Doxygen tutorials: basic structure
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Maksim Shabunin
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doc/tutorials/highgui/trackbar/trackbar.markdown
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doc/tutorials/highgui/trackbar/trackbar.markdown
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Adding a Trackbar to our applications! {#tutorial_trackbar}
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======================================
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- In the previous tutorials (about *linear blending* and the *brightness and contrast
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adjustments*) you might have noted that we needed to give some **input** to our programs, such
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as \f$\alpha\f$ and \f$beta\f$. We accomplished that by entering this data using the Terminal
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- Well, it is time to use some fancy GUI tools. OpenCV provides some GUI utilities (*highgui.h*)
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for you. An example of this is a **Trackbar**
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- In this tutorial we will just modify our two previous programs so that they get the input
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information from the trackbar.
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Goals
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-----
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In this tutorial you will learn how to:
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- Add a Trackbar in an OpenCV window by using @ref cv::createTrackbar
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Code
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----
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Let's modify the program made in the tutorial @ref Adding_Images. We will let the user enter the
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\f$\alpha\f$ value by using the Trackbar.
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@code{.cpp}
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#include <opencv2/opencv.hpp>
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using namespace cv;
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/// Global Variables
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const int alpha_slider_max = 100;
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int alpha_slider;
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double alpha;
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double beta;
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/// Matrices to store images
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Mat src1;
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Mat src2;
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Mat dst;
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/*
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* @function on_trackbar
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* @brief Callback for trackbar
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*/
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void on_trackbar( int, void* )
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{
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alpha = (double) alpha_slider/alpha_slider_max ;
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beta = ( 1.0 - alpha );
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addWeighted( src1, alpha, src2, beta, 0.0, dst);
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imshow( "Linear Blend", dst );
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}
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int main( int argc, char** argv )
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{
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/// Read image ( same size, same type )
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src1 = imread("../../images/LinuxLogo.jpg");
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src2 = imread("../../images/WindowsLogo.jpg");
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if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
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if( !src2.data ) { printf("Error loading src2 \n"); return -1; }
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/// Initialize values
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alpha_slider = 0;
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/// Create Windows
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namedWindow("Linear Blend", 1);
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/// Create Trackbars
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char TrackbarName[50];
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sprintf( TrackbarName, "Alpha x %d", alpha_slider_max );
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createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );
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/// Show some stuff
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on_trackbar( alpha_slider, 0 );
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/// Wait until user press some key
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waitKey(0);
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return 0;
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}
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@endcode
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Explanation
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-----------
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We only analyze the code that is related to Trackbar:
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1. First, we load 02 images, which are going to be blended.
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@code{.cpp}
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src1 = imread("../../images/LinuxLogo.jpg");
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src2 = imread("../../images/WindowsLogo.jpg");
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@endcode
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2. To create a trackbar, first we have to create the window in which it is going to be located. So:
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@code{.cpp}
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namedWindow("Linear Blend", 1);
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@endcode
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3. Now we can create the Trackbar:
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@code{.cpp}
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createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );
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@endcode
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Note the following:
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- Our Trackbar has a label **TrackbarName**
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- The Trackbar is located in the window named **"Linear Blend"**
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- The Trackbar values will be in the range from \f$0\f$ to **alpha_slider_max** (the minimum
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limit is always **zero**).
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- The numerical value of Trackbar is stored in **alpha_slider**
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- Whenever the user moves the Trackbar, the callback function **on_trackbar** is called
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4. Finally, we have to define the callback function **on_trackbar**
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@code{.cpp}
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void on_trackbar( int, void* )
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{
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alpha = (double) alpha_slider/alpha_slider_max ;
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beta = ( 1.0 - alpha );
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addWeighted( src1, alpha, src2, beta, 0.0, dst);
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imshow( "Linear Blend", dst );
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}
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@endcode
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Note that:
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- We use the value of **alpha_slider** (integer) to get a double value for **alpha**.
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- **alpha_slider** is updated each time the trackbar is displaced by the user.
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- We define *src1*, *src2*, *dist*, *alpha*, *alpha_slider* and *beta* as global variables,
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so they can be used everywhere.
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Result
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------
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- Our program produces the following output:
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- As a manner of practice, you can also add 02 trackbars for the program made in @ref
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Basic_Linear_Transform. One trackbar to set \f$\alpha\f$ and another for \f$\beta\f$. The output might
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look like:
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