.. _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 #include 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