OpenCV  3.0.0
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Adding a Trackbar to our applications!

Goals

In this tutorial you will learn how to:

Code

Let's modify the program made in the tutorial Adding (blending) two images using OpenCV. We will let the user enter the \(\alpha\) value by using the Trackbar.

#include <opencv2/opencv.hpp>
using namespace cv;
const int alpha_slider_max = 100;
int alpha_slider;
double alpha;
double beta;
Mat src1;
Mat src2;
Mat dst;
/*
* @function on_trackbar
* @brief Callback for trackbar
*/
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( int argc, char** argv )
{
src1 = imread("../../images/LinuxLogo.jpg");
src2 = imread("../../images/WindowsLogo.jpg");
if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
if( !src2.data ) { printf("Error loading src2 \n"); return -1; }
alpha_slider = 0;
namedWindow("Linear Blend", 1);
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;
}

Explanation

We only analyze the code that is related to Trackbar:

  1. First, we load 02 images, which are going to be blended.
    src1 = imread("../../images/LinuxLogo.jpg");
    src2 = imread("../../images/WindowsLogo.jpg");
  2. To create a trackbar, first we have to create the window in which it is going to be located. So:
    namedWindow("Linear Blend", 1);
  3. Now we can create the Trackbar:
    createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );
    Note the following:
    • Our Trackbar has a label TrackbarName
    • The Trackbar is located in the window named **"Linear Blend"**
    • The Trackbar values will be in the range from \(0\) to alpha_slider_max (the minimum limit is always zero).
    • The numerical value of Trackbar is stored in alpha_slider
    • Whenever the user moves the Trackbar, the callback function on_trackbar is called
  4. Finally, we have to define the callback function on_trackbar
    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 );
    }
    Note that:
    • We use the value of alpha_slider (integer) to get a double value for alpha.
    • alpha_slider is updated each time the trackbar is displaced by the user.
    • We define src1, src2, dist, alpha, alpha_slider and beta as global variables, so they can be used everywhere.

Result