OpenCV  4.10.0-dev
Open Source Computer Vision
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An example for creating histograms of an image

#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 histSize = 64;
int brightness = _brightness - 100;
int contrast = _contrast - 100;
* The algorithm is by Werner D. Streidt
* (
double a, b;
if( contrast > 0 )
double delta = 127.*contrast/100;
a = 255./(255. - delta*2);
b = a*(brightness - delta);
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);
calcHist(&dst, 1, 0, Mat(), hist, 1, &histSize, 0);
Mat histImage = Mat::ones(200, 320, CV_8U)*255;
normalize(hist, hist, 0, histImage.rows, NORM_MINMAX, CV_32F);
histImage = Scalar::all(255);
int binW = cvRound((double)histImage.cols/histSize);
for( int i = 0; i < histSize; i++ )
rectangle( histImage, Point(i*binW, histImage.rows),
Point((i+1)*binW, histImage.rows - cvRound(<float>(i))),
Scalar::all(0), -1, 8, 0 );
imshow("histogram", histImage);
const char* keys =
"{help h||}{@image|baboon.jpg|input image file}"
int main( int argc, const char** argv )
CommandLineParser parser(argc, argv, keys);
parser.about("\nThis program demonstrates the use of calcHist() -- histogram creation.\n");
if (parser.has("help"))
return 0;
string inputImage = parser.get<string>(0);
// Load the source image. HighGUI use.
image = imread(samples::findFile(inputImage), IMREAD_GRAYSCALE);
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);
return 0;
Designed for command line parsing.
Definition utility.hpp:832
n-dimensional dense array class
Definition mat.hpp:812
_Tp & at(int i0=0)
Returns a reference to the specified array element.
int cols
Definition mat.hpp:2138
int rows
the number of rows and columns or (-1, -1) when the matrix has more than 2 dimensions
Definition mat.hpp:2138
void convertTo(OutputArray m, int rtype, double alpha=1, double beta=0) const
Converts an array to another data type with optional scaling.
#define CV_8U
Definition interface.h:73
#define CV_32F
Definition interface.h:78
int cvRound(double value)
Rounds floating-point number to the nearest integer.
Definition fast_math.hpp:200
int waitKey(int delay=0)
Waits for a pressed key.
void namedWindow(const String &winname, int flags=WINDOW_AUTOSIZE)
Creates a window.
int createTrackbar(const String &trackbarname, const String &winname, int *value, int count, TrackbarCallback onChange=0, void *userdata=0)
Creates a trackbar and attaches it to the specified window.
CV_EXPORTS_W Mat imread(const String &filename, int flags=IMREAD_COLOR_BGR)
Loads an image from a file.
int main(int argc, char *argv[])
Definition highgui_qt.cpp:3
"black box" representation of the file storage associated with a file on disk.
Definition core.hpp:102
STL namespace.