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samples/cpp/tutorial_code/HighGUI/BasicLinearTransformsTrackbar.cpp

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+/**
+ * @file BasicLinearTransformsTrackbar.cpp
+ * @brief Simple program to change contrast and brightness
+ * @date Mon, June 6, 2011
+ * @author OpenCV team
+ */
+
+#include "opencv2/imgcodecs.hpp"
+#include "opencv2/highgui.hpp"
+
+// we're NOT "using namespace std;" here, to avoid collisions between the beta variable and std::beta in c++17
+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;
+
+/**
+ * @function on_trackbar
+ * @brief Called whenever any of alpha or beta changes
+ */
+static 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
+   String imageName("lena.jpg"); // by default
+   if (argc > 1)
+   {
+      imageName = argv[1];
+   }
+   image = imread( samples::findFile( imageName ) );
+
+   /// Initialize values
+   alpha = 1;
+   beta = 0;
+
+   /// Create Windows
+   namedWindow("Original Image", 1);
+   namedWindow("New Image", 1);
+
+   /// Create Trackbars
+   createTrackbar( "Contrast", "New Image", &alpha, alpha_max, on_trackbar );
+   createTrackbar( "Brightness", "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;
+}