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 Hu Chunming
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Adding a Trackbar to our applications! {#tutorial_trackbar}
======================================
@tableofcontents
@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](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp)
@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](https://github.com/opencv/opencv/tree/master/samples/java/tutorial_code/highgui/trackbar/AddingImagesTrackbar.java)
@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](https://github.com/opencv/opencv/tree/master/samples/python/tutorial_code/highgui/trackbar/AddingImagesTrackbar.py)
@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:
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