Adding (blending) two images using OpenCV

Goal

In this tutorial you will learn how to:

• What is linear blending and why it is useful.
• Add two images using :add_weighted:`addWeighted <>`

Cool Theory

From our previous tutorial, we know already a bit of Pixel operators. An interesting dyadic (two-input) operator is the linear blend operator:

g(x) = (1 - \alpha)f_{0}(x) + \alpha f_{1}(x)

By varying \alpha from 0 \rightarrow 1 this operator can be used to perform a temporal cross-disolve between two images or videos, as seen in slide shows and film production (cool, eh?)

Code

As usual, after the not-so-lengthy explanation, let's go to the code. Here it is:

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

using namespace cv;

int main( int argc, char** argv )
{
 double alpha = 0.5; double beta; double input;

 Mat src1, src2, dst;

 /// Ask the user enter alpha
 std::cout<<" Simple Linear Blender "<<std::endl;
 std::cout<<"-----------------------"<<std::endl;
 std::cout<<"* Enter alpha [0-1]: ";
 std::cin>>input;

 /// We use the alpha provided by the user iff it is between 0 and 1
 if( alpha >= 0 && alpha <= 1 )
   { alpha = input; }

 /// 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; }

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

 beta = ( 1.0 - alpha );
 addWeighted( src1, alpha, src2, beta, 0.0, dst);

 imshow( "Linear Blend", dst );

 waitKey(0);
 return 0;
}

Explanation

1. Since we are going to perform:

   g(x) = (1 - \alpha)f_{0}(x) + \alpha f_{1}(x)
   

   We need two source images (f_{0}(x) and f_{1}(x) ). So, we load them in the usual way:

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

   Warning

   Since we are adding src1 and src2, they both have to be of the same size (width and height) and type.

2. Now we need to generate the g(x) image. For this, the function :add_weighted:`addWeighted <>` comes quite handy:

   beta = ( 1.0 - alpha );
   addWeighted( src1, alpha, src2, beta, 0.0, dst);
   

   since :add_weighted:`addWeighted <>` produces:

   dst = \alpha \cdot src1 + \beta \cdot src2 + \gamma
   

   In this case, \gamma is the argument 0.0 in the code above.

3. Create windows, show the images and wait for the user to end the program.

Result