Adding a Trackbar

int createTrackbar(const string& trackbarname, const string& winname, int* value, int count, TrackbarCallback onChange=0, void* userdata=0)

Parameters:

• trackbarname – Name of the created trackbar.
• winname – Name of the window that will be used as a parent of the created trackbar.
• value – Optional pointer to an integer variable whose value reflects the position of the slider. Upon creation, the slider position is defined by this variable.
• count – Maximal position of the slider. The minimal position is always 0.
• onChange – Pointer to the function to be called every time the slider changes position. This function should be prototyped as void Foo(int,void*); , where the first parameter is the trackbar position and the second parameter is the user data (see the next parameter). If the callback is the NULL pointer, no callbacks are called, but only value is updated.
• userdata – User data that is passed as is to the callback. It can be used to handle trackbar events without using global variables.

The code provided below is slight modification of code given in OpenCV documentation.

Example:

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#include <opencv/cv.h>
#include <opencv2/highgui/highgui.hpp>

using namespace cv;

/// Global Variables
const int alpha_slider_max = 100;
int alpha_slider;
Mat src1,src2,dst;

void on_trackbar( int, void* )
{
 double alpha;
 double beta;

 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( )
{
 // Read image ( same size, same type )
 src1 = imread("image2.jpg",0);
 src2 = imread("image1.jpg",0);

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

 // Initialize values
 alpha_slider = 0;
 createTrackbar( "Alpha ", "Linear Blend", &alpha_slider,  alpha_slider_max, on_trackbar );

 // Initialize trackbar
 on_trackbar( 0, 0 );

 waitKey(0);
 return 0;
}

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