chore: import upstream snapshot with attribution

samples/cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp

@@ -0,0 +1,74 @@
+/**
+ * @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;
+}