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Adding a Trackbar to our applications! {#tutorial_trackbar}
======================================
- 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](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/HighGUI/AddingImagesTrackbar.cpp)
@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:
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