opencv/doc/tutorials/app/trackbar.markdown

Adding a Trackbar to our applications!

@next_tutorial{tutorial_raster_io_gdal}

Original author	Ana Huamán
Compatibility	OpenCV >= 3.0

• In the previous tutorials (about @ref tutorial_adding_images and the @ref tutorial_basic_linear_transform) 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 module) 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.

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

@add_toggle_java This tutorial code's is shown lines below. You can also download it from here @include java/tutorial_code/highgui/trackbar/AddingImagesTrackbar.java @end_toggle

@add_toggle_python This tutorial code's is shown lines below. You can also download it from here @include python/tutorial_code/highgui/trackbar/AddingImagesTrackbar.py @end_toggle

Explanation

We only analyze the code that is related to Trackbar:

• First, we load two images, which are going to be blended.

@add_toggle_cpp @snippet cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp load @end_toggle

@add_toggle_java @snippet java/tutorial_code/highgui/trackbar/AddingImagesTrackbar.java load @end_toggle

@add_toggle_python @snippet python/tutorial_code/highgui/trackbar/AddingImagesTrackbar.py load @end_toggle

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

@add_toggle_cpp @snippet cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp window @end_toggle

@add_toggle_java @snippet java/tutorial_code/highgui/trackbar/AddingImagesTrackbar.java window @end_toggle

@add_toggle_python @snippet python/tutorial_code/highgui/trackbar/AddingImagesTrackbar.py window @end_toggle

• Now we can create the Trackbar:

@add_toggle_cpp @snippet cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp create_trackbar @end_toggle

@add_toggle_java @snippet java/tutorial_code/highgui/trackbar/AddingImagesTrackbar.java create_trackbar @end_toggle

@add_toggle_python @snippet python/tutorial_code/highgui/trackbar/AddingImagesTrackbar.py create_trackbar @end_toggle

Note the following (C++ code): - 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 for C++ and Python code, using an anonymous inner class listener in Java

@add_toggle_cpp @snippet cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp on_trackbar @end_toggle

@add_toggle_java @snippet java/tutorial_code/highgui/trackbar/AddingImagesTrackbar.java on_trackbar @end_toggle

@add_toggle_python @snippet python/tutorial_code/highgui/trackbar/AddingImagesTrackbar.py on_trackbar @end_toggle

Note that (C++ code): - 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 set \f$\beta\f$. The output might look like: