opencv/samples/cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp

/**
 * @file AddingImagesTrackbar.cpp
 * @brief Simple linear blender ( dst = alpha*src1 + beta*src2 )
 * @author OpenCV team
 */

#include "opencv2/imgcodecs.hpp"
#include "opencv2/highgui.hpp"
#include <iostream>

using namespace cv;
using std::cout;

/** Global Variables */
const int alpha_slider_max = 100;
int alpha_slider;
double alpha;
double beta;

/** Matrices to store images */
Mat src1;
Mat src2;
Mat dst;

//![on_trackbar]
/**
 * @function on_trackbar
 * @brief Callback for trackbar
 */
static void on_trackbar(int pos, void* userdata) {
    (void) userdata;
    alpha = (double)pos / alpha_slider_max;
    beta = (1.0 - alpha);
    addWeighted(src1, alpha, src2, beta, 0.0, dst);
    imshow("Linear Blend", dst);
}
//![on_trackbar]

/**
 * @function main
 * @brief Main function
 */
int main(void)
{
    //![load]
    /// Read images (both must be of the same size and type)
    src1 = imread(samples::findFile("LinuxLogo.jpg"));
    src2 = imread(samples::findFile("WindowsLogo.jpg"));
    //![load]

    if (src1.empty()) { cout << "Error loading src1 \n"; return -1; }
    if (src2.empty()) { cout << "Error loading src2 \n"; return -1; }

    // Initialize trackbar value
    alpha_slider = 0;

    //![window]
    namedWindow("Linear Blend", WINDOW_AUTOSIZE); //Create Window
    //![window]

    //![create_trackbar]
    char TrackbarName[50];
    snprintf(TrackbarName, sizeof(TrackbarName), "Alpha x %d", alpha_slider_max);
    // Example userdata: Pass a pointer to an integer as userdata
    createTrackbar(TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar);
    //![create_trackbar]

    /// Show initial result
    on_trackbar(alpha_slider, nullptr);

    /// Wait for user input
    waitKey(0);
    return 0;
}