Blending two Images/Merging two Images/Adding two image

Image are basically matrices of pixel values.Thus Image Blending or Image Merging in layman terms simply means that  adding the pixel values at a particular co-ordinates of two images.
Hence here images should be of the same size.

For e.g if the pixel value of two gray-scale images are 120 and 35 respectively.Then after blending the pixel value at that particular co-ordinate would become 155.

Note:
Grayscale image is the one where each pixel is stored as a single byte(8 bits).
Thus the pixel values can range from 0 to 255.
Where 0 denotes black and 255 denotes white.

So what would happen if the pixel value of the two gray-scale images when merged exceed 255.
For e.g Let the pixel value at the particular co-ordinate is 250 (would appear white)and that of the other image at the same co-ordinate is 120 (would appear dark).
After merging it would seem dark because the pixel value at the respective co-ordinate of the merged image would be 255.Since 120+240>255)

Thus the lowest value of the pixel is 0. So even if we multiply a pixel value by -1.
The pixel value of the modified image would be 0.
i.e. If the pixel value at a particular co-ordinate is 255 (white) and if we multiply it by -1.
      Then the pixel at that point of image would become 0 (black).

We can check the above concept by accessing the pixel value of the merged image at a particular point.Refer:
http://opencvhub.blogspot.in/2015/06/accessing-pixel-value-of-image-using-vec3b-function-obtain-coordinates-get-location-point-display-opencv.html

Note:We can merge more than 2 images also.

The code for merging/blending the two images are as shown below:

#include <opencv2/core/core.hpp> 
#include <opencv2/highgui/highgui.hpp> 
#include <iostream>


using namespace cv;
using namespace std;

int main()
{
 
 Mat src1, src2, src3;
 /// Read image ( same size, same type )
 src1 = imread("C:\\Users\\arjun\\Desktop\\red.jpg");
 src2 = imread("C:\\Users\\arjun\\Desktop\\green.jpg");
 
 ///Comparing whether the two images are of same size or not
 int width1 , width2 , height1 , height2;
 width1 =src1.cols; 
 height1=src1.rows; 
 width2 =src2.cols; 
 height2=src2.rows; 
 
 if (width1!=width2 && height1!=height2)
 {
  printf("Error:Images must be of the same size \n");
  return -1;
 }
 
 //Merging two images
 src3=src1 + src2;

 if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src2.data ) { printf("Error loading src2 \n"); return -1; }
 if( !src3.data ) { printf("Error loading src1 \n"); return -1; }
  
 /// Create Windows
 namedWindow("First Image", 1);
 imshow( "First Image", src1 );

 namedWindow("Second Image", 1);
 imshow( "Second Image", src2 );

 namedWindow("Blend1 Image", 1);
 imshow( "Blend1 Image", src3 );

 waitKey(0);
 return 0;
}

Input Images:

Red

Green

Output Image:

For 3 images:
The modified code of merging the images is as shown below, where we have taken 3 images  of blue,green and red color and mixed them which other:
Note:Here we have assumed that the images are of same size.Hence we have not included the code for comparing the size of images which are to be merged

#include <opencv2/core/core.hpp> 
#include <opencv2/highgui/highgui.hpp> 
#include <iostream>


using namespace cv;
using namespace std;

int main()
{
 
 Mat src1, src2, src3,src4, src5, src6, src7;
 /// Read image ( same size, same type )
 src1 = imread("C:\\Users\\arjun\\Desktop\\red.jpg");
 src2 = imread("C:\\Users\\arjun\\Desktop\\green.jpg");
 src3  = imread("C:\\Users\\arjun\\Desktop\\blue.jpg");


 //Merging two images
 src4=src1 + src2;
 src5=src2 + src3;
 src6=src1 + src3;
 src7=src1 + src2 + src3;

 
 if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src2.data ) { printf("Error loading src2 \n"); return -1; }
 if( !src3.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src4.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src5.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src6.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src7.data ) { printf("Error loading src1 \n"); return -1; }
  
