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;
}
