.. _Adding_Trackbars:

Adding a Trackbar to our applications!
***************************************

* 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 :math:`\alpha` and :math:`beta`. 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** 

  .. image:: images/Adding_Trackbars_Tutorial_Trackbar.png
     :alt: Trackbar example
     :align: center 

* 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  :create_trackbar:`createTrackbar <>` 

Code
=====

Let's modify the program made in the tutorial :ref:`Adding_Images`. We will let the user enter the :math:`\alpha` value by using the Trackbar.

.. code-block:: cpp

   #include <cv.h>
   #include <highgui.h>

   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;
   }


Explanation
============

We only analyze the code that is related to Trackbar:

#. First, we load 02 images, which are going to be blended.

   .. code-block:: cpp

      src1 = imread("../../images/LinuxLogo.jpg");
      src2 = imread("../../images/WindowsLogo.jpg");

#. To create a trackbar, first we have to create the window in which it is going to be located. So:

   .. code-block:: cpp

      namedWindow("Linear Blend", 1);

#. Now we can create the Trackbar:

   .. code-block:: cpp

      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 :math:`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

#. Finally, we have to define the callback function **on_trackbar**

   .. code-block:: 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 );
      }

   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:

  .. image:: images/Adding_Trackbars_Tutorial_Result_0.jpg
     :alt: Adding Trackbars - Windows Linux
     :align: center 

* As a manner of practice, you can also add 02 trackbars for the program made in :ref:`Basic_Linear_Transform`. One trackbar to set :math:`\alpha` and another for :math:`\beta`. The output might look like:

  .. image:: images/Adding_Trackbars_Tutorial_Result_1.jpg
     :alt: Adding Trackbars - Lena
     :align: center 





