Adding a Trackbar to our applications! {#tutorial_trackbar}
======================================

-   In the previous tutorials (about *linear blending* and the *brightness and contrast
    adjustments*) you might have noted that we needed to give some **input** to our programs, such
    as \f$\alpha\f$ and \f$beta\f$. We accomplished that by entering this data using the Terminal
-   Well, it is time to use some fancy GUI tools. OpenCV provides some GUI utilities (*highgui.h*)
    for you. An example of this is a **Trackbar**

    ![](images/Adding_Trackbars_Tutorial_Trackbar.png)

-   In this tutorial we will just modify our two previous programs so that they get the input
    information from the trackbar.

Goals
-----

In this tutorial you will learn how to:

-   Add a Trackbar in an OpenCV window by using @ref cv::createTrackbar

Code
----

Let's modify the program made in the tutorial @ref tutorial_adding_images. We will let the user enter the
\f$\alpha\f$ value by using the Trackbar.
@code{.cpp}
#include <opencv2/opencv.hpp>
using namespace cv;

/// 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;

/*
 * @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 )
{
  /// Read image ( same size, same type )
  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; }

  /// Initialize values
  alpha_slider = 0;

  /// Create Windows
  namedWindow("Linear Blend", 1);

  /// Create Trackbars
  char TrackbarName[50];
  sprintf( TrackbarName, "Alpha x %d", alpha_slider_max );

  createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );

  /// Show some stuff
  on_trackbar( alpha_slider, 0 );

  /// Wait until user press some key
  waitKey(0);
  return 0;
}
@endcode

Explanation
-----------

We only analyze the code that is related to Trackbar:

-#  First, we load 02 images, which are going to be blended.
    @code{.cpp}
    src1 = imread("../../images/LinuxLogo.jpg");
    src2 = imread("../../images/WindowsLogo.jpg");
    @endcode
-#  To create a trackbar, first we have to create the window in which it is going to be located. So:
    @code{.cpp}
    namedWindow("Linear Blend", 1);
    @endcode
-#  Now we can create the Trackbar:
    @code{.cpp}
    createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );
    @endcode
    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 \f$0\f$ 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

-#  Finally, we have to define the callback function **on_trackbar**
    @code{.cpp}
    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 );
    }
    @endcode
    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
------

-   Our program produces the following output:

    ![](images/Adding_Trackbars_Tutorial_Result_0.jpg)

-   As a manner of practice, you can also add 02 trackbars for the program made in
    @ref tutorial_basic_linear_transform. One trackbar to set \f$\alpha\f$ and another for \f$\beta\f$. The output might
    look like:

    ![](images/Adding_Trackbars_Tutorial_Result_1.jpg)