opencv/doc/tutorials/highgui/trackbar/trackbar.markdown

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 \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.hpp) for you. An example of this is a Trackbar

  

• 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. This tutorial code's is shown lines below. You can also download it from here @include cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp

Explanation

We only analyze the code that is related to Trackbar:

-# First, we load two images, which are going to be blended. @snippet cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp load

-# To create a trackbar, first we have to create the window in which it is going to be located. So: @snippet cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp window

-# Now we can create the Trackbar: @snippet cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp create_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 \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 @snippet cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp on_trackbar

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:

  

• As a manner of practice, you can also add two 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: