How To Blur a Picture Using A Trackbar In OpenCV [C++]

Step 1 : importing HPP files (Headers):

#include <opencv2\highgui\highgui.hpp>
 
#include <opencv2\imgproc\imgproc.hpp>
 
#include 

• OpenCV has some good GUI utilities for us (highgui.hpp).
• We need imgproc.hpp to have access to plenty of functions to edit our pictures like blur for example.

Step 2 : creating namespaces :

using namespace cv;
 
using namespace std;

Using namespaces will let us save some time and effort. For example, writing certain terms like cv every time before calling an OpenCV function will not be necessary.

Step 3 : Creating a Trackbar Callback Function and declaring its necessary variables :

Mat image,output;
 
void onChange(int sliderValue, void* userData)
 
{
 
if (sliderValue <= 0)
 
return;
 
blur(image, output, Size(sliderValue, sliderValue));
imshow("BlurWindow", output);
 
}

• We are here creating empty matrices. The Input picture will be stored in the matrix named image and the Output picture in the matrix named output.
• onChange presents a pointer to the function to be called every time the slider position has been changed. So we don't need to have a loop to check every time the slider state.
• blur is a predefined function in OpenCV which blurs a picture using box filter method.
• imshow let us display the window.
• To avoid an exception in case sliderValue <=0,we need an if statement which returns nothing.

Step 4 : Writing the main function :

int main() {
 
// Code Here
 
return 0 ;
 
}

If you don't know what a main function is, it's simply a function that presents the entry point of the program that we are going to run.

Step 5: Declaring the rest of the necessary variable inside the main function :

int blurAmount = 0;

• 0 is the starting value of the slider

String imagefile = "C:\\mouse.jpg";

• The path of the local picture will be stored as string in the variable imagefile.
• Be aware to use \\ instead of \

Step 6: Loading the local image :

image = imread(imagefile);

Step 7: Creating a Display Window :

namedWindow("BlurWindow");

BlurWindow will be the window name.

Step 8 : Creating the trackbar inside the main function:

createTrackbar("Blur Value", "BlurWindow", &blurAmount, 100, onChange);

• 1st parameter : name of the trackbar
• 2nd parameter: name of the window where the trackbar must be attached
• 3rd parameter: starting value of the slider
• 4th parameter: maximum value of the slider
• 5th parameter: trackbar callback function name

Step 9 : Displaying the processed image:

imshow("Display window", image);

Step 10 : Setting up display time of the output window:

waitKey(0);  //exit on any key pressed

• waitKey(0) will display the window infinitely until you press any key(it is suitable for image display not for video display).

Code :

#include <opencv2\highgui\highgui.hpp>
 
#include <opencv2\imgproc\imgproc.hpp>
 
#include 
 
using namespace cv;
 
using namespace std;
 
Mat image, output;
 
void onChange(int sliderValue, void* userData) {
 
if (sliderValue <= 0) return;
 
// 1st param =input image ;2nd param =output image ,3rd param =blur size
 
blur(image, output, Size(sliderValue, sliderValue));
 
imshow("BlurWindow", output);
 
}
 
int main(){
 
int blurAmount = 0;
 
namedWindow("BlurWindow");
 
String imagefile = "C:\\mouse.jpg";
 
image = imread(imagefile);
 
createTrackbar("Blur Value", "BlurWindow", &blurAmount, 100, onChange);
 
imshow("BlurWindow", image);
 
waitKey(0);
 
}