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

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  and

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  . 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 createTrackbar

Code

Let’s modify the program made in the tutorial Adding (blending) two images using OpenCV. We will let the user enter the

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value by using the Trackbar.

#include <cv.h>
#include <highgui.h>

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;
}

Explanation

We only analyze the code that is related to Trackbar:

1. First, we load 02 images, which are going to be blended.

   src1 = imread("../../images/LinuxLogo.jpg");
   src2 = imread("../../images/WindowsLogo.jpg");
   

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

   namedWindow("Linear Blend", 1);
   

3. Now we can create the Trackbar:

   createTrackbar( TrackbarName, "Linear Blend", &alpha_slider, alpha_slider_max, on_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

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     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

   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 );
   }
   

   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 Changing the contrast and brightness of an image!. One trackbar to set

  System Message: WARNING/2 (\alpha)

  latex exited with error: [stderr] [stdout] This is pdfTeX, Version 3.1415926-1.40.10 (TeX Live 2009/Debian) entering extended mode (./math.tex LaTeX2e <2009/09/24> Babel <v3.8l> and hyphenation patterns for english, usenglishmax, dumylang, noh yphenation, loaded. (/usr/share/texmf-texlive/tex/latex/base/article.cls Document Class: article 2007/10/19 v1.4h Standard LaTeX document class (/usr/share/texmf-texlive/tex/latex/base/size12.clo)) (/usr/share/texmf-texlive/tex/latex/base/inputenc.sty (/usr/share/texmf-texlive/tex/latex/ucs/utf8x.def)) (/usr/share/texmf-texlive/tex/latex/ucs/ucs.sty (/usr/share/texmf-texlive/tex/latex/ucs/data/uni-global.def)) (/usr/share/texmf-texlive/tex/latex/amsmath/amsmath.sty For additional information on amsmath, use the `?’ option. (/usr/share/texmf-texlive/tex/latex/amsmath/amstext.sty (/usr/share/texmf-texlive/tex/latex/amsmath/amsgen.sty)) (/usr/share/texmf-texlive/tex/latex/amsmath/amsbsy.sty) (/usr/share/texmf-texlive/tex/latex/amsmath/amsopn.sty)) (/usr/share/texmf-texlive/tex/latex/amscls/amsthm.sty) (/usr/share/texmf-texlive/tex/latex/amsfonts/amssymb.sty (/usr/share/texmf-texlive/tex/latex/amsfonts/amsfonts.sty)) (/usr/share/texmf-texlive/tex/latex/tools/bm.sty) (/usr/share/texmf-texlive/tex/latex/euler/euler.sty Package: `euler’ v2.5 <1995/03/05> (FJ and FMi) ) (/usr/share/texmf-texlive/tex/latex/graphics/color.sty (/etc/texmf/tex/latex/config/color.cfg) (/usr/share/texmf-texlive/tex/latex/graphics/dvips.def) (/usr/share/texmf-texlive/tex/latex/graphics/dvipsnam.def)) ! LaTeX Error: File `bbm.sty’ not found. Type X to quit or <RETURN> to proceed, or enter new name. (Default extension: sty) Enter file name: ! Emergency stop. <read *> l.11 ...ackage{amsmath}\usepackage{bbm}\usepackage {colortbl}^^M No pages of output. Transcript written on math.log.

  and another for

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  . The output might look like: