1Adding a Trackbar to our applications! {#tutorial_trackbar}
2======================================
3
4-   In the previous tutorials (about *linear blending* and the *brightness and contrast
5    adjustments*) you might have noted that we needed to give some **input** to our programs, such
6    as \f$\alpha\f$ and \f$beta\f$. We accomplished that by entering this data using the Terminal
7-   Well, it is time to use some fancy GUI tools. OpenCV provides some GUI utilities (*highgui.h*)
8    for you. An example of this is a **Trackbar**
9
10    ![](images/Adding_Trackbars_Tutorial_Trackbar.png)
11
12-   In this tutorial we will just modify our two previous programs so that they get the input
13    information from the trackbar.
14
15Goals
16-----
17
18In this tutorial you will learn how to:
19
20-   Add a Trackbar in an OpenCV window by using @ref cv::createTrackbar
21
22Code
23----
24
25Let's modify the program made in the tutorial @ref tutorial_adding_images. We will let the user enter the
26\f$\alpha\f$ value by using the Trackbar.
27@code{.cpp}
28#include <opencv2/opencv.hpp>
29using namespace cv;
30
31/// Global Variables
32const int alpha_slider_max = 100;
33int alpha_slider;
34double alpha;
35double beta;
36
37/// Matrices to store images
38Mat src1;
39Mat src2;
40Mat dst;
41
42/*
43 * @function on_trackbar
44 * @brief Callback for trackbar
45 */
46void on_trackbar( int, void* )
47{
48  alpha = (double) alpha_slider/alpha_slider_max ;
49  beta = ( 1.0 - alpha );
50
51  addWeighted( src1, alpha, src2, beta, 0.0, dst);
52
53  imshow( "Linear Blend", dst );
54}
55
56int main( int argc, char** argv )
57{
58  /// Read image ( same size, same type )
59  src1 = imread("../../images/LinuxLogo.jpg");
60  src2 = imread("../../images/WindowsLogo.jpg");
61
62  if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
63  if( !src2.data ) { printf("Error loading src2 \n"); return -1; }
64
65  /// Initialize values
66  alpha_slider = 0;
67
68  /// Create Windows
69  namedWindow("Linear Blend", 1);
70
71  /// Create Trackbars
72  char TrackbarName[50];
73  sprintf( TrackbarName, "Alpha x %d", alpha_slider_max );
74
75  createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );
76
77  /// Show some stuff
78  on_trackbar( alpha_slider, 0 );
79
80  /// Wait until user press some key
81  waitKey(0);
82  return 0;
83}
84@endcode
85
86Explanation
87-----------
88
89We only analyze the code that is related to Trackbar:
90
91-#  First, we load 02 images, which are going to be blended.
92    @code{.cpp}
93    src1 = imread("../../images/LinuxLogo.jpg");
94    src2 = imread("../../images/WindowsLogo.jpg");
95    @endcode
96-#  To create a trackbar, first we have to create the window in which it is going to be located. So:
97    @code{.cpp}
98    namedWindow("Linear Blend", 1);
99    @endcode
100-#  Now we can create the Trackbar:
101    @code{.cpp}
102    createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );
103    @endcode
104    Note the following:
105
106    -   Our Trackbar has a label **TrackbarName**
107    -   The Trackbar is located in the window named **"Linear Blend"**
108    -   The Trackbar values will be in the range from \f$0\f$ to **alpha_slider_max** (the minimum
109        limit is always **zero**).
110    -   The numerical value of Trackbar is stored in **alpha_slider**
111    -   Whenever the user moves the Trackbar, the callback function **on_trackbar** is called
112
113-#  Finally, we have to define the callback function **on_trackbar**
114    @code{.cpp}
115    void on_trackbar( int, void* )
116    {
117     alpha = (double) alpha_slider/alpha_slider_max ;
118     beta = ( 1.0 - alpha );
119
120     addWeighted( src1, alpha, src2, beta, 0.0, dst);
121
122     imshow( "Linear Blend", dst );
123    }
124    @endcode
125    Note that:
126    -   We use the value of **alpha_slider** (integer) to get a double value for **alpha**.
127    -   **alpha_slider** is updated each time the trackbar is displaced by the user.
128    -   We define *src1*, *src2*, *dist*, *alpha*, *alpha_slider* and *beta* as global variables,
129        so they can be used everywhere.
130
131Result
132------
133
134-   Our program produces the following output:
135
136    ![](images/Adding_Trackbars_Tutorial_Result_0.jpg)
137
138-   As a manner of practice, you can also add 02 trackbars for the program made in
139    @ref tutorial_basic_linear_transform. One trackbar to set \f$\alpha\f$ and another for \f$\beta\f$. The output might
140    look like:
141
142    ![](images/Adding_Trackbars_Tutorial_Result_1.jpg)
143