OpenCV  4.1.2-dev
Open Source Computer Vision
samples/cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp

Check the corresponding tutorial for more details

#include <iostream>
using namespace cv;
using std::cout;
const int alpha_slider_max = 100;
int alpha_slider;
double alpha;
double beta;
Mat src1;
Mat src2;
Mat dst;
static void on_trackbar( int, void* )
{
alpha = (double) alpha_slider/alpha_slider_max ;
beta = ( 1.0 - alpha );
addWeighted( src1, alpha, src2, beta, 0.0, dst);
imshow( "Linear Blend", dst );
}
int main( void )
{
src1 = imread( samples::findFile("LinuxLogo.jpg") );
src2 = imread( samples::findFile("WindowsLogo.jpg") );
if( src1.empty() ) { cout << "Error loading src1 \n"; return -1; }
if( src2.empty() ) { cout << "Error loading src2 \n"; return -1; }
alpha_slider = 0;
namedWindow("Linear Blend", WINDOW_AUTOSIZE); // Create Window
char TrackbarName[50];
sprintf( TrackbarName, "Alpha x %d", alpha_slider_max );
createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );
on_trackbar( alpha_slider, 0 );
waitKey(0);
return 0;
}