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.h) 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. @code{.cpp} #include <opencv2/opencv.hpp> using namespace cv;

/// Global Variables const int alpha_slider_max = 100; int alpha_slider; double alpha; double beta;

/// Matrices to store images Mat src1; Mat src2; Mat dst;

/*

• @function on_trackbar
• @brief Callback for trackbar / void on_trackbar( int, void ) { alpha = (double) alpha_slider/alpha_slider_max ; beta = ( 1.0 - alpha );

addWeighted( src1, alpha, src2, beta, 0.0, dst);

imshow( "Linear Blend", dst ); }

int main( int argc, char** argv ) { /// Read image ( same size, same type ) src1 = imread("../../images/LinuxLogo.jpg"); src2 = imread("../../images/WindowsLogo.jpg");

if( !src1.data ) { printf("Error loading src1 \n"); return -1; } if( !src2.data ) { printf("Error loading src2 \n"); return -1; }

/// Initialize values alpha_slider = 0;

/// Create Windows namedWindow("Linear Blend", 1);

/// Create Trackbars char TrackbarName[50]; sprintf( TrackbarName, "Alpha x %d", alpha_slider_max );

createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar );

/// Show some stuff on_trackbar( alpha_slider, 0 );

/// Wait until user press some key waitKey(0); return 0; } @endcode

Explanation

We only analyze the code that is related to Trackbar:

1. First, we load 02 images, which are going to be blended. @code{.cpp} src1 = imread("../../images/LinuxLogo.jpg"); src2 = imread("../../images/WindowsLogo.jpg"); @endcode

2. To create a trackbar, first we have to create the window in which it is going to be located. So: @code{.cpp} namedWindow("Linear Blend", 1); @endcode

3. Now we can create the Trackbar: @code{.cpp} createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_trackbar ); @endcode 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

4. Finally, we have to define the callback function on_trackbar @code{.cpp} void on_trackbar( int, void* ) { alpha = (double) alpha_slider/alpha_slider_max ; beta = ( 1.0 - alpha );

   addWeighted( src1, alpha, src2, beta, 0.0, dst);

   imshow( "Linear Blend", dst ); } @endcode 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 02 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: