opencv/samples/cpp/tutorial_code/HighGUI/BasicLinearTransformsTrackbar.cpp

/**
 * @file LinearTransforms.cpp
 * @brief Simple program to change contrast and brightness
 * @date Mon, June 6, 2011
 * @author OpenCV team
 */

#include <cv.h>
#include <highgui.h>

using namespace cv;

/** Global Variables */
const int alpha_max = 5;
const int beta_max = 125;
int alpha; /**< Simple contrast control */
int beta;  /**< Simple brightness control*/

/** Matrices to store images */
Mat image;
Mat new_image;

/**
 * @function on_trackbar
 * @brief Called whenever any of alpha or beta changes
 */
void on_trackbar( int, void* )
{  
   Mat new_image = Mat::zeros( image.size(), image.type() );

   for( int y = 0; y < image.rows; y++ )
      { for( int x = 0; x < image.cols; x++ )
           { for( int c = 0; c < 3; c++ )
                {
		  new_image.at<Vec3b>(y,x)[c] = saturate_cast<uchar>( alpha*( image.at<Vec3b>(y,x)[c] ) + beta );
                }
	   }
      }
   imshow("New Image", new_image);
}


/**
 * @function main
 * @brief Main function 
 */
int main( int argc, char** argv )
{
   /// Read image given by user
   image = imread( argv[1] );

   /// Initialize values 
   alpha = 1;
   beta = 0;

   /// Create Windows
   namedWindow("Original Image", 1);
   namedWindow("New Image", 1);

   /// Create Trackbars
   createTrackbar( "Contrast Trackbar", "New Image", &alpha, alpha_max, on_trackbar );
   createTrackbar( "Brightness Trackbar", "New Image", &beta, beta_max, on_trackbar );   

   /// Show some stuff
   imshow("Original Image", image);
   imshow("New Image", image);
   
   /// Wait until user press some key
   waitKey();
   return 0;
}