    createTrackbar("contrast", "image", &_contrast, 200, updateBrightnessContrast);

    updateBrightnessContrast(0, 0);

    waitKey();

    return 0;

}