Image Scaling(Image Shrinking) Tutorial

Refer this code for shrinking the image by a factor of two:

// OpenCV Image Shrinking Tutorial 
#include <opencv2/core/core.hpp> 
#include <opencv2/highgui/highgui.hpp> 
#include <iostream>

using namespace cv;
using namespace std;

int main()
{
 Mat src1,src2;
 src1 = imread("C:\\Users\\arjun\\Desktop\\arj.jpg",CV_LOAD_IMAGE_COLOR);//Linux.jpg");//atom.jpg");//"Linux.jpg"); //f1.jpg
 if( !src1.data ) { printf("Error loading src1 \n"); return -1;}

 src2 = Mat::zeros((src1.rows/2)+1,(src1.cols/2)+1, CV_8UC3);
 cout<<"Shrinking factor is: 2"<<endl
 
for (float i=0; i<src1.cols ; i++)
{
  for (float j=0 ; j<src1.rows ; j++)
 { 
     Vec3b color1 = src1.at<Vec3b>(Point(i,j));
     Vec3b color2 = src2.at<Vec3b>(Point(i/2,j/2));
      color2.val[0] = color1.val[0];
      color2.val[1] = color1.val[1];
      color2.val[2] = color1.val[2];
 
      src2.at<Vec3b>(Point(i/2,j/2)) = color2;
 }
 }

namedWindow("Original Image",CV_WINDOW_AUTOSIZE); 
imshow("Original Image", src1);
//imwrite("C:\\Users\\arjun\\Desktop\\shrinked.jpg",src2);

namedWindow("Scaled Image",CV_WINDOW_AUTOSIZE); 
imshow("Scaled Image", src2); 

 waitKey(0);
 return 0;
}

Next we have done slight modifications in the code, where we take input from the user for shrinking an image by a particular factor:

// OpenCV Image Shrinking Tutorial 
#include <opencv2/core/core.hpp> 
#include <opencv2/highgui/highgui.hpp> 
#include <iostream>

using namespace cv;
using namespace std;

int main()
{
 Mat src1,src2;
 src1 = imread("C:\\Users\\arjun\\Desktop\\arj.jpg",CV_LOAD_IMAGE_COLOR);
    if( !src1.data ) { printf("Error loading src1 \n"); return -1;}
 
 int a=-1;
     while(a<=0){
 cout<<"Enter the Shrinking factor greater than 0 "<<endl;
 cin>>a;

  if( a<=0 )
 {
  cout<<"Invalid values... Please re-enter the correct values \n \n";
 }

      }

 src2 = Mat::zeros(src1.rows/a+1,src1.cols/a+1, CV_8UC3);
 cout<<"Shrinking factor is: "<<a<<endl;
for (int i=0; i<src1.cols-1 ; i++)
{
 for (int j=0 ; j<src1.rows-1; j++)
 {
  
     Vec3b color1 = src1.at<Vec3b>(Point(i,j));
   {
  {
  Vec3b color2 = src2.at<Vec3b>(Point(i/a,j/a));
  color2.val[0] = color1.val[0];
  color2.val[1] = color1.val[1];
  color2.val[2] = color1.val[2];
  src2.at<Vec3b>(Point(i/a,j/a)) = color2;
  
  }
   }
 }
 
 }

// imwrite("C:\\Users\\arjun\\Desktop\\shrinked.jpg",src2);
 namedWindow("Original Image",CV_WINDOW_AUTOSIZE); 
 imshow("Original Image", src1);

 namedWindow("Scaled Image",CV_WINDOW_AUTOSIZE); 
imshow("Scaled Image", src2); 

 waitKey(0);
 return 0;
}

OpenCV image shrinking tutorial with the help of a trackbar:

// OpenCV Image Shrinking Tutorial with trackbar
#include <opencv2/core/core.hpp> 
#include <opencv2/highgui/highgui.hpp> 
#include <iostream>

using namespace cv;
using namespace std;

Mat src1,src2;
int a=1;
const int shrink_slider_max=50;

void shrink_trackbar( int , void* )
{
 src2 = Mat::zeros(src1.rows/a+1,src1.cols/a+1, CV_8UC3);
 
for (int i=0; i<src1.cols-1 ; i++)
{
 for (int j=0 ; j<src1.rows-1; j++)
 {
  
     Vec3b color1 = src1.at<Vec3b>(Point(i,j));
   {
  {
  Vec3b color2 = src2.at<Vec3b>(Point(i/a,j/a));
  color2.val[0] = color1.val[0];
  color2.val[1] = color1.val[1];
  color2.val[2] = color1.val[2];
  src2.at<Vec3b>(Point(i/a,j/a)) = color2;
  
  }
   }
 }
 
 }
imshow("Scaled Image", src2); 
cout<<"Image Got Processed for Shrink factor="<<a<<endl;

}

int main()
{
 
 src1 = imread("C:\\Users\\arjun\\Desktop\\arj.jpg",CV_LOAD_IMAGE_COLOR);
 if( !src1.data ) { printf("Error loading src1 \n"); return -1;}
 
namedWindow("Scaled Image",CV_WINDOW_AUTOSIZE); 
createTrackbar( "Shrinking", "Scaled Image", &a,shrink_slider_max, shrink_trackbar );
shrink_trackbar( a, 0 );
// imwrite("C:\\Users\\arjun\\Desktop\\shrinked.jpg",src2);

namedWindow("Original Image",CV_WINDOW_AUTOSIZE); 
imshow("Original Image", src1);

 waitKey(0);
 return 0;
}