 //src4 = imwrite( "C:\\Users\\arjun\\Desktop\\new1.jpg",src4);
 //src5 = imwrite( "C:\\Users\\arjun\\Desktop\\new2.jpg",src5);
 //src6 = imwrite( "C:\\Users\\arjun\\Desktop\\new3.jpg",src6);
 //src7 = imwrite( "C:\\Users\\arjun\\Desktop\\new4.jpg",src7);
 //src4  = imread("C:\\Users\\arjun\\Desktop\\new1.jpg");
 //src5  = imread("C:\\Users\\arjun\\Desktop\\new2.jpg");
 //src6  = imread("C:\\Users\\arjun\\Desktop\\new3.jpg");
 //src7  = imread("C:\\Users\\arjun\\Desktop\\new4.jpg");
 
 /// Create Windows
 namedWindow("First Image", 1);
 imshow( "First Image", src1 );

 namedWindow("Second Image", 1);
 imshow( "Second Image", src2 );

 namedWindow("Third Image", 1);
 imshow( "Third Image", src3 );

 namedWindow("Blend1 Image", 1);
 imshow( "Blend1 Image", src4 );

 namedWindow("Blend2 Image", 1);
 imshow( "Blend2 Image", src5 );

 namedWindow("Blend3 Image", 1);
 imshow( "Blend3 Image", src6 );

 namedWindow("Blend4 Image", 1);
 imshow( "Blend4 Image", src7 );

 waitKey(0);
 return 0;
}

Input Image:

Red

Green

Blue

Red+Green

Green+Blue

Red+Blue

Multiplying Pixel Value By -1:

What would happen if we multiply the pixels by -1.
Since the minimum value of the pixel can be 0.Thus whole of the image would appear black.
Note:Here again we have assumed that the images are of same size.Hence we have not included the code for comparing the size of images which are to be merged

#include <opencv2/core/core.hpp> 
#include <opencv2/highgui/highgui.hpp> 
#include <iostream>


using namespace cv;
using namespace std;

int main()
{
 
 Mat src1, src2;
 /// Read image ( same size, same type )
 src1 = imread("C:\\Users\\arjun\\Desktop\\red.jpg");
 
 //Merging two images
 src2=src1 * (-1);
 
 if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src2.data ) { printf("Error loading src2 \n"); return -1; }
 
 //src2 = imwrite( "C:\\Users\\arjun\\Desktop\\new1.jpg",src2);
 //src2  = imread("C:\\Users\\arjun\\Desktop\\new1.jpg");

 /// Create Windows
 namedWindow("First Image", 1);
 imshow( "First Image", src1 );

 namedWindow("Second Image", 1);
 imshow( "Second Image", src2 );

 waitKey(0);
 return 0;
}

Output:

Think:
By including a for loop we can achieve a smooth transition effect between two images.Can't We?
Here is the code for it:

#include <opencv2/core/core.hpp> 
#include <opencv2/highgui/highgui.hpp> 
#include <iostream>


using namespace cv;
using namespace std;

int main()
{
 
 Mat src1, src2,src3;
 /// Read image ( same size, same type )
 src1 = imread("C:\\Users\\arjun\\Desktop\\red.jpg");
 src2 = imread("C:\\Users\\arjun\\Desktop\\green.jpg");

 //Checking whether images are loaded or not
 if( !src1.data ) { printf("Error loading src1 \n"); return -1; }
 if( !src2.data ) { printf("Error loading src2 \n"); return -1; }

 //Merging two images
 for (double i=0; i<=255;i=i+5)
 {
 src3=(i*src1)/255 + ((255-i)*src2)/255;

 /// Create Windows
 namedWindow("Blend Image", 1);
 imshow( "Blend Image", src3 );
 
 waitKey(250);
 }
 return 0;
}

Here is the Output:
You could note the smooth blending between the two images